بسم الله الرحمن الرحيم
Rare Earth
By: Peter Ward & Donald Brownlee
إعداد: أ. مصطفى نصر قديح
Why Life Might Be Widespread in the Universe
· The discovery that life is abundant and diverse in extreme environments is one of the most important of the Astrobiological Revolution. It gives us hope that microbial life may be present and even common elsewhere in the solar system and in our galaxy, for many environments on Earth that are now known to bear extremophile life are duplicated on other planets and moons of the solar system. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p3-4]
· Life is tougher than we thought. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p4]
· Although many types of archaeans have been found in hot-water settings, it is clear that they can live in other subterranean settings, including within solid rock itself. The first clue that life might exist hundreds to thousands of meters below Earth’s surface came in the 1920s, when geologist Edson Bastin of the University of Chicago began to wonder why water extracted from deep within oil fields contained hydrogen sulfide and bicarbonates. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p7]
· Extremophiles are not only adapted to hot and high-pressure conditions. Other groups are found in conditions thought too cold for life. All animal life eventually ceases at below-freezing temperatures. When the bodies of animals are cooled below the freezing point, they can enter a state of suspended animation, but the metabolic functions do not continue. Some extremophiles, however, circumvent this. Microbiologist James Staley of the University of Washington discovered a new suite of extremophiles living in icebergs and other sea ice. This habitat was long considered too cold to harbor life, yet life has found a way to live in the ice. This particular finding is as exciting and as relevant to the astrobiologist as the heat-loving extremophiles, for many places in the solar system are locked in ice. Other extremophiles relish chemical conditions inimical to more complex life, such as highly acidic or basic environments or very salty seawater. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p11]
· Conditions on the Martian surface today are highly inimical to life: subject to harsh ultraviolet radiation, lack of water, numbing cold. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p11]
· And even if life is now totally extinct on Mars, what of its past? Since the Viking landing of 1976, scientists have known that the ancient Mars had a much thicker atmosphere and had water on its surface, at least for a brief period of time. Three billion years ago, Mars could have been warmer because of its cloaking atmosphere. Such conditions still would have been too harsh for animal life, but judging from what we now know about the extremophiles on Earth, the early Martian environment would have been quite conducive to colonization by microbes. The extremophiles need water, nutrients, and a source of energy. All would have been present on Mars. It may be that life does not exist on Mars today. Yet there may be a great deal that we can learn about ancient Mars in its fossil record—a fossil record perhaps populated by Martian analogs to Earth’s extremophiles. Andrew Knoll of Harvard University has pointed out that for very old rocks, the fossil record may be fuller on Mars than it is on Earth, because there has been little erosion or tectonic activity on Mars to erase the billions of years of fossil records. Knoll has even told us where on Mars to search for fossils: on an ancient volcano named Apollinaris Pater, whose summit shows whitish patches interpreted to be the minerals formed by escaping gases, or in a place called Dao Vallis, a channel deposit on the flank of another ancient volcano where hot water may have flowed out from a hydrothermal system within the Martian interior. Mineral deposits there might yield a rich fossil record of ancient Martian extremophiles. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p12]
· The extremophiles have rendered the original concept of the habitable zone obsolete. In our solar system, surface water exists only on Earth (and perhaps on Europa), so if we assume that we will find life only on planets with water, then we would have to conclude that only these two bodies should harbor life of any sort. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p13]
Habitable Zones of the Universe
· We cannot know for certain what the limits are for life’s environments, but looking at what is needed to support Earth life provides a basis for estimating where in the Universe life might exist. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p15]
· One of Earth’s most basic life-supporting attributes is indeed its location, its seemingly ideal distance from the sun. In any planetary system there are regions—distances from the central star—where a surface environment similar to the present state of Earth could occur. The favorable region or distance from the star is the basis for defining the “habitable zone” (referred to by astrobiologists as the HZ), the region in a planetary system where habitable Earth clones might exist. Since its introduction, the concept of habitable zone has been widely adopted and has been the subject of several major scientific conferences, including one held by Carl Sagan near the end of his brilliant career. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p16]
· The defining aspect of the HZ is that it is the region where heating from the central star provides a planetary surface temperature at which a water ocean neither freezes over nor exceeds its boiling point . The actual width of the HZ depends on how Earth-like we decide a planet must be to be deemed habitable. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p16]
· Astronomers held the first discussions of the habitable zone in the 1960s. The range of the habitable zone was considered to be bounded by two effects: low temperature at the outer edge and high temperature at the inner edge. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p16]
· In 1978 the astrophysicist Michael Hart performed detailed calculations and reached a stunning conclusion. His work included the well-known fact that the sun becomes slightly brighter with time. About 4 billion years ago, the sun was about 30% fainter than at present. As the sun brightens, the HZ drifts outward. Hart called the small region wherein Earth would remain within the HZ over the entire age of the solar system the continuously habitable zone, or CHZ. His computations indicated that sometime during its history, Earth would have experienced runaway glaciation if it had formed 1% farther from the sun and would have experienced runaway greenhouse heating if it had formed 5% closer to the sun. Both of these effects were considered irreversible. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p18]
· The idea of a habitable zone is a very important concept of astrobiology, but being within an HZ is not an essential requirement for life. Life can exist outside the habitable zones of stars. Astronauts in an “ideally” supplied, powered, and designed spacecraft could survive almost anywhere in the solar system and (for that matter) almost anywhere in the vast, empty regions of the entire Universe. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p19]
· We believe that the concept of habitable zones should be expanded to include other categories. For planets like Earth, the animal habitable zone (AHZ) is the range of distances from the central star where it is possible for an Earth-like planet to retain an ocean of liquid water and to maintain average global temperatures of less than 50°C. This temperature appears to be the upper limit above which animal life cannot exist (at least animal life on Earth). Because water can exist on a planetary surface at temperatures up to the boiling point, a planet with liquid water on its surface (the original criterion of the habitable zone) might be much too hot to allow animal life. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p19-20]
· Although the habitable zone is described in terms of distance from a central star, it must also be thought of in terms of time. In the solar system, the HZs have definable widths; and as the sun constantly gets brighter, they move outward. Earth will eventually be left behind as the greenhouse effect causes it to become more like Venus. This will happen between 1 and 3 billion years from now, and Earth will have had about 5 to 8 billion years in the HZ. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p20]
· Biological evolution requires vast periods of time to arrive at complex organisms—periods on the order of hundreds of millions to billions of years. The AHZ and the MHZ are therefore both spatial and temporal domains. Our newly defined AHZ is obviously the most highly restrictive, but paradoxically, it also allows for the greatest diversity of life. Earth is in this zone [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p20]
Habitable zones in other stellar systems
· The concept of habitable zones is perhaps most interesting as applied to stars other than the sun. The brightness of the star determines the location of its habitable zone, but brightness in turn depends on the star’s size, type, and age. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p22]
· Ultraviolet (UV) light breaks the bonds of most biological molecules, and life must be shielded from it to survive. UV also can be disastrous for the atmospheres of Earth-like planets. It is strongly absorbed at the top of such atmospheres and is a potent high-altitude heat source than can lead to escape of the atmosphere. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p23]
· It is often said that the sun is a typical star, but this is entirely untrue. The mere fact that 95% of all stars are less massive than the sun makes our planetary system quite rare. Less massive stars are important because they are much more common than more massive ones. For stars less massive than the sun, the habitable zones are located farther inward. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p23]
· Astrobiologist Alan Hale, who has written on the problems of habitability in binary or multiple star systems, notes, “The effects of nearby stellar companions on the habitability of planetary environments must be considered in estimating the number of potential life-bearing planets within the Galaxy.” [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p24]
· The most pressing question is whether planets, once formed in a multiple star system, can achieve stable orbits. The rise of life (at least on Earth) seems to require long periods of constant conditions, which require stable or bits. Highly elliptical orbits wherein a planet moves in and out of the CHZ might allow microbial life to form and even flourish but probably would be lethal to animal life. In such systems planets might form, but their orbits would be perturbed by the various gravitational forces of more than a single star, which would eventually either eject the planets or cause them to fall into one of the stars. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p24-25]
· Finally, the residents of any planet in a multiple star system will have to deal with the stellar evolution of two or more suns. Our sun is getting brighter through time. This gradual brightening causes the habitable zones to migrate ever outward. With two or more suns undergoing the same process, we might expect habitable zones to migrate even faster through time. Although this might not adversely affect microbial life, it could inhibit animal life. All in all, it appears that multiple star systems might be regions that could support life, but perhaps not animal life. They are certainly less favorable habitats for animal life than solitary stars. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p25]
· Unusual stellar entities such as neutron stars and white dwarf stars are probably uninhabitable by any form of life. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p25]
· What of regions where star frequency (the number of stars in a volume of space) is very high? Such regions include open star clusters and globular star clusters. Open clusters are unlikely to be hospitable to animal life because they are too young. Most are composed of relatively new stars, where life at least advanced life such as higher plants and animals—would not yet have had a chance to develop. Many open clusters are dispersed by the time they have orbited their galaxy several times. Others are more long-lasting, but they too have problems. Because neighboring stars are so close, planetary orbits can be perturbed, causing planets to be ejected, to enter highly elliptical orbits, or even to fall into their suns. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p25-26]
· There would be no night on any planets in such clusters. There might be habitable stellar systems in such regions, but the very number of stars would make them more dangerous and less congenial to the maintenance of animal life than more widely separated stars; there is too much radiation and particles, too many chances for gravitational changes to affect the orbits of planets in any such mass. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p26]
· In 1974 a group of astronomers led by Frank Drake directed a radio signal toward the globular cluster M13. It was hoped that other radio-astronomers living around one of the 300,000 stars in the cluster might receive the message. Today, only a few decades later, we realize that there is no chance anyone will be there to take the call when the radio message arrives at M13, some 24,000 years from now. If the experiment were to be repeated, the beam would be directed toward stars more likely to have planets and life. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p26-27]
Habitable zones in the galaxy
· Our galaxy is a spiral galaxy (the other types are elliptical and irregular galaxies). In most galaxies the concentration of stars is highest in the center and diminishes away from the center. Spiral galaxies are dish-shaped (round, but flat if viewed from the side), with branching arms when viewed from the top. But viewed from the side they are quite flat. Our galaxy has an estimated diameter of about 85,000 light-years. Our sun is about 25,000 light-years from the center, in a region between spiral arms where star density is quite low compared to the more crowded interior. In this position we slowly orbit the central axis of the galaxy. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p27]
· Our star—by chance—is located in the “habitable zone” of the galaxy. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p28]
· Life is a very complex and delicate phenomenon that is easily destroyed by too much heat or cold and by too many gamma rays, X-rays, or other types of ionizing radiation. The center of any galaxy produces all of these. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p28]
· Among the lethal stellar members of any galaxy are the neutron stars called magnetars. These collapsed stars are small but astonishingly dense, and they emit X-rays, gamma rays, and other charged particles into space. Because energy dissipates as the square of distance, these objects are no threat to our planet. Closer to the center of the galaxy, however, their frequency increases. Any galactic center is a mass of stars, some the lethal neutron stars, and it seems most unlikely that any form of life as we know it could exist nearby. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p28]
· Any star going supernova would probably sterilize life within a radius of 1 light-year of the explosion and affect life on planets as far as 30 light-years away. The very number of stars in galactic centers increases the chances of a nearby supernova. Our sun and planet are protected simply by the scarcity of stars around us. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p28]
· our planet has a solid/liquid metal core that includes some radioactive material giving off heat. Both attributes seem to be necessary to the development of animal life: The metal core produces a magnetic field that protects the surface of the planet from radiation from space, and the radioactive heat from the core, mantle and crust fuels plate tectonics, which in our view is also necessary for maintaining animal life on the planet. No planet such as Earth can exist in the outer regions of the galaxy. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p29]
· Not only is Earth in a rare position in its galaxy; it may also be fortunate (at least as far as having life is concerned) in being in a spiral rather than an elliptical galaxy. Elliptical galaxies are regions with little dust which apparently exhibit little new star formation. The majority of stars in elliptical galaxies are nearly as old as the universe. The abundance of heavy elements is low, and although asteroids and comets may occur, it is doubtful that there are full-size planets. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p29]
Habitable zones, and times, in the universe
· life (at least life as we know it) requires many elements that had to be created after the Big Bang (the advent of the Universe, some 15 billion years ago). Twenty-six elements (including carbon, oxygen, nitrogen, phosphorus, potassium, sodium, iron, and copper) play a major role in the building blocks of advanced life, and many others (including the heavy radioactive elements such as uranium) play an important secondary role by creating, deep within Earth, heat indirectly necessary for life. All of these elements were created within the centers of stars—often in exploding stars, or supernovae—rather than in the Big Bang itself, so they were not present in sufficient abundance for perhaps the first 2 billion years or more of the Universe. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p29-30]
· Then, the “habitable zone” of the Universe, in the sense of time, began only after its first 2 billion years. The early history of the Universe was also dominated by objects known as quasars, which would have been very dangerous [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p30]
· The early Universe must have been lifeless or at least empty of advanced life, and quite remarkably, there are also limits on the time during which the Universe can exhibit Earth-like planets that provide adequate life support for advanced life. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p30]
· In our galaxy, stars that form at present have less of these radioisotopes than the sun did when it formed 4.6 billion years ago. It is entirely possible that any true Earth clones now forming around other stars would not have enough radioactive heat to drive plate tectonics, a key process that helps stabilize Earth’s surface temperature. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p30]
· The most distant galaxies visible in these photos probably date to some time during the first few billion years after the start of the Universe, and hence they may antedate life anywhere. It is unlikely that any of the stars in these galaxies could have Earth-like planets because the heavy elements to build them were not yet abundantly available. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p32]
The end of planetary habitability
· In other planetary systems, primitive life might flourish but never advance to the point where forests and flying animals even get a serious chance to evolve. Stars with short lifetimes, unstable planetary atmospheres, changes in orbital or spin axis, massive extinctions, impacts, crustal catastrophes, the cessation of plate tectonics, or any of a whole raft of other problems could prevent the evolution of advanced life or its prolonged survival. And on Earth itself, complex life has thrived only for the last 10% of the planet’s existence. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p32]
· Life on our planet will eventually be roasted out of existence. The sun is slowly getting brighter. It is now 30% brighter than it was in the early history of the planet. Over the next 4 billion years it will double in brightness. Even if life survives this travail, it will soon be stilled. About 4 billion years from now, the sun will begin to expand rapidly in size, and its brightness will dramatically increase. The sun will become a red giant, as did the stars Antares in the constellation Scorpio and Betelgeuse in the constellation Orion. In a billion-year time span, its brightness will increase over 5000 times. At the very beginning of this process, Earth’s oceans will vaporize, driving our precious water supply into space. In the final stages of its transformation into a red giant, the sun will expand to the point where it will nearly reach the orbit of Earth. The Universe will be one living planet poorer. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p32]
SUMMARY
· A review of habitable zones—for animals as well as microbes, and in the galaxy and Universe as well as around our sun—leads to an inescapable conclusion: Earth is a rare place indeed. Perhaps the most intriguing finding of this line of research is that Earth is rare as much for its abundant metal content as for its location relative to the sun. the metal-rich core of our Earth is responsible for much of its hospitality to life. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p33]
Building a Habitable Earth
· The Earth is the only world known, so far, to harbor life. There is nowhere else, at least in the near future, [to which] our species could migrate. (Carl Sagan, Pale Blue Dot). [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p35]
· Most of the Universe is too cold, too hot, too dense, too vacuous, too dark, too bright, or not composed of the right elements to support life. Only planets and moons with solid surface materials provide plausible oases for life as we know it. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p35]
· Earth is unique in both its physical properties and its proven ability to sustain life. The success of Earth in supporting life for billions of years is the result of a remarkable sequence of physical and biological processes; knowledge of these processes is our main source of insight into the possibilities of life elsewhere. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p35]
· Using Earth to generalize about what life requires is, of course, fraught with uncertainty. Lacking knowledge of any extraterrestrial life forms, we cannot be confident that we understand the optimal or even the minimal conditions necessary to support life beyond this planet. But our planet is an uncontested success in terms of the abundance and variety of life it sports, even though it was certainly sterile soon after its formation. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p36]
· Earth is the only location in the universe that is known to have life, but it is only one of perhaps millions of habitats in our galaxy, and trillions in the Universe, that might also harbor life. From the biased viewpoint of Earthlings, however, it does appear that Earth is quite a charmed planet. It has the right properties for the only type of life we know, it formed in the right place in the solar system, and it underwent a most remarkable and unusual set of evolutionary processes. Several of its neighbors in the solar system even played highly fortuitous, supporting roles in making Earth a congenial habitat for life. The near-ideal nature of Earth as a cradle of life can be seen in its prehistory, its origin, its chemical composition, and its early evolution. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p36]
· What are the most important factors that allowed Earth to support advanced life? Earth has offered (1) at least trace amounts of carbon and other important life-forming elements, (2) water on or near the surface, (3) an appropriate atmosphere, (4) a very long period of stability during which the mean surface temperature has allowed liquid water to exist on its surface, and (5) a rich abundance of heavy elements in its core and sprinkled throughout its crust and mantle regions. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p36]
· Earth is actually the final product of an elaborate sequence of events that occurred over a time span of some 15 billion years, three times the age of Earth itself. Some of these events have predictable outcomes, whereas others are more chaotic, with the final outcome controlled by chance. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p36]
· The evolutionary path that led to life included element formation in the Big Bang and in stars, explosions of stars, formation of interstellar clouds, formation of the solar system, assembly of Earth, and the complex evolution of the planet’s interior, surface, oceans, and atmosphere. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p36-37]
· it is unlikely that Earth could ever be truly replicated. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p37]
· Earth-like planets could certainly be made, but each outcome would differ in critical ways. This is well illustrated by the fantastic variety of planets and satellites that formed in the solar system. They all started with similar building materials, but the final products are vastly different from each other. Just as the more familiar evolution of animal life involved many evolutionary pathways with complex and seemingly random branch points, the physical events that led to the formation and evolution of the physical Earth also required an intricate set of nearly irreproducible circumstances. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p37]
· Any construction project requires that building materials be on site before the actual construction begins. The formation of Earth was no different. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p37]
Creation of the elements
· The elements are the building blocks of both planets and life. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p37]
· The temperature of the Universe, during its first half-hour, was above 50 million degrees Celsius. At this temperature, positively charged protons (the nuclei of hydrogen) could occasionally collide with enough energy to overwhelm the electrostatically repulsive effects of their like positive charges and fuse together to form helium. This simple fusion process is the secret of the stars. It is the reason why the night sky is not dark, the reason why Earth’s surface is not frozen, the reason why planets can exist; it is the energy source that powers life on Earth. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p38-39]
· During the first 15% of the age of the Universe, a period of over 2 billion years, stars could form, but there was not enough dust and rocks for them to have terrestrial planets. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p39]
· The trick for getting from helium to the generation of planets, and ultimately to life, was the formation of carbon, the key element for the success of life and for the production of heavy elements in stars. Carbon could not form in the early moments following the Big Bang, because the density of the expanding mass was too low for the necessary collisions to occur. Carbon formation had to await the creation of giant red stars, whose dense interiors are massive enough to allow such collisions. Because stars become red giants only in the last 10% of their lifetimes (when they have used up much of the hydrogen in their cores), there was no carbon in the Universe for hundreds of millions to several billion years after the Big Bang—and hence no life as we know it for that interval of time. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p40]
· Carbon formation requires three helium atoms (nuclei) to collide at essentially the same time: a three-way collision. What actually happens is that two helium atoms collide to form the beryllium-8 isotope, and then, within a tenth of a femtosecond (1/10,000,000,000,000,000 second) before this highly radioactive isotope decays, it must collide with and react with a third helium nucleus to produce carbon. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p40]
· The sequence of element production in the Big Bang and in stars provided not only the elements necessary for the formation of Earth and the other terrestrial planets but also all of the elements critical for life—those actually needed to form living organisms and their habitats. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p40]
· Among the most important of these elements were: iron, magnesium, silicon, and oxygen to form the structure of Earth; uranium, thorium, and potassium to provide radioactive heat in its interior; and carbon, nitrogen, oxygen, hydrogen, and phosphorus, the major “biogenic” elements that provide the structure and complex molecular chemistry of life. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 40-41]
· The elements carbon, oxygen, hydrogen, and nitrogen dominate its biotic inhabitants, or life. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p41]
· Carbon is a trace element in Earth, but as we have noted, it is the key element for terrestrial life, and its rich chemical properties are probably the basis of any alien life as well. Hydrogen is also a trace element in planet Earth; still its gifts include the oceans and all water, the essential fluid of terrestrial life. Other important trace elements are uranium, potassium, and thorium. The decay of these radioactive elements heats Earth’s interior and fuels the internal furnace that drives volcanism, the vertical movement of matter within its interior, and the drift of continents on its surface. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 42]
· There are also systematic variations within the Milky Way galaxy. Stars in the center of the galaxy are richer in metals (astronomical slang for elements heavier than helium) than stars at the outer regions. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 42-43]
· The abundance of heavy elements enters into Rare Earth considerations because it influences the mass and size of planets. If Earth had formed around a star with lower heavy-element abundance, it would have been smaller because there would have been less solid matter in the annular ring of debris from which it accumulated. Smaller size can adversely influence a planet’s ability to retain an atmosphere, and it can also have long-term effects on volcanic activity, plate tectonics, and the magnetic field. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 43]
· If the sun were older, if it were further from the center of the galaxy, or even if it were a typical onesolar-mass star (equal to the mass of the sun), then Earth would probably be smaller. If Earth were just a little smaller, would it have been able to support life for long periods of time? [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 43]
· Of all these properties of the solar system, perhaps the most curious—and at the same time the least appreciated—is that it is so rich in metals. Recent studies by Guillermo Gonzalez and others have shown that the sun is quite rare in this respect. Metals are necessary attributes of planets: Without them there would be neither magnetic fields nor internal heat sources. And metals may also be a key to the development of animal life: They are necessary to important organic constituents of animals (such as copper and iron blood pigments). How did we get our surplus treasure trove of metals? [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 43]
Construction of planet Earth
· Let’s take a closer look at what happened. The formation process began when a mass of interstellar material became dense and cool enough to grow unstable and gravitationally collapse into itself to form a flattened, rotating cloud—the solar nebula. As the nebula evolved, it quickly assumed the form of a disk-shaped distribution of gas, dust, and rocks orbiting the proto-sun, a short-lived juvenile state of the sun when it was larger, cooler, and less massive and was still gathering mass. The planets formed from this nebula, even though the nebula itself existed for only about 10 million years before the majority of its dust and gas either formed large bodies or was ejected from the solar system. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 44]
· In the solar nebula, 99% of the mass was gas (mostly hydrogen and helium), and the heavier elements that could exist as solids made up the remaining 1%. Some of the solids were surviving interstellar dust grains; others were formed in the nebula by condensation. This gas played a major role in forming the sun, Jupiter, and Saturn. All of the other planets, the asteroids, and the comets formed primarily from the solids. Solids were only a trace component of the nebula as a whole, but they could undergo a concentration process that gas could not. As the nebula evolved, dust, rocks, and larger solid bodies separated from the gas and became highly concentrated, forming a disk-like sheet in the mid-plane of the solar nebula, in some ways resembling the rings of Saturn. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 44-45]
· One of the fundamental processes that led to the production of planets was accretion, the collision of solids and their sticking to one another to form larger and larger bodies. This complex process involved the formation, evolution, destruction, and growth of vast numbers of bodies ranging in size from sand grains to planets. Most of the mass of a planet was accreted from materials in its “feeding zone”—a ring section of the solar nebula disk that extended roughly halfway to the nearest neighboring planets. If viewed from above, the concentric feeding zones could be imagined as a target, with one planet forming in each radial band. The composition of solids varied with distance from the sun, so the nature of each planet was critically influenced by its feeding zone. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 45]
· The accretion process was responsible for unique and very important aspects of Earth. An enigma of Earth’s formation is its composition and particular location in the solar system. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 45]
· Earth formed within the habitable zone of the sun. A grand paradox of terrestrial planets is that if they form close enough to the star to be in its habitable zone, they typically end up with very little water and a dearth of primary life-forming elements such as nitrogen and carbon, compared to bodies that formed in the outer solar system. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 45]
· In other words, the planets that are in the right place, and thus have warm surfaces, contain only minor amounts of the ingredients necessary for life. The accretion process accumulated solids from the nebula, but the composition of solid dust, rocks, and planetesimals in the nebula varied with distance from the sun. At Earth’s distance from the center of the solar nebula, the temperature was too high for abundant carbon, nitrogen, or water to be bound in solid materials that could accrete to form planetesimals and planets. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 45-46]
· Ice and carbon/nitrogen-rich solids were too volatile and had no means of efficiently forming solids in the warm inner regions of the nebula. Thus Earth has only trace amounts of these volatile components, compared to bodies that formed farther from the sun. An excellent example is the case of the carbonaceous meteorites, thought to be samples of typical asteroids formed between Mars and Jupiter. These bodies contain up to 20% water (in hydrous minerals similar to talc) and up to 4% carbon. The bulk of Earth, by comparison, is only 0.1% water and 0.05% carbon. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 46]
· Such gravitational effects from encounters with planets can cause asteroidal and cometary debris, rich in light elements, to assume earth-impacting orbits. This “cross-talk” caused some degree of mixing between different feeding zones and provided a means of bringing the building blocks of life to what might otherwise have been a lifeless planet lacking in many biogenic elements because it formed too close to the sun. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 48]
· Life on Earth formed from organic compounds, and it is possible that prebiotic compounds from the outer solar system stimulated the first steps toward the origin of life on Earth. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 48]
· In the final assembly stages, however, many large bodies orbited within the feeding zone, some as large as the planet Mars. The dramatic collision of these large bodies with the young Earth played a role in determining the initial tilt values of Earth’s spin axis, the length of the planet’s day, the direction of its spin, and the thermal state of its interior. It is widely believed that the impact of a Mars-sized body was responsible for formation of the Moon, an oddly large satellite relative to the size of its mother planet. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 50]
· The final composition of Earth had several crucial structural effects. First, enough metal was present in the early Earth to allow formation of an iron- and nickel-rich innermost region, or core, that is partially liquid. This enables Earth to maintain a magnetic field, a valuable property for a planet sustaining life. Second, there were enough radioactive metals such as uranium to make for a long period of radioactive heating of the inner regions of the planet. This endowed Earth with a long-lived inner furnace, which has made possible a long history of mountain building and plate tectonics—also necessary, we believe, to maintaining a suitable habitat for animals. Finally, the early Earth was compositionally able to produce a very thin outer crust of low-density material, a property that allows plate tectonics to operate. The thicknesses and stability of Earth’s core, mantle, and crust, could have come about only through the fortuitous assemblage of the correct elemental building blocks. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 50-51]
· There is no direct information about Earth’s early history because no rocks older than 3.9 billion years have survived. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 51]
· It is the heritage of terrestrial life that violent events and truly hostile environments preceded it. The violent events of these times may have determined the final abundance of water and carbon dioxide, two compounds that play crucial roles in the ability of Earth to maintain an environment where life can survive. It is interesting to speculate about what would have happened if the final abundance of these had varied. If Earth had had just a little more water, continents would not extend above sea level. Had there been more CO2, Earth would probably have remained too hot to host life, much like Venus. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 52]
· Without an atmosphere there would be no life on Earth. Its composition over Earth history is one of the reasons why our planet has remained a life-supporting habitat for so long. Today the atmosphere is highly controlled by biological processes, and it differs greatly from those of other terrestrial planets, which range from essentially no atmosphere (Mercury) to a CO2 atmosphere a hundred times denser (Venus) and a CO2 atmosphere a hundred times less dense (Mars). [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 52]
· Even viewed from a great distance, Earth’s strange atmospheric composition would provide a strong clue that life is present. Composed of nitrogen, oxygen, water vapor, and carbon dioxide (in descending order of abundance), it is not an atmosphere that could be maintained by chemistry alone. Without life, free oxygen would rapidly diminish in the atmosphere. Some of the O2 molecules would oxidize surface materials, and others would react with nitrogen, ultimately forming nitric acid. Without life, the CO2 abundance would probably rise, resulting in a nitrogen and CO2 atmosphere. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 52]
· To an alien astronomer, Earth’s atmospheric composition would be clearly out of “chemical equilibrium.” This situation would provide convincing evidence of life and a vigorous ecosystem capable of controlling the chemical composition of the atmosphere. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 52]
· The atmosphere was formed by outgassing from the interior, a process that released volatiles originally carried to Earth in planetesimal bodies as well as by delivery from impacting comets. The composition and density of the atmosphere are influenced by the amount and nature of the original accreted materials, but in Earth’s case they are most strongly affected by processes that recycle atmospheric components in and out of the atmosphere. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 52-53]
· Land provides a home for nonaquatic life, and the vast regions of shallow water that surround land offer crucial and complex habitats where oceanic life can flourish. Shallow water is also a setting where interactions between ocean and atmosphere alter the composition of the atmosphere. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 53]
· Earth’s topography and the total amount of water determine what fraction of Earth’s surface is land. The oceans contain enough water to cover a spherical Earth to a depth of about 4000 meters. If the surface of the planet varied only a few kilometers in elevation, Earth would be devoid of land. It is easy to imagine an Earth covered by water, but it is difficult to imagine that, with its present water supply, it could ever be dominated by land. To make more land or even produce an Earth dominated by land, the oceans would have to be deeper to accommodate the same volume of water in spite of having less total surface area. Thus the planet’s remarkable mixture of land and oceans is a balancing act. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 53]
· Land formation on Earth has throughout its history occurred by two principal means: simple volcanism creating mountains and the more complex processes related to plate tectonics. Simple volcanism leads to the formation of small islands such as Hawaii and the Galapagos archipelagos. Volcanic islands similar to Hawaii were probably the predominant landform on the early Earth. These were lifeless islands with no plant roots to slow the ravages of erosion. Low islands would have been bleak and desert-like, sterile surfaces bombarded by intense ultraviolet radiation from the sun unfiltered by Earth’s early atmosphere. If climatic conditions were anything like today, the higher islands would have had copious rainfall leading to extensive erosion. Although Earth evolved beyond the stage where its only land consisted of eroding and doomed islands, many of the water-covered planets elsewhere probably only have transient basaltic islands, at best. At worst they have no land at all. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 53-54]
· In the case of our planet, Earth managed to form continents that could endure for billions of years. This required the formation of land masses made of relatively low-density materials that could permanently “float” on the denser underlying mantle while parts of them extended above the sea. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 54]
· How did the first continents form? Early continental land masses may have formed when the impact of large comets and asteroids melted the outer region of Earth to form a “magma ocean,” a planet-smothering layer of molten rock. The concept of a global magma ocean grew out of studies of the Moon. The heat generated from the rapid accretion of many planetesimals into our solid Earth appears have melted the upper 400 kilometers of the Moon’s surface. In the lunar case, as the magma ocean cooled, myriad small crystals of a mineral called plagioclase feldspar (a low-density mineral rich in calcium, aluminum, and silicon) formed and floated upward to create a low-density crust nearly 100 kilometers thick. This ancient crust is still preserved and can even be seen with the naked eye as the bright, mountainous lunar “highlands.” In like fashion, a magma ocean on Earth, may have led to formation of the first continents. Alternatively, the processes leading to the formation of the first continental land may have occurred beneath large volcanic structures. The initial land mass was small and was not until half way through its history that land covered more than 10% of the Earth’s surface. In any event, the outcome was a planet with both land and sea. This fortuitous combination may be the most important factor that ultimately made life possible. By about 4.5 billion years ago Earth was built. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 54]
4- Life’s First Appearance on Earth
· Scientists now realize that habitats suitable for microbial life are far more widely distributed in our solar system, and surely in the Universe as well, than was considered possible even in the most optimistic views of the 1980s and before. On the other hand, these same studies are showing that complex life—such as higher animals and plants—may have fewer suitable habitats than was previously thought. But just because life could exist in a place doesn’t mean it is actually there. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 55]
· Life can be widely distributed in the Universe only if it can come into being easily. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p 55-56]
HOW DID LIFE BEGIN?
· The great British biologist J.B.S. Haldane pointed out that there are about as many living cells in a human being as there are atoms in a cell. Individual atoms themselves are not alive, though. “The line between living and dead matter is therefore somewhere between a cell and an atom,” Haldane concluded. Somewhere in between atoms and the living cell there is the entity known as a virus. Viruses are smaller than the smallest living cells and do not seem to be alive when isolated (they cannot reproduce), yet they are capable of infecting and then changing the internal chemistry of the cells they invade. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p56]
· We are also sure that all life on Earth is based on the DNA molecule. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p56]
· The order of bases on each strand of DNA supplies the language of life; these are the genes that code for all information about a particular life form. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p56-57]
· The fact is that all organisms on Earth share the same genetic code is the strongest evidence that all life here derives from one common ancestor. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p57]
· Life seems to have appeared on this planet somewhere between 4.1 and 3.9 billion years ago, or some 0.5 to 0.7 billion years after Earth originated. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p57]
· The Earth formed about 4.5 to 4.6 billion years ago from the accretion of variously sized “planetesimals,” or small bodies of rock and frozen gases. For the first several hundred million years of its existence, a heavy bombardment of meteors pelted the planet with lashing violence. Both the lava-like temperatures of Earth’s forming surface and the energy released by the barrage of incoming meteors during this heavy bombardment phase would surely have created conditions inhospitable to life. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p57-58]
· About 4.5 billion years ago, Earth began to differentiate into different layers. The innermost region, a core composed largely of iron and nickel, became surrounded by a lower-density region called the mantle. A thin crust of still lesser density rapidly hardened over the mantle, while a thick, roiling atmosphere of steam and carbon dioxide filled the skies. In spite of its being waterless on the surface, great volumes of water would have been locked up in Earth’s interior, and water would have been present in the atmosphere as steam. As lighter elements bubbled upward and heavier ones sank, water and other volatile compounds were expelled from the interior and added to the atmosphere. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p58]
· The heavy bombardment by comets and asteroids lasted more than half a billion years and finally began to diminish around 3.8 billion years ago as the majority of debris was incorporated into the planets and moons of our solar system. During the period of heaviest impact, the steady bombardment would have scarred our planet by craters in the same manner as the moon. Yet the comets and asteroids raining in from space delivered an important cargo with each blow. Some astronomers believe that much, or even most, of the water now on our planet’s surface arrived with the incoming comets; others think that only a minority of Earth’s water arrived in this fashion. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p58]
· Cooling by radiation into space would take place, but a new ocean produced by condensing rain would not fully form for thousands of years after the event.Much of the revolutionary detective work behind these conclusions was described in 1989 by Stanford University scientist Norman Sleep, who realized that the impact of such a large asteroid or comet could evaporate an ocean 10,000 feet deep, sterilizing Earth’s surface in the process. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p59]
· Somehow, the fact that we have not yet detected life on Mars seems consistent with its satellite images. A waterless world fits our picture of a lifeless world. Even when the young Earth was covered with water, however, it was still devoid of life. But not for long. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p60]
A RECIPE FOR LIFE
· Most scientists are confident that life had already arisen 3.8 to 3.9 billion years ago, at about the time when the heavy bombardment was coming to an end. The evidence indicative of life’s appearance is not the presence of fossils but the isotopic signatures of life extracted from rocks of that age in Greenland. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p60]
· Determining how the first DNA molecules appeared on Earth has been a very difficult scientific problem, and it is still far from solved. No one has yet discovered how to combine various chemicals in a test tube and arrive at a DNA molecule. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p62]
· To build DNA—and, ultimately, life—requires the following ingredients and conditions: energy, amino acids, factors that make chemical concentration possible, catalysts, and protection from strong radiation or excess heat. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p62]
· The chemical evolution of life entails four steps: 1. The synthesis and accumulation of small organic molecules, such as amino acids and molecules called nucleotides. The accumulation of chemicals called phosphates (one of the common ingredients in plant fertilizer) would have been an important requirement, because these are the “backbone” of DNA and RNA. 2. The joining of these small molecules into larger molecules such as proteins and nucleic acids. 3. The aggregation of the proteins and nucleic acids into droplets that took on chemical characteristics different from their surrounding environment. 4. The replicating of the larger complex molecules and the establishment of heredity. The DNA molecule can accomplish both, but it needs help from other molecules, such as RNA. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p62]
· RNA molecules are similar to DNA in having a helix and bases. But they differ in having but a single strand, or helix, rather than the double helix of DNA. They also differ in the makeup of their base composition: Instead of thymine, they contain a base called uracil. Most RNA is used as a messenger, sent from DNA to the site of protein formation within a cell, where the specific RNA provides the information necessary to synthesize a particular protein. To do this, a DNA strand partially unwinds, and an RNA strand forms and keys into the base-pair sequence on the now-exposed DNA molecule. This new RNA strand matches with the base pairs of the DNA and, in so doing, encodes information about the protein to be built. This process is called translation. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p63]
· De Duve paints the following picture of our planet more than 4 billion years ago. ” Brought down by rainfall and by comets and meteorites, the products of these chemical re-shufflings progressively formed an organic blanket around the lifeless surface of our newly condensed planet. Everything became coated with a carbon-rich film, openly exposed to the impacts of falling celestial bodies, the shocks of earthquakes, the fumes and fires of volcanic eruptions, the vagaries of climate, and daily baths of ultraviolet radiation. Rivers and streams carried these materials down to the sea where the materials accumulated until the primitive oceans reached the consistency of hot dilute soup, to quote a famous line from the British geneticist J.B.S. Haldane. In rapidly evaporating inland lakes and lagoons, the soup thickened to a rich puree. In some areas, it seeped into the inner depths of the Earth, violently gushing back as steamy geysers and boiling underwater jets. All these exposures and churning induced many chemical modifications and interactions among the original components showered from the skies. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p64]
· Both of these ideas about the first development of life have at their crux the need to bring various chemical components together somehow and then, from these aggregates, assemble very complex molecules. In the RNA model, the various chemicals assemble in liquid; in the second model, a mineral template becomes an assembly site. There is as yet no consensus on which of these alternatives is correct—or even on whether they are the only alternatives. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p66]
How long did it take?
· Miller thinks the transition from “prebiotic soup” to cyanobacteria (the microbes we find today sliming swamps and ponds) may have taken as little as 10 million years. Miller based his conclusion on three lines of evidence: the rate of plausible chemical reactions leading to the formation of the building blocks of life; the relative stability of these building blocks, once made (the number of years they remain intact before decomposition); and the rates of new gene formation through “amplification” in modern bacteria. The first of these, the rate of amino acid synthesis, is very fast—from minutes to tens of years at the most. Once formed, most organic compounds (such as sugars, fatty acids, peptides, and even RNA and DNA) can last from tens of years to thousands of years. Thus these are not rate-limiting steps; putting the pieces together was what took time. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p66-67]
· Miller sees three bottlenecks: (1) the origin of replicating systems—essentially the formation of RNA and then that of DNA, which could duplicate itself; (2) the emergence of protein biosynthesis, or the ability of the RNA molecule to begin synthesizing proteins, the actual material of cells; and (3) the evolutionary development of the various, essential cell operations, such as DNA replication, production of ATP (the energy source within cells), and other basic metabolic pathways. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p67]
· In a 1996 article written with Antonio Lazcano, Miller argues that the time necessary to go from soup to bugs may have been far less than 10 million years. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p67]
· Making life may be a rapid operation—a key observation supporting our contention that life may be very common in the Universe. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p67]
· But if not in a pond or tide pool, where could these components have come together to produce life? Here is an alternative view from microbiologist Norman Pace, one of the great pioneering microbiologists interested in life’s evolution: We can now imagine, based on solid results, a fairly credible scenario for the terrestrial events that set the stage for the origin of life. It seems fairly clear, now, that the early earth was, in essence, a molten ball with an atmosphere of high-pressure steam, carbon dioxide, nitrogen, and other products of volcanic emissions from the differentiating planet. It seems unlikely that any landmass would have reached above the waves (of a global ocean) to form the “tide pools” invoked by some theories for the origin of life. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p68-69]
· The “where” of life’s origination is obviously controversial, and as pointed out by University of Washington astronomer Guillermo Gonzalez, the favored habitats appear to depend on a given scientist’s discipline. In his delightful 1998 essay “Extraterrestrials: A Modern View,” Gonzalez noted, The kind of origin of life theory a scientist holds to seems to depend on his/her field of specialty: oceanographers like to think it began in a deep sea thermal vent, biochemists like Stanley Miller prefer a warm tidal pool on the Earth’s surface, astronomers insist that comets played an essential role by delivering complex molecules, and scientists who write science fiction part time imagine that the Earth was “seeded” by interstellar microbes. The fact that life appeared soon after the termination of the heavy bombardment about 3.8 billion years ago tells little about the probability of the origin of life—it could have been a unique event requiring extraordinary conditions. However, there are a few very basic ingredients that are required by any conceivable kind of life, overactive imaginations notwithstanding. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p69]
· What about the transfer of microbes between stellar systems? Although microbes are killed by radiation in space some bacteria or viruses embedded in dust grains might be shielded sufficiently to survive. If so, they might possibly “seed” regions of a galaxy through the process known as Panspermia, as suggested by Fred Hoyle and his collaborators in the early 1980s. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p73]
· Once any planet in a particular planetary system is “infected” with life, natural processes may spread that life to other systems. Of course, this process can work only on organisms that can withstand the raw vacuum of outer space. Animal life cannot spread in this fashion. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p73]
· Whatever and wherever its source, life was rooted and probably pervasive on the planet Earth by 3.5 billion years ago. Evolution was at work, and a host of new species proliferated as life began to exploit new food, new habitat, and new opportunities. The possible ways in which life may have first formed and the speed of its formation suggest that life may not be a unique property of this planet. Perhaps it can be found on any planet or moon with heat, hydrogen, and a little water in a rocky crust. Such environments are common in our solar system and probably in other parts of the galaxy and Universe, so life itself may indeed be widespread. The lesson of Earth is that not only can it live in extreme environments, it can probably form in such places as well. Life—but not animal life. How that further step to animal life occurred on Earth—and whether we can use this essential step in Earth’s history as a means of modeling the evolution and formation of analogs to animal life on other planets. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p79-80-81]
· In summary, we can characterize eukaryotic cells as having seven major characteristics that distinguish them from prokaryotes. 1. In eukaryotes, DNA is contained within a membrane-bounded organelle, the nucleus. 2. Eukaryotes have other enclosed bodies within the cell—the organelles such as mitochondria (which produce energy) and chloroplasts (tiny inclusions that allow photosynthesis). 3. Eukaryotes can perform sexual reproduction. 4. Eukaryotes have flexible cell walls that enable them to engulf other cells through a process known as phagocytosis. 5. Eukaryotes have an internal scaffolding system composed of tiny protein threads that allow them to control the location of their internal organelles. This cytoskeleton also helps eukaryotes replicate their DNA into two identical copies during cell division. This system is both more complicated and more precise than the simple splitting of DNA that occurs in prokaryotic cells. 6. Eukaryotic cells are nearly always much larger than prokaryotes; they usually have cell volumes at least 10,000 times greater than that of the average prokaryotic cell. An internal architecture and salt balance systems far more advanced than those found in prokaryotes that make this larger size possible. 7. Eukaryotes have much more DNA than prokaryotes—usually 1000 times as much. The DNA in the eukaryotic cell is stored in strands, or chromosomes, and is usually present in multiple copies. Between the first evolution of life and the first cell to sport a eukaryotic grade of organization, perhaps more than 1.5 billion years would pass. Why did it take so long? [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p90-91]
7-The Enigma of the Cambrian Explosion
· Our planet was without animal life for the first 3.5 billion years of its existence and was without animals large enough to leave a visible fossil record for nearly 4 billion years. But when, 550 million years ago, sizable and diverse animal life finally burst into the oceans, it did so with a figurative bang—in a relatively sudden event known as the Cambrian Explosion. Over a relatively short interval of time, all of the animal phyla (the categories of animal life characterized by unique body plans, such as arthropods, mollusks, and chordates) either evolved or first appear in the fossil record. Undoubted fossils of metazoan animals have never been found in 600-million-year-old sedimentary strata, no matter where on Earth we go. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p125]
· The Cambrian Explosion presents a great challenge to astrobiology [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p126]
· This sudden appearance of larger animals in the fossil record is the most dramatic aspect of the Cambrian Explosion. It drove Charles Darwin to distraction and challenged the newly evolving field of geology, which had taken as its guiding principle the idea that important events in Earth history unfolded gradually, not abruptly. Yet even to the earliest geologists, the Cambrian Explosion seemed anything but gradual. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p128]
· Sedgewick defined the base of the Cambrian as the stratal level where the first trilobite fossils could be found, and that view prevailed for over a century. Anywhere in the world where trilobite-bearing strata overlay unfossiliferous strata was considered to mark the base of the Cambrian. Recently, however, the way in which the base of the Cambrian is recognized has changed. It is now marked at a level that Sedgewick would have considered below the “base” of the Cambrian.Today geologists use the first occurrence of a particular trace fossil (the fossilized record of animal behavior, rather than the preserved hard parts of the animal itself) as the base of the Cambrian system. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p129]
· Sedgewick’s discovery of the seemingly instantaneous appearance of complex fossils convinced most scientists of his time that life was spontaneously created—put on Earth through the action of some deity; this observation is still cited by creationists as evidence against the theory of evolution. This observation was perhaps the most difficult for Charles Darwin to reconcile with his newly proposed theory of evolution, for the apparently sudden appearance of large, complex animals in the fossil record ran utterly contrary to his expectations. In On the Origin of Species, he speculated that the Precambrian interval must have been of long duration and “swarmed with living creatures.” Yet where were the fossils of these swarms? Surely, if Darwin was correct, a long period of evolutionary change with simpler precursors would have been necessary to produce the complex creatures collected by Sedgewick and others in the lowest strata now known as the Cambrian. Darwin was never able to refute this stringent criticism of his theory. Instead, he railed against the “imperfections” of the fossil record, believing that there must be a missing interval of strata just beneath the first trilobite-bearing beds everywhere on Earth. He was convinced that there must be Precambrianaged fossils. As it turns out, he was right, but he went to his grave unvindicated. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p129-130]
· The twentieth century has witnessed a revolution in the science of geology. No longer are fossils the sole means of dating rock. Sophisticated laboratory analyses of volcanic and some sedimentary rock give accurate ages in years, and the entire rock record (including the Cambrian) has been far more accurately dated. In the 1960s the base of the Cambrian was determined to be 570 million years old, and this date appears on age compilations even into the late 1980s. Recently, however, there have been significant improvements in radiometric dating techniques. The Precambrian/Cambrian boundary is now dated at 543 million years old. The “Middle” Cambrian is dated at about 510 million years ago, whereas the oldest trilobites are no more than 522 million years old, which suggests that the bulk of Cambrian time was “pre-trilobite.” [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p130-131]
· Interestingly enough, although the “base” of the Cambrian has gotten younger; its “top” has not changed in age. The Cambrian Explosion remains a relatively sudden and signal outburst of animals an unleashing of abundant and voracious creatures upon the earlier bacterial world, which continues, unabated, more than half a billion years later. With the exception of life’s first formation, it remains the most profound biological event to have occurred on this planet. And we propose that the Cambrian Explosion has an even greater significance than Charles Darwin (or modern scholars of the fossil and evolution record) realized: We believe that it yields crucial evidence for estimating the frequency of animal life in the Universe. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p131]
· Thus the stage was set for the great evolutionary drama we call the Cambrian Explosion. It was grand theater, composed of four acts, each with its own set of characters, although some of them hung around for successive acts before exiting—by going extinct! [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p132]
· Act 1: The Ediacarans: The first act introduced a truly odd assemblage of creatures that looked like bizarre jellyfish, mutated worms, and quilted air mattresses somehow brought to life. This opening cast of characters is collectively known as the Ediacaran fauna. We now know that the Ediacarans opened Act 1 about 580 million years ago and were largely gone by 550 million years ago (although a few appear in much younger rocks). [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p132]
· In a 1988 essay, Stephen Jay Gould proposes that these odd animals are indeed the flowering of the “diploblastic,” or two-cell-layer body plan, a type of body plan found today only in the corals and jellyfish. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p132-133]
· The Ediacarans were not discovered until the 1940s, when an Australian geologist named R.C. Sprigg noticed some odd-looking fossil remains on scattered slabs of sandstone mines in the Ediacaran Hills of southern Australia, a desolate and isolated locality in very arid country. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p133]
· The fossil record is still enigmatic. In some places on Earth, the Ediacarans are gone before the first “Cambrian” animals appear, which suggests that the new animals were simply filling the niches of the extinct Ediacarans. But as we have said, in other places there is a clear overlap between the two, suggesting that a competitive interaction ensued. The Ediacarans do make good theater—enigmatic, mysterious, and the first on the stage. They were a hard act to follow, but a great wave of diversification was occurring at the end of their time in the limelight, a wave that continues on planet Earth still. With the second act of the Cambrian Explosion, undoubted animals appear on the scene. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p136]
· Acts 2 and 3: Trace Fossils and Small Shellys: We can combine the next two acts, because the cast of characters is both incomplete and poorly characterized. In Act 2, a new group of players, seemingly wearing masks to disguise their true identity, replace most of our opening troupe. We detect them only by the footprints they left on the stage itself, for we have no true “body” fossils (usually the remains of skeletal hard parts). The second assemblage of life making up the Cambrian Explosion has left only squiggles and tracks in the ancient sediment. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p136]
· Our Act 3 introduces an assortment of tiny calcareous tubes, knobs, and twisted spines, none larger than about 1⁄2 inch, all coming from animals that it is still impossible to reconstruct completely. Some are the remains of larger skeletons that have been fragmented into pieces, but most are single elements of some sort of a multielement skeleton, like individual spines coming from a porcupine. Collectively, they are known as small shelly fossils, or SSFs. The small shellys are first found in rocks dated to around 545 million years ago. These extremely significant fossils tell us that another great biological breakthrough had been achieved: The SSFs are the first large animals with mineralized skeletons. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p137]
· Act 4: The Trilobite Faunas: Act 4 of our play is a grand finale featuring fossil icons much more familiar to us than the previous actors. They include the first trilobites, brachiopods, and a host of newly evolved mollusks and echinoderms. The characters are now far larger—and greater in number—than in any of the three previous acts, and ironically, these actors were long thought to mark the start of the Cambrian Explosion, rather than its end. This last group didn’t appear until about 530 million years ago. Its diversification proceeded for another 30 million years. By about 500 million years ago, the Cambrian Explosion was finished. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p137]
· The trilobites are by far the most diverse and obvious part of this assemblage. The oldest trilobites, of which the genus Olenellus is diagnostic, were spiny, somewhat resembled annelid worms, and had large crescentshaped eyes. They all had walking legs and gills, and all appear to have fed by ingesting sediment or particulate material on the sea floor. They showed little adaptation for defense against predation. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p137]
· The remarkable Burgess Shale fauna of British Columbia has yielded extraordinary insights into animals living among the trilobites. Because of the lack of oxygen in this ancient environment, even soft parts were preserved, and these remains offer us an unparalleled window into the past. The Burgess Shale reveals how diverse the marine ecosystems were by the time trilobites evolved. Yet by the time of the Burgess fauna, some 505 million years ago, the majority of animal phyla appear to have been present. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p138]
WAS THE CAMBRIAN EXPLOSION INEVITABLE ?
· Darwin’s theory of evolution describes two of the most important scientific discoveries ever made: (1) that all life has descended from a single common ancestor, and (2) that the various species descending from this ancestral creature have descended with modification. The great advances of physics and chemistry are milestones in human understanding, but they do not themselves describe life. We are life, and we have appeared on this planet through the processes of evolution; it is a central law affecting us. Yet for all its importance, the theory of evolution remains one of the most misunderstood of scientific views. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p138]
· Of the three domains of life, only one, Eucarya, has undertaken wholesale experimentation in new morphology and body plans. If the process of life’s creation were to be repeated innumerable times, it is not at all certain that eucaryan equivalents (lineages exploiting the morphological route of adaptation, rather than the chemical route utilized by the archaeans and bacteria) would appear each time—or even ever again. But on this planet, at least, the eucaryans did arise, and it was from this group that the multicellular animals now dominating planet Earth arose. The pattern and timing of their evolution on Earth may provide major clues to understanding whether, and how often, equivalents of our planet’s complex animals could have arisen on other planets. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p139]
· There are important astrobiological implications in this: Will animal life (or some other type of complex life) inevitably develop on all worlds in a planetary habitable zone? In our estimation, it has always been assumed that forming the first life was the hardest aspect, but that once life originated, it inevitably proceeded “up” gradients of complexity, culminating in very complex animals. Yet the actual history of life on this planet tells a different story. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p139]
· The first life appeared about 4 billion years ago. Eukarytotic organisms did not appear for another 1.5 billion years, and multicellular animals did not appear until more than 3 billion years after the first life. On the basis of this information alone, we would have to conclude that forming animal life is a much more difficult—or at least a more time consuming—project than the initial formation of nonanimal life. Perhaps the timing observed on Earth was just chance; perhaps on any number of other Earth-like planets with newly evolved prokaryotic equivalents, animals would appear not billions, but millions, of years after life originated. Abundant evidence from our planet’s history casts doubt on this possibility, however. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p139-140]
· On Earth it is clear that the evolution of animals occurred not as a gradual process but as a series of long periods of little change, punctuated by great advances. This pattern of evolutionary “thresholds” was succinctly described by paleontologists Douglas Erwin, James Valentine, and David Jablonski in a 1997 article in American Scientist: “The fossil record of the last 3.5 billion years shows not a gradual accumulation of biological forms, but a relatively abrupt transition from body plans of single cells to those of a rich diversity of animal phyla.” Evolution thus did not gradually create complex metazoans. They evolved quickly, probably in response to a set of environmental conditions quite different from those that allowed the evolution of life in the first place. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p140]
· There were several of these “great leaps forward.” One was the evolution of the eukaryotic cell type with its enclosed nucleus; another was the initial radiation of the animal phyla, described in the last chapter. The most profound, however, was the Cambrian Explosion, that short burst of evolutionary innovation that resulted in the appearance of the larger, complex animals we believe to be so rare in the Universe. In this single, approximately 40-millionyear interval, all major animal phyla (all of the basic body plans found on our planet) appeared, each represented by some number of species. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p140]
· This event has profound implications for the possibility of life on other planets. Is the pattern on Earth—a single, short-lived diversification of larger animals—unique or the standard for all planets? And why did the Cambrian Explosion not take place until 3 billion years after life’s first appearance on our planet? Does evolution always require 3 billion years to transform a bacterium into a multicellular animal, or was evolution simply waiting for the environment to become conducive to the proliferation of animal life? This may be one of the most critical questions facing the emerging field of astrobiology. The Cambrian Explosion signaled a major change in the tempo of evolution then prevailing on Earth. Prior to this, our planet’s most complex life consisted of algae, slime molds, and single-celled animals characterized by low rates of evolutionary change. There was little morphological change, and few new species arose over vast stretches of time (see Figure 7.2). The first evolution of metazoans changed all this. The staid tempo of evolutionary change that had characterized the first 3.5 billion years of life’s history shifted into a higher gear. New species appeared at a far more rapid rate. They wehave been diversifying at breakneck speed ever since. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p140-141]
· If the Cambrian Explosion was necessary for animals to become so diverse on this planet, and if the inertial interchange event occurred as postulated, and if the Cambrian IIE event contributed to the Cambrian Explosion or even somehow was required for the Cambrian Explosion to take place, then Earth as a habitat for diverse animal life is rare indeed. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p147]
· The Cambrian Explosion marked not only the start of the majority of phyla as recognized in the fossil record but also the end of evolutionary innovation at the phyla level: Since the Cambrian, not a single new phylum has evolved. The extraordinary fact is that the diversification of new animal body plans started and ended during the Cambrian Period. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p151]
· The lesson of Earth’s Cambrian Explosion is that two parallel preparatory steps must be taken if complex metazoans—animals— are to appear. First, an oxygen atmosphere must be constructed. This is surely the most critical environmental step. Second, a very large number of evolutionary adaptations must be concluded to allow the evolution of an ocean liner—our animals—from the toy sailboat—the bacteria—that began it all. Both of these parallel tracks require time. There do not appear to be any shortcuts. On Earth, one or both required several billion years. And during that time, Earth had to maintain a temperature that allowed the presence of liquid water and avoid what we might call “planetary disasters” of sufficient magnitude to sterilize the evolving root stocks of animals. In the next chapter, we shall see why no such disaster put an end to animal evolution on planet Earth. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p156]
8- Mass Extinctions and the Rare Earth Hypothesis
· Just 65 million years ago, such an impact event did end the Mesozoic era, and it ended the Age of Dinosaurs as well. It was but one of many impacts and other assorted global catastrophes that have imperiled complex life on Earth over the past 500 million years. Such events must happen on planets elsewhere in the Universe, and they would surely be the greatest obstacle to the continued existence of any complex metazoan that might exist there. Extinction events are an important aspect of the Rare Earth Hypothesis. Although the animals and plants of Earth have suffered grievously in the assorted mass extinction events through time, the damage could have been worse—and on many other planets where life may have evolved, it probably has been, or will be. If hit at an inopportune time, a planet’s higher life might be snuffed out—or it might never be allowed to evolve in the first place. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p159-160]
· the Earth of 500 million years ago was teeming with complex animals and plants. Attaining such a world, for the first time populated by animals, required a large number of evolutionary and environmental changes and took 3 to 3.5 billion years. Maintaining these organisms required other conditions. Complex metazoans tolerate a far narrower range of environmental conditions than do microbes; there are no extremophile or anaerobic complex metazoans, for example. Complex metazoans are also far more susceptible to extinction caused by short-term environmental deterioration. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p160]
· The frequency of animal life in the Universe must be some function of how often it arises and of how long it survives after evolving. We believe that both of these factors are significantly influenced by the frequency and intensity of what are termed mass extinctions, brief intervals when significant proportions of a planet’s biota are killed off. There is no mystery about what kills organisms: too much heat or cold; not enough food (or other necessary nutrients); too little (or too much) water, oxygen, or carbon dioxide; excess radiation; incorrect acidity in the environment; environmental toxins; and other organisms. Mass extinctions occur when one or some combination of these factors kills a significant percentage of the planet’s biota. There has been no shortage of them in the past. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p160]
· Surface life, on the other hand (even bacterial surface life), is surely susceptible to major planetary catastrophes, such as the impact of truly large comets or asteroids. It may be that life on Earth’s surface was repeatedly sterilized during the period of heavy bombardment about 4 billion years ago, only to be reseeded by the deep-earth microbes or by the return of rocks ejected by the collisions. But for animal life, quite the opposite is true. Animals are not capable of the safer subterranean existence or of hibernating in the vacuum of space. If they are wiped out by catastrophe, they cannot be immediately restocked from some underground reserve. They have to evolve again in a slow, step-by-step process that lasts hundreds of millions or even billions of years. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p162]
· On every planet, sooner or later, a planetary catastrophe can be expected that either seriously threatens the existence of animal life or wipes it out altogether. Earth is constantly threatened by planetary catastrophe mainly by impact from comets and asteroids crossing the Earth’s orbit, but also from other hazards of space. Yet it is not only the hazards of outer space that threaten the diversity of life on this planet and on any others where it exists. There are Earth-borne causes of catastrophe as well as extraplanetary causes. Both types have brought about mass extinction on this planet in the past and would be likely to do so on other planets as well. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p162]
· The immediate, or direct, cause of all mass extinctions appears to be changes in the “global atmosphere inventory.” Changes to the atmospheric gases (which may be changes in volume or in the relative constituents of the atmosphere) can be caused by many things: asteroid or comet impact, degassing of carbon dioxide or other gases into the oceans and atmosphere during flood basalt extrusion (when great volumes of lava flow out onto Earth’s surface), degassing caused by liberation of organic-rich ocean sediments during changes in sea level, and changes in the patterns of ocean circulation. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p162]
· The killing agents arise through changes in the makeup and behavior of the atmosphere or through factors such as temperature and circulation patterns that are dictated by properties of the atmosphere. Planetary disasters can occur for a great number of reasons. We shall examine a few, in no particular order of importance. • Changing a planet’s spin rate We take Earth’s 24-hour spin rate for granted, when in fact it appears to be unusual if we compare it to other planets and satellites in our solar system. Jupiter and Saturn, for instance, each far greater in mass and diameter than Earth, spin much faster. Many other bodies, however, such as Venus and Mercury (and even our own Moon), spin much more slowly. In lower-mass stars, planets in the habitable zone become “tidally locked” by the gravitational force of the larger star or planet they revolve around. When one side always faces the star in question, that particular face becomes very hot, whereas the other side is always facing cold space and becomes frigid. Either environment would be lethal to surface life and prevent its evolution. Planets can change spin rates, and when they do, any life already adapted to a particular spin regime would be likely to face planetary disaster because of the major temperature changes it would encounter. Earth itself has been gradually slowing, a phenomenon that has probably altered the distribution of cloud cover over time. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p162-163]
· Moving out of the animal “habitable zone” Animal life needs liquid water, so it requires a mean global temperature that allows liquid water to exist. Any movement of a planet out of an orbit that allows such temperatures will create a planetary disaster. Though such changes of orbit are unlikely, they could be caused by another planet in a stellar system. Such perturbations would be common in open star clusters. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p163]
· Changing the energy output of the sun (star) Complex animal life on any planet is dependent on stellar energy. If stellar output either increases or decreases such that liquid water can no longer exist, the result will be disastrous to animal life or to the prospects for its evolution. Short-term and long-term changes in stellar energy output may be one of the most common forms of planetary extinction—and even sterilization. Some scientists are convinced that the end of life on Earth will be caused by an increase in the sun’s energy output. This is nothing new. As we have seen, the amount of energy being produced by the sun—and indeed, by most stars—increases over time. On Earth, the maintenance of a relatively constant temperature has been attained through a gradual reduction in greenhouse gases as the amount of energy from the sun has increased, thus keeping temperatures in check. We seem to be nearing the end of this type of planetary temperature regulation, however. There are now very small volumes of carbon dioxide in the atmosphere compared to earlier periods of geological time, and the sun’s energy output continues to increase. Some scientists have predicted that temperatures on Earth will become too high for animal life within several hundred million years from now. That event, when it comes to pass, will produce the last greatest mass extinction on Earth, its sterilization. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p163-164]
· Impact of a comet or asteroid Any planetary system is rife with cosmic debris: asteroids and comets, the residue left over from planetary formation. Great quantities of this material will eventually strike all members of a planetary system, and the energy released can spell planetary disaster. Such disasters are now known to have caused mass extinctions on Earth. In 1980, Luis and Walter Alvarez, Frank Asaro, and Helen Michel from the University of California at Berkeley proposed that one of the greatest of all mass extinctions, the 65-millionyear-old event that killed off the dinosaurs and many other species living near the end of the Mesozoic Era, was caused by the impact of a large meteor or comet striking Earth, as described at the beginning of this chapter. As evidence for this view mounted, most scientists realized that collision with a meteor or comet could cause a biotic crisis on any planet and that it has done so at least once (and probably other times as well) during Earth’s past. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p164]
· Nearby supernova: Another mechanism that could produce a mass extinction is the occurrence of a supernova in the sun’s galactic neighborhood. Two astronomers from the University of Chicago calculated in 1995 that a star going supernova within 10 parsecs (30 light-years) of our sun would release fluxes of energetic electromagnetic and charged cosmic radiation sufficient to destroy Earth’s ozone layer in 300 years or less. Much recent research on ozone depletion in the present-day atmosphere suggests that removal of the ozone layer would prove calamitous to the biosphere and to the species residing within. A depleted ozone layer would expose both marine and terrestrial organisms to potentially lethal solar ultraviolet radiation. Photosynthesizing organisms, including phytoplankton and reef communities, would be particularly affected. Judging by the number of stars within 10 parsecs of the sun in the last 530 million years, and by the rates of supernova explosions among stars, astronomers have concluded that it is very plausible that one or more supernova explosions have occurred within 10 parsecs of Earth during the last 500 million years. They also believe such explosions are likely to occur every 200 to 300 million years. The probability of nearby supernovae would be much greater closer to the galactic center, as suggested in Chapter 2. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p167]
· Sources of gamma rays: Astronomers have detected sudden bursts of intense gamma radiation being emitted from various galaxies (gamma rays are the most dangerous radiation emitted by atomic bombs). Although very little is yet known about these short but extremely violent releases of energy, they would be lethal to any life on nearby planetary systems. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p167]
· Cosmic ray jets and gamma ray explosions: A new entry into the mass-death rogues gallery is lethal bursts of radiation produced by violent stellar collisions. Cosmic ray jets and gamma rays might both result from the same source: merging neutron stars. Astronomers Arnon Dar, Ari Laor, and Nir Shaviv have postulated that cosmic ray jets may account for several of the major mass extinctions and might explain the rapid evolutionary events that follow them. They propose that high-energy fluxes of cosmic rays follow the merger or collapse of neutron stars, themselves the residues of supernovae. These explosions are the most powerful in the Universe, releasing in a few seconds as much energy as the entire output of a supernova. When two of these objects coalesce, they create a broad beam of high-energy particles that, if it hit Earth, would be capable of stripping away the ozone layer and bombarding the planet with lethal doses of radiation. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p167-168]
· Catastrophic climate change: Icehouse and Runaway Greenhouse: Under certain circumstances, radical changes in climate can cause mass extinction. Major glaciations and greenhouse heating are examples, and both depend on the amount of carbon dioxide or other greenhouse gases in the atmosphere. These are the actual killing mechanisms brought about by the reduction or increase of stellar output or by a planet’s orbit becoming either closer to or farther from its sun. Climate changes intense enough to threaten the biosphere with major mass extinction would involve great swings in mean planetary temperature, as well as relocation of oceanic current systems and shifts in planetary rainfall patterns. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p168]
· The emergence of intelligent organisms: There is abundant evidence that the emergence of humanity as a globally distributed species armed with technology has triggered a new episode of mass extinction on Earth. It can be argued that the emergence of any intelligent species co-opting a planet’s resources in the service of advanced technology and agriculture will necessarily cause a planetary mass extinction. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p169]
· How often might we expect the event that kills off the entire biosphere of the planet—the complete sterilization of Earth of all its huge diversity of living things? “I once tried extreme-value statistics on extinction data to ask, ‘How often should we expect extinction of all species on Earth?.’ I don’t have much confidence in the results, but they are at least comforting: Extinctions sufficient to exterminate all life should have an average spacing of well over 2 billion years.” Yet this should not be a comforting figure. Indeed, it goes to the heart of the Rare Earth Hypothesis. If we might expect a planetary catastrophe to exterminate all life on this planet every 2 billion years, and if life has already lasted 4 billion years, we are truly pressing our luck! And luck may be just what animals need to evolve for a long time when the grim reaper of planetary extermination is put off only through blind chance. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p170-171]
THE EFFECT OF MASS EXTINCTIONS
· Are mass extinctions that stop short of complete sterilization necessarily detrimental to planetary diversity? Perhaps it can be argued that instead of being deleterious to diversity, they are actually forces that increase diversity. For example, it can be argued that the various Paleozoic extinctions caused archaic reef communities to be reassembled with more modern types of corals. Mass extinctions paved the way for a takeover of bottom communities previously dominated by brachiopods (archaic shellfish) by the more modern (and more diverse) mollusks. In another case, the extinction of the dinosaurs paved the way for the evolution of many new types of mammals, and it appears that there are more types of mammal species than there were dinosaur species. If these mass extinctions had not occurred, would planetary diversity (the number of extant species) be higher or lower than it is today, other variables (the history of continental drift, for instance) remaining the same? [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p171]
· We can illustrate the enigma of mass extinctions and their effect on global biodiversity as follows: The Cambrian Explosion results in a sudden rise in diversity, followed by an approximately steady state during the Paleozoic. The mass extinctions during the Ordovician and Devonian cause shortterm drops in diversity, but these are soon compensated for by evolution of new forms. The great mass extinction that ended the Permian creates a longer-term deficit in diversity, but eventually, in the Mesozoic era, it also is compensated for. In fact, after every mass extinction that occurred on Earth over the past 500 million years, biodiversity has not just returned to its former value but has exceeded that value. Today, in our world, biodiversity is higher than it has been at any time in the past 500 million years. If there had been twice the number of mass extinctions, would there be an even higher level of diversity than there is on Earth now? Perhaps mass extinctions exert a positive effect, creating new opportunities and fostering evolutionary innovation by weeding out decadent or poorly adapted but entrenched and resource-hogging species. On the other hand, perhaps just the opposite is true: If the mass extinctions had not occurred, biodiversity would be higher than it is today. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p171-172]
The History Of Mass Extinctions On Earth
· Our understanding of the various events is in inverse proportion to their age: The older they are, the more mystery still surrounds them. The modern extinction that is still under way will also be dealt with only briefly. The most recent of the ancient events (the K/T event) is by far the most studied and best known. Accordingly, we will discuss it in greatest detail. The bombardment extinctions, 4.6 to 3.8 billion years ago The period of heavy bombardment is thought to have sterilized Earth’s surface at least several times. There is no other information known. The advent of oxygen—Snowball Earth, 2.5 to 2.2 billion years ago The rise of oxygen certainly doomed to extinction most anaerobic bacterial species then on Earth. There is little or no fossil record of this phenomenon, which may have coincided with the first “Snowball Earth” event. Snowball Earth events of 750 to 600 million years ago We have almost no information about these events, which may have included three or four separate extinctions coinciding with repeated glaciations. There did appear to be wholesale extinction among stromatolites and planktonic organisms called acritarchs. The lack of fossilized animals from this time period obscures these events. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p177]
· The Cambrian mass extinctions, 560 to 500 million years ago The extinctions that took place immediately before and then during the Cambrian period remain the most enigmatic of all extinction episodes. We believe that they are also the most important, in terms of their effects on animal life on this planet. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p177-178]
· The Ordovician and Devonian mass extinctions, 440 and 370 million years ago During the Paleozoic era there were two other major mass extinctions. About 370 million years ago, in the Devonian period, and more than 430 million years ago, in the Ordovician, major mass extinction events decimated the marine faunas of the time. Because our record of land life is poor for both of these intervals, we have much to learn about the severity of these events on land. However, it is clear that the majority of species in the sea went extinct. Both extinctions eliminated more than 20% of marine families •
The Permo-Triassic event, 250 million years ago On the basis of various measures of extinction (percent of existing species, genera, or families eliminated worldwide during the event), the Permo-Triassic mass extinction of 250 million years ago appears to have been the most catastrophic of all mass extinctions that have occurred on Earth. Specialists in compiling extinction records through time, such as Jack Sepkoski and David Raup of the University of Chicago, point out that this particular event stands alone in severity compared to all other such events. More than 50% of marine families died out, and this figure is more than twice that for any other extinction. Estimates of the percent of species (belonging to various families) that went extinct in this event vary from nearly 80% to more than 90%. It is clear that the vast majority of animal and plant life on the Earth was extinguished. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p179]
· The end-Triassic mass extinction, 202 million years ago: The end of the Triassic period witnessed a significant mass extinction in which about 50% of genera were eliminated. There is still a very poor record for the fate of land life at this extinction boundary, but it is clear that marine life at this time was extensively and catastrophically affected. This mass extinction, like the K/T event described at the beginning of this chapter, was thought to have been brought about by the impact of a large extraterrestrial body, either comet or asteroid. The Manicouagan Crater in Quebec is 100 kilometers in diameter (compared to the approximately 200-kilometer diameter of the Chicxulub Crater associated with the K/T event). This crater has been dated at 214 million years in age, which is older than the age of the Triassic/ Jurassic boundary. Environmental changes other than impact have been associated with this extinction event as well, most notably oceanographic changes creating anoxia in many shallow-water environments at the end of the Triassic. It is difficult, however, to see how such changes could have affected land life, which also suffered significant extinction at this time. The probable cause remains unknown. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p180]
· The Cretaceous/Tertiary boundary event, 65 million years ago: The mass extinction of the dinosaurs, as well as 50% or more of the other species then on Earth, has been recognized for more than a century and a half as one of the most devastating periods of mass death in Earth’s long history. Although numerous explanations for this event have been proposed, an asteroid impact cause is now largely accepted. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p181]
· The modern extinction: Suggesting that we have entered a mass extinction episode would have been controversial at best during the 1980s, but most investigators now concede that the number of extinctions that have occurred since the end of the last glacial period, some 12,000 years ago, clearly defines the Holocene time interval as one of pronounced and elevated extinction rates. There are many estimates of how many species are currently going extinct each year, though there are few hard data for many regions. What is clear is that the world’s forests are being felled inexorably to make way for agriculture and that the removal of forests leads to extinction. At the moment the seas are more insulated, and there is little evidence of major extinction occurring there at present, although this could change quickly as pressure on the world’s fish stocks increases. As runoff and chemical pollution increase over the next several centuries, the extinction rate in the sea may rise substantially. Estimates of the faunal tally vary, but all carry the grave message that Earth is losing a great number of species rather quickly. Perhaps the most sobering estimate comes from Peter Raven of the National Academy of Sciences, who has suggested that two-thirds of the world’s species may be lost by the year 2300. The ultimate cause of this extinction is the runaway population of Homo sapiens. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p182]
9-The Surprising Importance of Plate Tectonics
· As we return to Earth from this trip, we ponder what is unique about Earth that may offer us clues to why animal life exists here but not on other planets and their moons in our solar system. A crucial difference, its seems, is Earth’s unique possession of liquid water at its surface. Water, the universal solvent, seems indispensable for animal life. Earth has other unique attributes, too, including its oxygen-rich atmosphere and a temperature range that allows liquid water to exist. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p193]
· Another unique attribute of Earth at first glance seems extraneous to animal life but may indeed be crucial to it: linear mountain ranges. There are, of course, giant mountains elsewhere in the solar system, the tallest being the great volcano Olympus Mons on Mars. Yet such mountains are always single and never occur in chains, unlike most mountains on Earth. There is no equivalent to the Rockies, the Andes, the Himalayas, or the score of other linear mountain chains we are so familiar with. Even at this crude level of observation, oceans, mountain chains, and life make Earth unique in this solar system. Life has had little to do with creating oceans and mountain chains. Yet these features of Earth may have been crucial to the origin of life. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p193]
· we argue that all three of these precious attributes of Earth are connected in a complex interrelationship. All three, furthermore, may be the result of plate tectonics. This process, the movement of the planetary crust across the surface of the planet, is found in our solar system only on Earth, and it may be vanishingly rare in the Universe as a whole. It is not the mountains as such that are so important to life on Earth but the process that creates them: plate tectonics. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p194]
· It may seem odd to think that plate tectonics could be not only the cause of mountain chains and ocean basins but also, and most enigmatically, a key to the evolution and preservation of complex metazoans on Earth. But there are several reasons to consider this view. First, plate tectonics promotes high levels of global biodiversity. In the last chapter, we suggested that the major defense against mass extinctions is high biodiversity. Here we argue that the factor on Earth that is most critical to maintaining diversity through time is plate tectonics. Second, plate tectonics provides our planet’s global thermostat by recycling chemicals crucial to keeping the volume of carbon dioxide in our atmosphere relatively uniform, and thus it has been the single most important mechanism enabling liquid water to remain on Earth’s surface for more than 4 billion years. Third, plate tectonics is the dominant force that causes changes in sea level, which, it turns out, are vital to the formation of minerals that keep the level of global carbon dioxide (and hence global temperature) in check. Fourth, plate tectonics created the continents on planet Earth. Without plate tectonics, Earth might look much as it did during the first billion and a half years of its existence: a watery world, with only isolated volcanic islands dotting its surface. Or it might look even more inimical to life; without continents, we might by now have lost the most important ingredient for life, water, and in so doing come to resemble Venus. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p194]
· Finally, plate tectonics makes possible one of Earth’s most potent defense systems: its magnetic field. Without our magnetic field, Earth and its cargo of life would be bombarded by a potentially lethal influx of cosmic radiation, and solar wind “sputtering” (in which particles from the sun hit the upper atmosphere with high energy) might slowly eat away at the atmosphere, as it has on Mars. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p194]
WHY IS PLATE TECTONICS IMPORTANT TO LIFE ?
· The rate of continental growth is of major importance to life and its ecosystems. The majority of Earth’s biodiversity is today found on continents, and there is no reason to believe that this relationship has changed over the last 300 million years. As continents have grown through time, they have affected global climate, including the planet’s overall albedo (its reflectivity to sunlight), the occurrence of glaciation events, oceanic circulation patterns, and the amount of nutrients reaching the sea. All of these factors have biological consequences and affect global biodiversity. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p202]
· In the last chapter we proposed that diversity (roughly the number and relative abundance of species on the planet at any given time) is a major hedge, or defense, against planetary extinction or sterilization of life: High levels of diversity can counter the loss of body plans during mass extinctions. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p203]
· Plate tectonics can augment diversity by increasing the number and degree of separation of habitats (which promotes speciation). For example, as continents break apart, the seaways forming between them create barriers to dispersal. This in turn reduces gene flow and enhances the formation of new species through geographic isolation. Plate tectonics also increases the nutrients available to the biosphere, which may (or may not) also promote increased biotic diversity. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p203]
· Plate tectonics promotes environmental complexity—and thus increased biotic diversity—on a global scale. A world with mountainous continents, oceans, and myriad islands such as those produced by plate tectonic forces is far more complex, and offers more evolutionary challenges, than would either totally land- or ocean-dominated planets without plate tectonics. James Valentine and Eldredge Moores first pointed out this relationship in a series of classic papers in the 1970s. They showed that changes in the position and configuration of the continents and oceans would have far-reaching effects on organisms, causing both increased diversification and extinction. Changes in continental position would affect ocean currents, temperature, seasonal rainfall patterns and fluctuations, the distribution of nutrients, and patterns of biological productivity. Such varying conditions would cause organisms to migrate out of the new environments—and would thus promote speciation. The deep sea would be least affected by such changes, but the deep sea is the area on Earth today with the fewest species. Over two-thirds of all animal species live on land, and the majority of marine species live in the shallow-water regions that would be most affected by plate tectonic movements. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p203]
What Would Happen If Plate Tectonics Ceased?
· The fossil record suggests that there are more species of animals and plants alive on Earth today than at any time in the past; estimates vary between about 3 and 30 million species. This great diversity has come about through many physical and evolutionary factors. We contend that the effects of plate tectonics are among the most important. But once created, does high biodiversity require the continued presence of plate tectonics? We can examine this question with a thought experiment. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p204]
An End to Volcanism
· Imagine that all volcanism on Earth’s surface suddenly ceases. This will stop the many dozens of volcanic eruptions that occur on the continents each year (usually causing great media fanfare and little damage). But the cessation of volcanism will have a far more profound effect. If all volcanism stops, so does sea floor spreading—and thus plate tectonics as well. And if plate tectonics stops, Earth eventually (through erosion) loses most or all of the continents where most terrestrial life exists. In addition, CO2 is removed from the atmosphere via weathering, causing our planet to freeze. Of all of the attributes that make Earth rare, plate tectonics may be one of the most profound and—in terms of the evolution and maintenance of animal life—one of the most important. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p205]
· Only cessation of the flow of heat up from Earth’s interior or thickening of the crust would stop volcanism. It is this heat that causes convective motion of the interior, the subterranean motor of plate tectonics. To stop plate tectonics would require eliminating these great lithic boiling pots, and that cannot be done unless all heat emanating from the Earth’s interior is stopped (which would require that all the radioactive minerals locked away there decay to stable daughter products) or the composition of Earth’s crust or upper mantle changes such that movement can no longer occur. This could happen if the crust became too thick or the mantle too viscous to allow movement. None of these conditions is likely to occur on Earth in the foreseeable future, but there is speculation that just such events occurred on both Venus and Mars in the past. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p205]
· If Earth’s tectonic plates did suddenly stop moving, subduction would no longer occur at the contacts between colliding plates. Mountains—and mountain chains—would cease to rise. Erosion would begin to eat away at their height. Eventually, the world’s mountains would be reduced to sea level. How long would it take? The problem is a bit more complicated than simply measuring average erosion rates and calculating the number of years required for the mountains to disappear. This is because of the principle of isostacy. Mountains (and continents) are a bit like icebergs: If you cut off the top, the bottom rises up relative to sea level, causing the entire iceberg (or mountain) to rise. Eventually, however, even this isostatic rebound effect would be overcome by the extent of the erosion. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p205-206]
· Paradoxically, the increase of ocean area would probably also be accompanied by extinctions in the sea. Ocean life depends on nutrients, and most nutrients come from the land as runoff from rivers and streams. With the disappearance of land, the total amount of nutrients (though initially higher as so much new sediment entered the ocean system) would eventually lessen, and with fewer resources, there would be fewer marine animals and plants. How long before such a water world would be achieved? Tens of millions of years would be required for the mountains and continents to erode to sea level. Yet mass extinction would ensue long before that. Planetary calamity for complex life would occur shortly after the cessation of plate movement, for plate tectonics is not only the reason we have mountains; it turns out to control our planet’s climatic thermostat as well. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p206]
Loss of Planetary Temperature Control
· The temperature of Earth must remain in a range suitable for the existence of liquid water if animal life is to be maintained. The range of temperature that Earth experiences is the result of many factors. One is the existence of the atmosphere. The average temperature of the Moon is 18°C, for example, well below the freezing point of water, simply because it has no appreciable atmosphere. If Earth did not have its cloaking atmosphere, including such insulating gases as water vapor and carbon dioxide (producing the much discussed Greenhouse Effect), its temperature would be about the same as that of the Moon. Yet the Earth, thanks to the greenhouse gases, has an average global temperature of 15°C (33°C warmer than the Moon). Greenhouse gases are keys to the presence of fresh water on this planet and thus are keys to the presence of animal life—and many scientist now believe that the balance of greenhouse gases in Earth’s atmosphere is directly related to existence of plate tectonics. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p206-207]
Plate Tectonics as Global Thermostat
· Over and over again we come back to to a common theme: the importance of liquid water. For animal life based on DNA to exist and evolve, water must be present and abundant on a planet’s surface. Even on the water-rich Earth today, slight differences in water content obviously affect life. In desert regions there is little life; in rainforests at the same latitude, life teems in abundance. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p208]
· For complex life to be attained (and then maintained), a planet’s water supply (1) must be large enough to sustain a sizable ocean on the planet’s surface, (2) must have migrated to the surface from the planet’s interior, (3) must not be lost to space, and (4) must exist largely in liquid form. Plate tectonics plays a role in all four of these criteria. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p208]
· Earth is about one-half of 1% water by weight. Much of this water arrived among the planetesimals that took part in Earth’s formation and accretion. Other volumes of it were dumped here by incoming comets after Earth accreted. The relative importance of these two processes is largely unknown at this time. Once liquid water is established on the surface of a planet, its maintenance becomes the primary requirement for attaining (and then support ing) animal life. The maintenance of liquid water is controlled largely by global temperatures, which are a by-product of the greenhouse gas volumes of a planet’s atmosphere. The temperature of Earth’s (and of any planet’s surface) is a function of several factors. The first is related to the energy coming from its sun. The second is a function of how much of that energy is absorbed by the planet (some might be reflected into space, and this relationship is dictated by a planet’s reflectivity, or albedo). The third is related to the volume of “greenhouse gases” maintained in a planet’s atmosphere. Greenhouse gases have a residence time in any atmosphere and are eventually broken down or undergo a change in phase. If their supplies are not constantly replenished, the planet in question (such as Earth) will grow colder gradually until the freezing temperature of water is reached, at which point it will grow colder rapidly (as we have noted, when a planet starts accumulating ice, its albedo increases, boosting its rate of cooling). [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p208-209]
· Greenhouse gases are thus vastly important in maintaining a planet’s thermostatic reading. Both plate tectonic and non–plate-tectonic planets regularly produce greenhouse gases, because the most important source of these planetary insulators is volcanic eruption, which occurs on most or all planets. On Earth, volcanoes daily exhale vast volumes of carbon dioxide from deep within. Even so-called “dormant” volcanoes are venting carbon dioxide into the atmosphere. On any planet with volcanism there is usually an abundance of greenhouse gases—too much in some cases, and this is where plate tectonics becomes crucial. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p208-209]
· The planetary thermostat requires a balance between the amount of CO2 being pumped into the atmosphere through volcanic action and the amount being taken out through the formation of limestone. On non–platetectonic worlds, buried limestone stays buried, thus removing calcium from the system and producing increases in carbon dioxide. On Earth, at least, plate tectonics plays an integral part in maintaining a stable global temperature by recycling limestone into the system. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p212]
Plate Tectonics and The Magnetic Field
· Outer space is not a particularly friendly place. One of its hazards is cosmic rays, which are elementary particles—electrons, protons, helium nuclei, and heavier nuclei—traveling at velocities approaching the speed of light. They come from many sources, including the sun and cosmic rays from distant supernovae, the explosions of stars. These catastrophic events send great numbers of particles hurtling through space. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p212]
· In The Search for Life in the Universe, D. Goldsmith and T. Owen speculate that without some sort of protection, life on Earth’s surface would be extinguished within several generations by cosmic rays hitting our planet’s surface. However, the vast majority of cosmic rays are deflected by Earth’s magnetic field. The innermost layer of our planet, its core, is made up mainly of iron, which in the outermost region of the core is in a liquid state. As Earth spins, it creates convective movement in this liquid that produces a giant magnetic field surrounding the entire planet. What produces the convection cells in the core is loss of heat. Heat must be exported out of the core, and this liberation of heat appears to be greatly influenced by Earth’s plate tectonic regime. Joseph Kirschvink of Cal Tech has suggested that without plate tectonics, there would not be enough temperature difference across the core region to produce the convective cells necessary to generate Earth’s magnetic field; no plate tectonics, no magnetic field. The magnetic field also reduces “sputtering” of the atmosphere, a process whereby the atmosphere is gradually lost into space. No magnetic field, perhaps no animal life. Plate tectonics to the rescue again. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p213]
Why Does Earth (But Not Mars or Venus) Have Plate Tectonics?
· Why is there plate tectonics on Earth? The recipe for plate tectonics seems simple enough at first glance. You need a planet differentiated into a thin, solid crust sitting atop an underlying region that is hot, fluid, and mobile. You need this underlying region to be undergoing convection, and for that you need heat emanating from even deeper in the planet. And you are likely to need water—oceans of water: Much new research suggests that without water you cannot have plate tectonics (though perhaps it is simply that without water you cannot get continents). As in so much else in planetary geology, there is still a great deal we don’t know about why our planet (and, more important, any planet) develops and then maintains plate tectonics. Because ours is still the only planet we know that has plate tectonics, we have nothing with which to compare it. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p213]
Most Crucial Element of The Rare Earth Hypothesis?
· Plate tectonics plays at least three crucial roles in maintaining animal life: It promotes biological productivity; it promotes diversity (the hedge against mass extinction); and it helps maintain equable temperatures, a necessary requirement for animal life. It may be that plate tectonics is the central requirement for life on a planet and that it is necessary for keeping a world supplied with water. How rare is plate tectonics? We know that of all the planets and moons in our solar system, plate tectonics is found only on Earth. But might it not be even rarer than that? One possibility is that Earth has plate tectonics because of another uncommon attribute of our planet: the presence of a large companion moon, the subject of the next chapter. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p220]
10-The Moon, Jupiter, and Life on Earth
· Perhaps an astronomer’s greatest fear is that sooner or later, someone will mistake him or her for an astrologer. The ancient belief system known as astrology posits that the stars and planets exert a major influence on our daily lives—a set of beliefs frequently and fervently disputed by astronomers. Recent research, however, has in an odd way proved the astrologers slightly correct. Two heavenly bodies, the Moon and Jupiter, do, in fact, play pivotal roles in our very existence as a species. Without the Moon, and without Jupiter, there is a strong likelihood that animal life would not exist on Earth today. Both are thus key elements in the Rare Earth Hypothesis, but for different reasons. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p221-222]
LUNA
· Without the Moon there would be no moonbeams, no month, no lunacy, no Apollo program, less poetry, and a world where every night was dark and gloomy. Without the Moon it is also likely that no birds, redwoods, whales, trilobites, or other advanced life would ever have graced Earth. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p222]
· Although there are dozens of moons in the solar system, the familiar ghostly white moon that illuminates our night sky is highly unusual, and its presence appears to have played a surprisingly important role in the evolution of life. The Moon is just a spherical rock 2000 miles in diameter and 250,000 miles away, but its presence has enabled Earth to become a long-term habitat for life. The Moon is a fascinating factor in the Rare Earth concept because the likelihood that an Earth-like planet should have such a large moon is small. The conditions suitable for moon formation were common for the outer planets but rare for the inner ones. Of the many moons in the solar system, nearly all orbit the giant planets of the outer solar system. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p222]
· The warm, Earth-like planets that are close to the sun and that fall within the habitable zone, are nearly devoid of moons. The only moons of the terrestrial planets are ours and Phobos and Diemos, the two tiny (10 kilometers in diameter) moons of Mars. Some of the solar system’s moons are huge. Jupiter’s Ganymede is nearly as large as Mars, and Saturn’s Titan is nearly that large and has an atmosphere denser than our own, though much colder. Our Moon is somewhat of a freak because of its large size in comparison to its parent planet. The Moon is nearly a third the size of Earth, and in some ways it is more of a twin than a subordinate. The only other case in the solar system where a moon is comparable in size to its planet is Pluto and its moon, Charon. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p222]
Tilt!
· The Moon plays three pivotal roles that affect the evolution and survival of life on Earth. It causes lunar tides, it stabilizes the tilt of Earth’s spin axis, and it slows the Earth’s rate of rotation. Of these, the most important is its effect on the angle of tilt of Earth’s spin axis relative to the plane of its orbit, which is called “obliquity.” Obliquity is the cause of seasonal changes. For most of Earth’s recent history, its obliquity has not varied by more than a degree or two from its present value of 23 degrees. Although the direction of the tilt varies over periods of tens of thousands of years as the planet wobbles, much like the precession of a spinning top, the angle of the tilt relative to the orbit plane remains almost fixed. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p223]
· This angle is nearly constant for hundreds of millions of years because of gravitational effects of the Moon. Without the Moon, the tilt angle would wander in response to the gravitational pulls of the sun and Jupiter. The monthly motion of our large Moon damps any tendencies for the tilt axis to change. If the Moon were smaller or more distant, or if Jupiter were larger or closer, or if Earth were closer to or farther from the sun, the Moon’s stabilizing influence would be less effective. Without a large moon, Earth’s spin axis might vary by as much as 90 degrees. Mars, a planet with the same spin rate and axis tilt, but no large moon, is believed to have exhibited changes to its tilt axis of 45 degrees or more. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p223]
· Because tilt of a planet’s spin axis determines the relative amounts of sunlight that land on the polar and on the equatorial regions during the seasons, it strongly affects a planet’s climate. On planets with moderate tilts, the majority of solar energy is absorbed in the equatorial regions, where the noon sun is always high in the sky. Each pole is in total darkness for half a year and has constant illumination for half a year. The highest altitude that the sun reaches in the sky at the pole is exactly equal to the number of degrees of the tilt of the spin axis. For moderate tilt angles, the sun is never high in the polar sky, and ground heating by sunlight is low even in the middle of the summer. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p223]
· The planet Mercury provides a spectacular example of what can happen on a planet whose spin axis is nearly perfectly perpendicular to the plane of its orbit. Mercury is the closest planet to the sun and most of its surface is hellishly hot, but radar imaging from Earth has shown that the poles of the planet are covered with ice. The planet is very close to the sun, but as viewed from the poles, the sun is always on the horizon. In contrast to Mercury’s lack of tilt, the planet Uranus has a 90-degree tilt; and one pole is exposed to sunlight for half a year while the other experiences cryogenic darkness. Although our viewpoint is certainly biased, our planet’s tilt axis seems to. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p223-224]
· be “just right.” Constancy of the tilt angle is a factor that provides long-term stability of Earth’s surface temperature. If the polar tilt axis had undergone wide deviations from its present value, Earth’s climate would have been much less hospitable for the evolution of higher life forms. One of the worst possibilities is that excessive axis tilt could have led to the total freezing over of the oceans, a situation that might be very difficult to recover from. Extensive ice cover increases the reflectivity of the planet, and with less absorption of sunlight, the planet continues to cool. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p224]
· Astronomer Jacques Laskar, who made many of the calculations that led to the surprising discovery of the Moon’s importance in maintaining Earth’s stable obliquity, summarized the situation as follows: These results show that the situation of the Earth is very peculiar. The common status for all the terrestrial planets is to have experienced very large scale chaotic behavior for their obliquity, which, in the case of the Earth and in the absence of the Moon, may have prevented the appearance of evoluted forms of life. . . . [W]e owe our present climate stability to an exceptional event: the presence of the Moon. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p224]
· High obliquity has remarkable and seemingly counterintuitive effects on planets (see Figure 10.1). Consider a planet that is tipped 90 degrees. Averaged over the year, the poles would receive exactly as much solar energy as the equator would with no tilt angle. The north pole would become the Sahara! For the 90-degree tilt, however, the equatorial regions would receive much less energy averaged over the year and would become colder. If a planet is tilted more than 54 degrees, its polar regions actually receive more energy input from sunlight than the equatorial regions. If the Earth were tilted more than this amount, the equatorial oceans might freeze and the polar regions would be warmer: a topsy-turvy world. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p224-226]
· Recently uncovered evidence has revealed that equatorial ice sheets did exist about 800 to 600 million years ago, and ice-rafted sediments of this age have been found in formerly equatorial regions. This has led to the “Snowball Earth” hypothesis that Earth may have actually have frozen over, as we saw in Chapter 6. It has been suggested that this may have been due to high tilt angle during a period of time when the Moon did not have full control. We do not know for sure how long the Moon has been successful in stabilizing Earth’s obliquity. In the distant future, the Moon will lose its ability to stabilize Earth’s spin axis. The Moon is slowly moving outward from Earth (at a rate of about 4 centimeters a year), and within 2 billion years it will be too far away to have enough influence to stabilize Earth’s obliquity. Earth’s tilt angle will begin to change as a result, and the planet’s climate will follow suit. Further complicating the future is the slow but unrelenting increase in the brightness of the sun. At the time when our planet’s spin axis begins to wander, the sun will be hotter, and both effects will decrease the habitability of Earth. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p226]
· There is currently much speculation about how rapid such changes of planetary obliquity might be in the absence of the Moon. Estimates for the time it would take Earth to “roll” on its side range from tens of millions of years to far shorter periods. Astronomer Tom Quinn of the University of Washington has suggested to us that the time of obliquity change could occur on scales as short as hundreds of thousands, rather than millions, of years. Such large-scale fluctuations would probably lead to very rapid and violent climate change. If the tropical regions became locked in a permanent ice cover in 100,000 years or less, there would certainly be a mass extinction of great severity. Is the lack of a large moon sufficient to prevent microbial life from evolving into animal life? We have no information, but because deep-sea regions are insulated from climate change, it seems doubtful that rapid obliquity changes would deprive a planet of animal life. What it could do, however, is deprive a planet of complex life on land. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p226]
Lunar Tides
· A second benefit of Earth’s large moon is tides, which are due to gravitational effects of both the sun and the Moon. The pull of these two bodies produces bulges in the ocean pointing both toward and away from the Moon and the sun. The complexity of Earth’s present tidal effects is well illustrated by the tidal charts cherished by clam diggers, anglers, and sailors. The daily variations seen in the charts are caused by the interplay of both lunar and solar tides. Both the Moon and the sun cause ocean bulges on their respective near and far sides of the planet. As Earth spins under the bulges, the sea rises and falls at any particular location. When the Moon lines up with the sun every 2 weeks, the tidal ranges are at a maximum, and when they are 90 degrees apart in the sky (the quarter moon is overhead at sunrise or sunset), the range of tidal change is minimized. With a smaller or more distant moon, the lunar tides would be lesser and would have a different annual variation. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p227]
· Soon after the Moon formed, it was perhaps 15,000 miles from Earth. Instead of being a few meters high, as they are today, it is possible that lunar tides rose hundreds of meters or higher. The extreme effects of such a close moon could have strongly heated Earth’s surface. The ocean tides (and land tides) from a nearby Moon would have been enormous, and the flexing of Earth’s crust, along with frictional heating, may have actually melted the rocky surface. However severe their effects, the enormous tidal variations would have been short-lived because the forces responsible for tides also cause the Moon to move outward, thus diminishing the effect. Early land tides may have been a kilometer high, but they dropped to moderate levels in less than a million years. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p227]
· The retreat of the Moon is a natural consequence of gravitational pull between the Moon and the tidal bulges. The Earth’s lunar tidal bulges don’t actually correspond with a line from Earth to the Moon but, rather, lead ahead as the Moon orbits the planet (see Figure 10.2). This offset produces a torque that causes Earth’s spin rate to decrease slowly and the distance be tween Earth and Moon to increase. Besides measurement by laser, the recession of the Moon can also be detected in the fossil record. Daily and annual layers in Devonian horn corals show that about 400 million years ago there were 400 days in each year, the Moon was closer, and Earth was spinning faster. The coupling of these two effects is due to conservation of angular momentum, the same physical law that allows ice skaters to spin faster by pulling their arms against their bodies. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p227-228]
· The outward movement of the Moon would be reversed if the Moon happened to be orbiting in the opposite direction. Instead of retreating, it would approach Earth and would eventually collide with it. Although we have nothing to fear from Earth’s moon in this respect Triton, the large moon of Neptune, is in a retrograde orbit and will collide with Neptune within a few hundred million years. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p228-229]
A New Account of the Moon’s Origin
· A quite remarkable aspect of the Moon is that its formation appears to have been highly unlikely, a rare chance happening. The origin of the Moon has inspired endless speculation ever since people first gazed into the sky, but interest in this subject peaked in 1969, when Apollo 11 landed on the Moon and returned lunar rocks to earthbound laboratories. A major goal of the feverish research activity on these samples was to determine how the Moon, the “Rosetta Stone of the solar system,” had formed. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p229]
· Before the Apollo rocks were brought back, the most popular notion was that the Moon had formed cold and consequently would retain records of the earliest history of the solar system. When the lunar samples were returned, there was breathless anticipation that the samples would finally show how the Moon formed. However, no one satisfactorily solved the mystery of the Moon’s origin in 1969 or even in the next decade. Ironically, the assembly of a well-agreed-upon theory for the lunar origin was slowed by the fabulous wealth of highly detailed data from the samples. Extensive work did show that the Moon had a violent, high-temperature history and was thus not a benevolent body for preserving records of its earliest past, as had been hoped. The rocks did, however, record exquisite details of the Moon’s history between 3 and 4 billion years ago, a time interval when Earth’s history is very poorly known. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p229]
· During the Apollo program, everyone talked about the origin of the Moon; in the following years, though, most lunar scientists worked on details, and the “big picture” was little discussed. As happens at times in science, this situation radically changed when a sort of cosmic convergence occurred at a meeting on the origin of the Moon held at Kona, Hawaii, in 1984. At this meeting, many details of the analysis of lunar samples and advances in theory came to light, and many scientists left Kona convinced that the Moon had a quite peculiar and improbable origin. Theories on the lunar origin usually fit into three categories: it formed in place, it formed elsewhere and then was captured, or it was somehow ejected from Earth. The new idea was, in a sense, a combination of all three (see Figure 10.3). The model was that during its formation, Earth was hit by a Mars-sized (half the diameter of Earth) projectile. Debris from this collision was ejected into space, and some of it remained in orbit, where self-collisions would cause it to form a thin, orbiting ring of rocks analogous to the rings of Saturn. The moon would form from this ring by collision and sticking, the processes of accretion that built most of the bodies in the solar system. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p229-230]
· Several important aspects of this process were consistent with the properties of the Moon deduced from lunar sample studies. One was that the great violence of the event would deplete the Moon of the so-called volatile elements. Compared to meteorites, the moon is depleted in elements such as zinc, cadmium, and tin. These relatively volatile elements would have been vaporized in the impact, and the resulting vapor would have had difficulty completely recondensing from hot gas. Stuck in the gas phase, the volatile elements would have been swept into space and lost to the Earth–Moon system. Included in the lost elements and compounds were nitrogen, carbon, and water. One of the surprising initial findings of Apollo was that the lunar samples were exceedingly dry. Unlike terrestrial rocks, they contained no detectable water. Another remarkable property of lunar samples is that they are highly depleted in the siderophile, or iron-loving, elements that tend to concentrate in the metallic iron cores of planets. When planetary cores form by the molten iron sinking to the planet’s center, the siderophile elements (such as platinum, gold, and iridium) are incorporated into the falling iron and are highly depleted from the crustal and mantle materials left above. That the lunar rocks were depleted in siderophile elements was unexpected, because the Moon cannot have a substantial iron core. The mean density of the Moon is 3.4 times the density of water, very similar to that of rocks on the lunar surface and much lower than that of Earth (5.5 times the mean density of water). If it had a substantial core of dense metallic iron, the mean density of the Moon would be higher than is observed. Seismic and magnetic data also show no evidence of a significant core. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p230]
· The collision model solves the siderophile mystery by suggesting that metallic cores formed in both Earth and the projectile before the collision. In the collision, both cores ended up in the center of Earth, and the debris ejected into orbit was mainly from the mantles of both bodies. This sequestering of siderophiles explains why gold and platinum are so rare on the Moon and in the crustal rocks of Earth. The impact ejection of mantle materials from both the giant impactor and the target Earth is consistent with some of the remarkable similarities between the trace element content of the Moon and that of rocks from Earth’s mantle. It is also consistent with Earth and the Moon being identical in isotopic composition. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p232]
· A collisional origin is very attractive, but did it actually happen? In earlier times, it was often imagined that the accretion of planets occurred in a regular fashion by collision with small bodies. This was why the Moon was thought to have formed cold. A body that grows by accretion of small objects does not bury heat in its interior. Even though the bodies may collide at high velocity, if they are small they produce small craters, and the energy of impact can be largely radiated back into space. For an impact to eject enough material to form the Moon, the colliding body has to be huge, a Mars-size body. Theoretical modeling by George Wetherill, a Medal of Science–winning planetary scientist at the Carnegie Institute of Washington Department of Terrestrial Magnetism, showed that a natural consequence of the accretion process is that several large bodies do form in each planet’s accretion zone. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p232]
· The growth process includes the impact of several bodies each of which carries more than 10% of the mass of the final planet. In the case of Earth, these big bodies were the size of Mars and larger. Their impact not only ejected material into space to form the Moon but also injected considerable amounts of heat into Earth’s mantle. This heat input and great violence led to the forging of Earth’s core during the accretion phase, before the planet was fully formed. This is in contrast with a planet that formed cold, by slow accretion of small bodies. Core formation requires high internal temperatures so that blobs of molten iron descend through the mantle to reach the core. Such a planet could form a core only after long. term buildup of radioactive heat from the decay of uranium, potassium, and thorium. In Earth’s case, the early heat from accretion of large bodies led to core formation as accretion occurred. The Moon’s formation occurred after its core formed. Both bodies had differentiated and already had metal cores at the time of collision. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p232-233]
· Recent computer simulations by A. Cameron and colleagues provide the best fit with the actual properties of Earth and the Moon when the Moonforming collision occurs when Earth has only grown to about half of its final mass and the impactor mass is about a fourth of the final mass of Earth. When the collision occurred, the effects on both bodies were incredible. They briefly fused, but then inertial effects on the resulting plastic mass literally ripped the assembly into two major pieces. The fragments separated for several hours but then fell back in response to the forces of gravity. After a few violent oscillations, the planet finally settled down. Like droplets ejected from a stone tossed into a pond, a small amount of material was ejected and formed a debris ring around Earth. Materials in the ring were derived from the silicate mantles of both bodies. The Moon formed from that disk by accretion of solid particles over a period of a few tens of thousands of years. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p233]
· When it finally formed, the Moon was only 15,000 miles from the planet’s surface. With the Moon so close, Earth would have been spinning at such a rate that the day would be only 5 hours long. The height of the tides would have been fantastic, and as noted earlier, the resulting heat probably melted the surface of the planet. The importance of this heat may be purely academic, because Earth would still have been exceedingly hot just from the great impact. The impact event was so energetic that it actually vaporized rocks, forming a “silicate atmosphere” that survived for a short time before it cooled and condensed as a silicate rain. All of these effects would have been detrimental to any attempts by life to establish an early presence on the planet. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p233]
· Although this is somewhat of a conjecture, it is possible that Earth’s violent early history seeded the eventual development of plate tectonics. The large-scale heating would have led to formation of a magma ocean covering the surface of the planet. Differentiation of this “ocean” may have spawned the first rocks that could form long-term continents. This violent early history must also have had severe effects on the ocean and atmosphere. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p233-234]
· If the Earth’s formation could be replayed 100 times, how many times would it have such a large moon? If the great impactor had resulted in a retrograde orbit, it would have decayed. It has been suggested that this may have happened for Venus and may explain that planet’s slow rotation and lack of any moon. If the great impact had occurred at a later stage in Earth’s formation, the higher mass and gravity of the planet would not have allowed enough mass to be ejected to form a large moon. If the impact had occurred earlier, much of the debris would have been lost to space, and the resulting moon would have been too small to stabilize the obliquity of Earth’s spin axis. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p234]
· If the giant impact had not occurred at all, the Earth might have retained a much higher inventory of water, carbon, and nitrogen, perhaps leading to a Runaway Greenhouse atmosphere. Of the many elements of the Rare Earth Hypothesis, the presence of our huge Moon seems to be one of the most important and yet most perplexing. Without the large Moon, Earth would have had a very unstable atmosphere, and it seems most unlikely that life could have progressed as successfully as it has. Even with Earth’s relatively stable long-term climate, it still took over 90% of the planet’s lifetime to date for land animals to develop. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p234]
· Unfortunately, there is no evidence on how common large moons are for warm terrestrial planets close to their parent stars. We just don’t know, and we probably won’t for some time. Detection of terrestrial planets will be possible in the coming decades, but detecting their satellites will be much more difficult. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p234]
· The Moon, our closest neighbor in space, has thus figured prominently in the origin and evolution of life on Earth. Other solar system bodies, though far more distant, have also had effects remarkable enough to suggest that the conditions that promoted development of life on Earth are rare if not unique. A fascinating case in point is a body over 500 million miles from us, the planet Jupiter. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p234]
· If Earth had been a little closer to Jupiter, or if Jupiter had had a somewhat larger mass, then the “Jupiter effect” that aborted the formation of the asteroid planet and nearly ruined the formation of Mars could also have affected Earth, rendering it a smaller planet. And if Earth had been smaller, its atmosphere, hydrosphere, and long-term suitability for life would surely have been less than ideal. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p237]
· If the early Mars did have oceans, they may have been effectively ejected into space by impacts. Even if there were more water on the young Mars, most of the early impact history of Mars occurred on a planet that was dry compared to Earth. If lifeevolved on both planets, it may have been destroyed on Earth, once or even several times, while it survived on Mars. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p237-238]
· There are sound reasons to believe that life may have formed during a limited window of opportunity. This window of time may have closed before the end of heavy bombardment on Earth. It is thus possible that the present life on Earth is of Martian origin, transported to Earth by meteorites ejected by major impacts. If Mars had been Earth-like, with oceans, then it too would have been sterilized by impact. If Mars had been larger and had had a denser atmosphere, it would also have been much more difficult for impacts to eject meteorites into space. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p238]
· In other words, the bigger it gets, the faster it grows. If gas could be continually fed to it, it would gobble up the Universe in a relatively short time! What actually happens is that Jovian planet formation depletes its feeding zone of matter, which in turn truncates planet formation. And although the general properties of this process might be modeled, it just seems to have been by chance that our Jupiter formed as it did. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p240]
· Because it cleans our solar system of dangerous Earth orbit–crossing asteroids and comets, Jupiter has had a beneficial influence on life on Earth. However, it appears that we have been quite lucky that the Jupiter in our solar system has maintained a stable orbit around the sun. A Jupiter and a giant neighbor like Saturn are a potentially deadly couple that can lead to disastrous situations where a planetary system can literally be torn apart. Recently, it has become possible to use powerful computers to determine the stability of the orbits of Jupiter and Saturn over the lifetime of the planetary system. There are minor chaotic changes but no major changes, and the solar system, at least to a first approximation, is stable over its lifetime. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p240]
· However, this would not be the case if either Jupiter or Saturn were more massive or if the two were closer together. It would also be dangerous to have a third Jupiter-sized planet in a planetary system. In an unstable system the results can be catastrophic. Gravitational perturbations among the planets can radically change orbits, make them noncircular, and actually lead to the loss of planets ejected into interstellar space. It is possible for chaotic disruption to occur even after a system has been stable for billions of years, and in the worst cases, planets can be spun out of the planetary system, escaping the gravitational hold of the star. A life-bearing planet ejected into galactic space would have no external heat source to warm its surface and no sunlight to provide energy for photosynthesis. Although instability might start with just two planets, the effects would spread to them all. In less severe cases, the orbits of the planets would become highly elliptical, and the changing distance between planets and the central star would prevent the persistence of conditions required for stable atmospheres, oceans, and complex life. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p240]
· When a Jupiter spirals inward, the inner planets precede it and are pushed into the star. If our Jupiter had done this, Earth would have been vaporized long before life-tolerant conditions were ever established on its surface. Lin has suggested that our solar system may have had several Jupiters that actually did spiral into the sun, only to be replaced with a newly formed planet. Perhaps Jupiter is at its “right” distance from the sun only because it was the last one to form and it formed at a time when the solar nebula had weakened to the point where orbital decay ceased to be important. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p242]
· On the other hand, it appears that most other “Jupiters” so far detected would have prevented the development of animal life anywhere in their respective solar systems. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p242]
· The Moon and Jupiter are two factors causing us to believe that complex life requires disparate influences. We shall see, in the next chapter, how we might put this hypothesis to the test. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p242]
11-Testing the Rare Earth Hypothesis
· The Rare Earth Hypothesis is the unproven supposition that although microscopic, sludge-like organisms might be relatively common in planetary systems, the evolution and long-term survival of larger, more complex, and even intelligent organisms are very rare. The observations on which this hypothesis is based are as follows: (1) Microbial life existed as soon as Earth’s environment made it possible, and this nearly invincible form of life flourished over most of Earth history, populating a broad range of hostile terrestrial environments. (2) The existence of larger and more complex life occurred only late in Earth history, it occurred only in restricted environments, and the evolution and survival of this more fragile variant of terrestrial life seem to require a highly fortuitous set of circumstances that could not be expected to exist commonly on other planets. This hypothesis can be tested. Throughout human history, people have wondered what lies beyond the limits of the known world. This instinctive obsession has driven humans (and perhaps other species) to expand their own territories. This haunting question permeates mythology and religion and has provoked some of the deepest of human thoughts. In earlier times, the phrase beyond the known world may have referred to regions only hundreds to thousands of miles distant. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p243]
· We see no evidence of advanced life in the solar system, except on Earth, so the main search for advanced life will focus on planetary systems around nearby stars. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p244]
Advanced Life
· An alien astronomer, viewing Earth from a great distance, could detect the presence of life on the planet with comparative ease. This could not be done directly, by imaging, but rather indirectly, by spectral analysis of the composition of the atmosphere. Even with the most grandiose alien telescopes, it is doubtful that extraterrestrial astronomers could directly detect organisms, groups of organisms, or even the most immense structures life has formed: coral reefs, forests, forest fires, red tides, city lights, the Great Wall of China, freeways, and dams. At best, images of Earth would only slightly resolve what Carl Sagan referred to as the “pale blue dot.” The principal clue alerting distant astronomers to the presence of life would be a spectacular and unmistakable signature. Spectral analysis of infrared light would reveal that life plays such a major role on the planet that it controls the composition of the atmosphere. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p244-245]
· Spectrum of a Life-Bearing Planet Earth’s atmosphere would actually be quite “unnatural” for a nonbiotic planet. It is clearly different from the nearly pure carbon dioxide atmospheres of its neighbors, Mars and Venus. The mix of nitrogen, oxygen, and water vapor is chemically unstable and would never arise on a dead planet. Without life, nitrogen and oxygen in the presence of water would combine to form nitric acid and become a dilute acidic component of the ocean. Earth’s peculiar atmosphere is not in chemical equilibrium, and it succeeds in disobeying natural chemical laws only because of the presence of life. The most peculiar aspect of the atmosphere is the abundance of free oxygen. Oxygen is the most abundant element in the whole Earth (45% by weight and 85% by volume!), but in the atmosphere, it is a highly reactive gas that would exist only at trace levels in the atmosphere of a terrestrial planet devoid of life. Oxygen is a poisonous gas that oxidizes organic and inorganic materials on a planetary surface; it is quite lethal to organisms that have not evolved protection against it. The source of atmospheric oxygen is photosynthesis, the miraculous biological process that utilizes the energy of sunlight to convert carbon dioxide to pure oxygen and organic material. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p245]
· Ozone (O3) is a molecule composed of three oxygen atoms. It is a highly chemically active molecule, so it is not very stable. It is produced in the atmosphere by the interaction of ultraviolet light from the sun and normal oxygen (O2). Light splits O2 into individual atoms, and they in turn react with O2 to form ozone. Only a minuscule fraction of the atmosphere’s oxygen is in the form of ozone, but it provides strong infrared absorption. Ozone implies the presence of oxygen, which, in sufficient concentration, implies the existence of life. Maintaining moderate oxygen levels requires continuous production to balance the many processes that lead to its removal from the atmosphere. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p248]
· Unfortunately, it is very difficult to know if SETI is an effective use of resources. If the Rare Earth Hypothesis is correct, then it clearly is a futile effort. If life is common and it commonly leads to the evolution of intelligent creatures that have long, prosperous planetary tenures, then it is possible that enlightened aliens might be beaming signals off into space. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p250-251]
· The Viking landers did not convincingly detect Martian life, but they did show how difficult it is to identify microbial life, with unknown properties, on a planet that is quite different from Earth. Viking was capable of detecting organisms in most of Earth’s surface materials, but our planet teems with life, and a gram of typical soil contains over a billion individual organisms. Viking found that the surface soils on Mars did not and could not support any of the forms of life found on Earth. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p254]
12-Assessing the Odds
· How rare is Earth? We have arrived at the end of a long grocery list of ingredients seemingly necessary to make a planet teeming with complex life. It involves material, time, and chance events. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p257]
· Let us begin by imagining we have the power to observe 100 solar nebulae coalesce into stars and the planets that will encircle them. How many of these events will yield an Earth-like planet with animal life? As we have seen, the first step in preparing the way for a habitable environment is the formation of a suitable star: one that will burn long enough to let evolution work its wonders, one that does not pulse or rapidly change its energy output, one without too much ultraviolet radiation, and most important, perhaps, one that is large enough. Of the 100 applicants, perhaps only two to five will yield a star as large as our sun. The vast majority of stars in the Universe are smaller than our sun, and although smaller stars could have planets with life, most would be so dim that Earth-like planets would have to orbit very close to their star to receive energy sufficient to melt water. But being close enough to get adequate energy from a small star leads to another problem: tidal lock, the condition where the same side of the planet always faces the sun. A tidally locked planet is probably unsuitable for animal life. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p258]
· What if we increased the number to 1000 planetary systems, so that we might expect 20 stars of our sun’s size or greater to be born? Even these numbers are too small to yield a high probability that we will find a truly Earthlike planet. Perhaps a better way to envision the various odds is to re-create the scenario that led to the formation of our solar system and then run through the process once again in a thought experiment. Stephen Jay Gould used this type of mental reconstruction in his interpretation of the Cambrian Explosion. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p258]
· In his 1989 book Wonderful Life, Gould described the exercise as follows: I call this experiment “replaying the tape.” You press the rewind button and, making sure you thoroughly erase everything that actually happened, go back to any time and place in the past—say, to the seas of the Burgess Shale. Then let the tape run again and see if the repetition looks at all like the original. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p258]
· If we rerun this particular tape, we will in all probability not get a repeat of our solar system with its nine planets, its one failed planet (now the asteroid belt), a Jupiter and three other gas giants orbiting outside four terrestrial planets; and a halo of comets surrounding the entire mix. Now we enter the realm of multiple contingencies. Of the 1000 newly formed planetary systems, none is likely to be identical to our solar system today—just as no two people are identical. In a coalescing planetary system many processes, including planetary formation, may be chaotic. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p259]
· Planets form in what are known as “feeding zones,” regions where various elements come together and eventually coalesce into planetesimals, which finally aggregate into a planet. Recent work by planetary scientists shows that the spacing of planets will probably be fairly regular. There might be as few as six planets or as many as ten or even more. James Kasting of Penn State University believes that planetary spacing is not accidental—that the positions of planets are highly regulated, and that if the solar system were to re-form many times, we would get the same number of planets each time. Yet the observational evidence to date does not back up the theory. The extrasolar planets that have been discovered exhibit an enormous diversity of spacing and orbits; their positions are not nearly so orderly as the theory suggests they should be. Ross Taylor, an astronomer who received the prestigious Leonard Award in 1998, disputes Kasting’s views. “Clearly,” he maintains, “the conditions that existed to make our system of planets are not easily reproduced. Although the processes of forming planets around stars are probably broadly similar, the devil is in the details.” [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p259]
· the most important lesson from Earth’s history is that it takes time to make animals—long periods of environmental stability with global temperatures staying well below theboiling point of water. Hence we need to add the time component to each question. For instance, what percentage of planets that have oceans keep them for a billion years, or 4 billion years, or 10 billion years, for that matter? [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p260-261]
· Of all the factors important in assessing the odds of once again getting (or finding) a world with animal life, one factor stands paramount: water. Earth succeeded in acquiring its ark-load of animals and complex plants—and then keeping them—for more than half a billion years (so far) because it retained its oceans for more than 4 billion years. Moreover, if our analysis of the sedimentary record is correct, for the last 2 billion years it maintained the oceans at average temperatures less than 50°C. Also—at least for the last 2 billion years—the oceans have been maintained at a chemical composition conducive to the existence of complex animal life: at a salinity and pH favorable to the formation and maintenance of proteins. The oceans are clearly the cradle of animal life—not fresh water, not the land, but the saltwater oceans have spawned every animal phylum, every basic body plan that exists or has existed on our planet. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p261]
· Discovering how Earth acquired its supply of water is one of the most critical concerns of the new field of astrobiology. As we pointed out in an earlier chapter, water was not abundant in the inner regions of the solar system when the planets formed. There was far more water in the outer regions of the solar system than among the inner planets. Where did our water come from? Although where our oceanic water came from is still the subject of debate, everyone agrees that it must have arrived during planetary accretion, with perhaps significant volumes added during the period of heavy bombardment. Ironically, the volume of water eventually found on Earth may be related to the formation of Earth’s core. When the iron- and nickel-rich core formed, most of the water found in the coalescing planet was consumed in oxidation processes whereby oxygen bound up in water was used to make iron and nickel oxides. It is the residual water that makes up the oceans. Perhaps that residual quantity was significantly enhanced by water carried by comets after Earth’s initial formation, perhaps not. In either case, the oceans reached approximately their present volume by 3.8 billion years ago. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p261]
· There is evidence of deep-water, inorganic limestone formation in very old rocks on Earth, as demonstrated by John Grotzinger and his team from M.I.T. These studies showed that the early Earth’s ocean may have been saturated in the compounds that can produce limestone and thus could have precipitated limestone in deeper water at that time, removing carbon dioxide from the atmosphere as a consequence. However, Grotzinger points out that occurrences of carbonate rocks during the Early Archaean—roughly the first billion years of Earth’s existence—are rare. And this is only partly due to the rarity of rocks of this age. It looks as though the central mode of removing carbon dioxide from the atmosphere—the formation of carbonate rocks—seldom occurred. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p262-263]
· If plate tectonics on Earth had not created increasingly large land areas (and, as a by-product of that, massive areas next to the continents with shallow-water regions where limestone could easily form), Earth might well have reached global temperatures greater than animal life could tolerate. And had global temperatures exceeded 100°C, the oceans would have boiled away, the gigantic volumes of water becoming steam in the atmosphere. This would have spelled a catastrophic ending of all life on the surface of planet Earth. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p263]
· What about the situation where the oceans are lower in volume than they were on Earth? If the continents covered two-thirds of Earth’s surface (rather than their present day one-third), would we have animal life? The great mass extinction of the late Permian almost ended animal life because of high temperatures. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p264]
· The relative areas of land and sea affect more than just planetary temperature. If plate tectonics is not operating, there will be no continents, only large numbers of seamounts and islands (whose number will be dictated by the amount of volcanicity, itself a function of a planet’s heat flow). And without continents, a planet’s ocean may never achieve a chemistry suitable for animal life. Sherwood Chang of NASA cites an example of this. In 1994 Chang proposed that without substantial weathering (which can occur only when there is substantial land area to weather), the early ocean of an Earthlike planet would remain acidic—a poor environment for the development of animal life. It seems that water worlds might be quite fecund habitats for short periods of time but might not achieve the long-term temperature or chemical stability conducive to animal life. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p265]
· Kasting notes that whether habitable planets exist around other stars “depends on whether other planets exist, where they form, how big they are, and how they are spaced.” Kasting stresses, as we do, the importance of plate tectonics in creating and maintaining habitable planets, and he suggests that the presence of plate tectonics on any planet can be attributed to the planet’s composition and position in its solar system. But one of Kasting’s most intriguing comments is related to our Moon. Kasting notes that the obliquity (the angle of the axis of spin of a planet) of three of the four “terrestrial” planets of our solar system—Mercury, Venus, and Mars—has varied chaotically. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p266]
· Earth is the exception, but only because it has a large moon. . . . If calculations about the obliquity changes in the absence of the moon are correct, Earth’s obliquity would vary chaotically from 0 to 85 degrees on a time scale of tens of millions of years were it not for the presence of the Moon. . . . Earth’s climatic stability is dependent to a large extent on the existence of the Moon. The Moon is now generally believed to have formed as a consequence of a glancing collision with a Mars-sized body during the later stages of the Earth’s formation. If such moon-forming collisions are rare . . . habitable planets might be equally rare. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p266]
· We have accumulated a laundry list of potentially low-probability events or conditions necessary for animal life: not only Earth’s position in the “habitable zone” of its solar system (and of its galaxy), but many others as well, including a large moon, plate tectonics, Jupiter in the wings, a magnetic field, and the many events that led up to the evolution of the first animal. Let us explore what these conditions might mean for life beyond Earth. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p266]
The Odds of Animal Life Elsewhere, and of Intelligence
· In the 1950s, astronomer Frank Drake developed a thought-provoking equation to predict how many civilizations might exist in our galaxy. The point of the exercise was to estimate the likelihood of our detecting radio signals sent from other technologically advanced civilizations. This was the beginning of sporadic attempts by Earthlings to detect intelligent life on other planets. Now called the Drake Equation in its creator’s honor, it has had enormous influence in a (perhaps necessarily) qualitative field. The Drake Equation is simply a string of factors that, when multiplied together, give an estimate of the number of intelligent civilizations, N, in the Milky Way galaxy. As originally postulated, the Drake Equation is.
N* x fs x fp x ne x fi x fc x fl x N. where: N* = stars in the Milky Way galaxy. fs = fraction of sun-like stars. fp = fraction of stars with planets. ne = planets in a star’s habitable zone. fi = fraction of habitable planets where life does arise. fc = fraction of planets inhabited by intelligent beings. fl = percentage of a lifetime of a planet that is marked by the presence of a communicative civilization Our ability to assign probable values to these terms varies enormously. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p267]
· When Drake first published his famous equation, there were great uncertainties in most of the factors. There did (and does) exist a good estimate for the number of stars in our galaxy (over 300 billion). The number of star systems with planets, however, was very poorly known in Drake’s time. Although many astronomers believed that planets were common, there was no theory that proved star formation should include the creation of planets, and many believed that the formation of planetary systems was exceedingly rare. During the 1970s and later, however, it was assumed that planets were common; in fact, Carl Sagan estimated that an average of ten planets would be found around each star. Even though no extra solar planets were found until the 1990s, their discovery seemed to vindicate those who believed planets were common. But is it so? A new look at this problem suggests that planets may indeed be quite rare—and thus the presence of animal life rarer still. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p267-268]
Are Stars with Planets Anomalous?
· We now know that planetary formation outside our own system does indeed occur. The recent and spectacular discovery of extrasolar planets, one of the great triumphs of astronomical research in the 1990s, has proved what has long been assumed: that other stars have planets. But at what frequency? It may be that a substantial fraction of stars have planetary systems. To date, however, astronomers have succeeded in detecting only giant, “Jupiter-like” planets; available techniques cannot yet identify the smaller, rocky, terrestrial worlds. Now that numerous stars have been examined, it appears that only about 5% to 6% of examined stars have detectable planets. Because only large gas-giant planets can be detected, this figure really shows that Jupiter clones close to stars or in elliptical orbit are rare. But perhaps it indicates that planets as a whole are rare as well. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p268]
· The evidence that planets may be rare comes not so much from the direct-observation approach of the planet finders (such as the Marcy/Butler group) but from spectroscopic studies of stars that appear similar to our own sun. The studies of those stars around which planets have been discovered have yielded an intriguing finding: They, like our sun, are rich in metals. According to astronomers conducting these studies, there seems to be a causal link between high metal content in a star and the presence of planets. Our own star is metal-rich. In a study of 174 stars, astronomer G. Gonzalez discovered that the sun was among the highest in metal content. It appears that we orbit a rare sun. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p268-269]
· It is still impossible to observe smaller, rocky planets orbiting other stars. Perhaps such planets—which we believe are necessary for animal life—are quite common. But perhaps this is a moot point. We have hypothesized that animal life cannot long exist on a planet unless there is a giant, Jupiter-like planet within the same planetary system—and orbiting outside the rocky planets—to protect against comet impacts. It may be that Jupiters like our own, in regular orbits, are rare as well. To date, all tend to be in orbital positions that would be lethal, rather than beneficial, to any smaller, rocky planets. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p269]
Planet Frequency and The Drake Equation
· All predictions concerning the frequency of life in the Universe inherently assume that planets are common. But what if the conclusions suggested by emerging studies—that Earth-like planets are rare, and planets with metal rarer still—are true? [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p270]
· This finding has enormous significance for the final answer to the Drake Equation. Any factor in the equation that is close to zero yields a near-zero final answer, because all the factors are multiplied together. Carl Sagan, in 1974, estimated that the average number of planets around each star is ten. Goldsmith and Owen, in their 1992 The Search for Life in the Universe, also estimated ten planets per star. But the new findings suggest greater caution. Perhaps planetary formation is much less common than these authors have speculated. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p270]
· To estimate the frequency of intelligent life, the Drake Equation hinges on the abundance of Earth-like planets around sun-like stars. The most common stars in the galaxy are M stars, fainter than the sun and nearly 100 times more numerous than solar-mass stars. These stars can generally be ruled out because their “habitable zones,” where surface temperatures could be conducive to life, are uninhabitable for other reasons. To be appropriately warmed by these fainter stars, planets must be so close to the star that tidal effects from the star force them into synchronous rotation. One side of the planet always faces the star, and on the permanently dark side, the ground reaches such low temperatures that the atmosphere freezes out. Stars much more massive than the sun have stable lifetimes of only a few billion years, which might be too short for the development of advanced life and evolution of an ideal atmosphere. As we noted earlier, each planetary system around a 1-solar-mass star will have space for at least one terrestrial planet in its habitable zone. But will there actually be an Earth-sized planet orbiting its star in that space? When we take into account factors such as the abundance of planets and the location and lifetime of the habitable zone, the Drake Equation suggests that only between 1% and 0.001% percent of all stars might have planets with habitats similar to those on Earth. But many now believe that even these small numbers are overestimated. On a universal viewpoint, the existence of a galactic habitable zone vastly reduces them. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p270-271]
· Such percentages seem very small, but considering the vastness of the Universe, applying them to the immense numbers of stars within it can still result in very large estimates. Carl Sagan and others have mulled these various figures over and over. They ultimately arrived at an estimate of one million civilizations of creatures capable of interstellar communication existing in the Milky Way galaxy at this time. How realistic is this estimate? If microbial life forms readily, then millions to hundreds of millions of planets in the galaxy have the potential for developing advanced life. (We expect that a much higher number will have microbial life.) [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p271]
· However, if the advancement to animal-like life requires continental drift, the presence of a large moon, and many of the other rare Earth factors discussed in this book, then it is likely that advanced life is very rare and that Carl Sagan’s estimate of a million communicating civilizations is greatly exaggerated. If only one in 1000 Earth-like planets in a habitable zone really evolves as Earth did, then perhaps only a few thousand have advanced life. Although it could be argued that this is too pessimistic, it may also be much too optimistic. Even so, we cannot rule out the possibility that Earth is not unique in the galaxy as an abode of life that has just recently developed primitive technologies for space travel and interplanetary radio communication. Perhaps we can suggest a new equation, which we can call the “Rare Earth Equation,” tabulated for our galaxy: N* x fp x ne x fi x fc x fl N. where: N* = stars in the Milky Way galaxy. fp = fraction of stars with planets. ne =planets in a star’s habitable zone. fi = fraction of habitable planets where life does arise. fc = fraction of planets with life where complex metazoans arise fl = percentage of a lifetime of a planet that is marked by the presence of complex metazoans [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p271-272]
· And what if some of the more exotic aspects of Earth’s history are required, such as plate tectonics, a large moon, and a critically low number of mass extinctions? When any term of the equation approaches zero, so too does the final result. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p272]
· With this in mind, let us return to our Rare Earth Equation and flesh it out a bit by adding some of the other factors featured in this book. N* x fp x fpm x ne x ng x fi x fc x fl x fm x fj x fme x N. where: N* = stars in the Milky Way galaxy. fp = fraction of stars with planets. fpm = fraction of metal-rich planets. ne = planets in a star’s habitable zone. ng = stars in a galactic habitable zone. fi = fraction of habitable planets where life does arise. fc = fraction of planets with life where complex metazoans arise. fl = percentage of a lifetime of a planet that is marked by the presence of complex metazoans. fm = fraction of planets with a large moon. fj = fraction of solar systems with Jupiter-sized planets. fme = fraction of planets with a critically low number of mass extinction events. With our added elements, the number of planets with animal life gets even smaller. We have left out other aspects that may also be implicated: Snowball Earth and the inertial interchange event. Yet perhaps these too are necessary. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p274-275]
· Again, as any term in such an equation approaches zero, so too does the final product. How much stock can we put in such a calculation? Clearly, many of these terms are known in only the sketchiest detail. Years from now, after the astrobiology revolution has matured, our understanding of the various factors that have allowed animal life to develop on this planet will be much greater than it is now. Many new factors will be known, and the list of variables involved will undoubtedly be amended. But it is our contention that any strong signal can be perceived even when only sparse data are available. To us, the signal is so strong that even at this time, it appears that Earth indeed may be extraordinarily rare. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p275]
· The Universe seems to be finite; there are not an infinite number of planets circling the vast number of stars in the ocean of space. But the numbers are immense beyond understanding. We are one of many planets. But as we have tried to show in this book, perhaps not so many as we might hope—and perhaps not so many that we will ever, however long the history of our species, find any extraterrestrial animals among the stars surrounding our sun. That is a fate not foreseen by Hollywood—that we may find nothing but bacteria, even on planets orbiting distant stars. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p278]
· If the Rare Earth Hypothesis is correct—that is, if microbial life is common but animal life is rare—there will be societal implications, or at least some small personal implications. What will be the effect if news comes back from the next Mars mission that there is life on Mars after all—microbial to be sure, but life. Or what if, after astronauts voyage repeatedly to other planets in our solar system, or even to the dozen nearest stars, we find nothing more advanced than a bacterium? What if, at least in this quadrant of the galaxy, we are quite alone, not just as the only intelligent organisms but also as the only animals? How much of our striving to travel into space is the hope of discovering—and perhaps talking to—other animalia? [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p279]
· The great danger to our thesis (that Earth is rare because of its animal life, the factors and history necessary to arrive at this point as a teeming, animal- and plant-rich planet being highly improbable) is that it is a product of our lack of imagination. We assume in this book that animal life will be somehow Earth-like. We take the perhaps jingoistic stance that Earth-life is every-life, that lessons from Earth are not only guides but also rules. We assume that DNA is the only way, rather than only one way. Perhaps complex life—which we in this book have defined as animals (and higher plants as well)—is as widely distributed as bacterial life and as variable in its makeup. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p282]
· Perhaps Earth is not rare after all but is simply one variant in a nearly infinite assemblage of planets with life. Yet we do not believe this, for there is so much evidence and inference—as we have tried to show in the preceding pages—that such is not the case. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p282]
Our Rare Earth
· Let us recap why we think Earth is rare. Our planet coalesced out of the debris from previous cosmic events at a position within a galaxy highly appropriate for the eventual evolution of animal life, around a star also highly appropriate—a star rich in metal, a star found in a safe region of a spiral galaxy, a star moving very slowly on its galactic pinwheel. Not in the center of the galaxy, not in a metal-poor galaxy, not in a globular cluster, not near an active gamma ray source, not in a multiple-star system, not even in a binary, or near a pulsar, or near stars too small, too large, or soon to go supernova. We became a planet where global temperatures have allowed liquid water to exist for more than 4 billion years—and for that, our planet had to have a nearly circular orbit at a distance from a star itself emitting a nearly constant energy output for a long period of time. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p282-283]
· Our planet received a volume of water sufficient to cover most—but not all—of the planetary surface. Asteroids and comets hit us but not excessively so, thanks to the presence of giant gas planets such as Jupiter beyond us. In the time since animals evolved over 600 million years ago, we have not been punched out, although the means of our destruction by catastrophic impact is certainly there. Earth received the right range of building materials—and had the correct amount of internal heat—to allow plate tectonics to work on the planet, shaping the continents required and keeping global temperatures within a narrow range for several billion years. Even as the Sun grew brighter and atmosphere composition changed, the Earth’s remarkable thermostatic regulating process successfully kept the surface temperature within livable range. Alone among terrestrial planets we have a large moon, and this single fact, which sets us apart from Mercury, Venus, and Mars, may have been crucial to the rise and continued existence of animal life on Earth. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p283]
· The continued marginalization of Earth and its place in the Universe perhaps should be reassessed. We are not the center of the Universe, and we never will be. But we are not so ordinary as Western science has made us out to be for two millennia. Our global inferiority complex may be unwarranted. What if Earth is extremely rare because of its animals (or, to put it another way, because of its animal habitability)? [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p283]
· The possibility that animal life may be very rare in the Universe also heightens the tragedy of the current rate of extinction on our planet. Earlier, we suggested that the rise of an intelligent species on any planet might be a common source of mass extinction. That certainly seems to be the case on Earth. And if animals are as rare in the Universe as we suspect, it puts species extinction in a whole new light. Are we eliminating species not only from our planet but also from a quadrant of the galaxy as a whole? [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p283]
· The discovery of a phenomenon such as the magnetar is an object lesson that suggests a great deal more than life’s rarity: There is still so much more to learn about the heavens surrounding us. We humans are like 2-yearolds, just beginning to comprehend the immensity, wonder, and hazards of the wide world. So too with our understanding of astrobiology. It is clearly just beginning. [Peter Ward & Donald Brownlee: Rare Earth, Why Complex Life Is Uncommon in the Universe, Copernicus Books, 2000, p287]
الحمد لله الذي بنعمته تتمّ الصَّالِحات