As we have tried to show in the preceding chapters, 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 the boiling 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?
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.
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. But this does not mean they were at their present area. Don Lowe of Stanford University has estimated that before 3 billion years ago, less than 5% of the surface was land. Until about 2.5 billion years ago, the chemistry of this world-girding ocean was controlled largely by interactions with the oceanic crust beneath it and with Earth's mantle, whose by-products interacted in the oceans at mid-ocean ridges and rifting areas. It is estimated that because this early Earth was much warmer than the Earth we know, the area of this zone of ocean-mantle contact was as much as six times that found today.
Earth's atmosphere was also very different from that of today. There was no oxygen, and there was a great deal more carbon dioxide—perhaps 100 to 1000 times as much as today. Earth's surface temperature was higher than it is now because more heat was emanating from the interior and because of the warming generated by the extensive CO2 and other greenhouse gases in the atmosphere. Earth's internal generation of heat was an important factor; the sun at this time was much fainter, and delivering perhaps a third less energy, than at the present time.
What would have happened if Earth had stayed a water world? Probably global temperatures would have remained high or even increased. For animal life to form, the temperature had to drop from the levels acknowledged to have been characteristic of Archaean time. A drop in global temperature while the sun was getting hotter required a drastic reduction of atmospheric CO2—a reduction of the greenhouse effect. Thus some means of removing CO2 had to be brought to bear. As we saw in Chapter 9, the most effective way to do this is through the formation of limestone, which uses CO2 as one of its building blocks and thus scrubs it from the atmosphere. But significant volumes of limestone form today only in shallow water; the most effective limestone formation occurs in depths of less than 20 feet. In deeper water, high concentrations of dissolved CO2 slows or inhibits the chemical reactions that lead to limestone formation. 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.
To form limestone in significant volumes, then, shallow water is needed, but on a planet without continents, shallow water is in short supply. If the volume of water on a planet is low enough that significant areas of shallow water are available even without continents, there is no problem. On Earth and other planets with significantly deep oceans, however, without continents the shallow-water regions would not be large enough for the necessary limestone formation. Thus when planets have too much water on their surfaces—when their oceans are too deep—there is no natural brake on carbon dioxide buildup. Water temperatures will rise as planetary temperatures rise.
What about underwater weathering? James Kasting has pointed out to us that an all-water world can indeed regulate its temperature. He rightly notes that as oceanic water temperature rises, it eventually causes weathering of limestone on the bottom of the sea. Although much less efficient than the weathering of continental material, this mechanism will indeed produce a feedback mechanism. Yet to heat water temperatures sufficiently to serve as a global thermostat a planet may well exceed the critical 40°C mark that is the upper temperature limit of animal life. 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.
The removal of carbon dioxide is called CO2 drawdown. On Earth, it was accomplished because of continent formation, which took place during a relativity brief interval of Earth history. From perhaps 2.7 to 2.5 billion years ago there occurred a rapid buildup of continental areas. This buildup resulted in the land surface increasing from perhaps 5% to about 30%. This marked change had equally profound effects on the atmosphere-ocean system.
With the formation of continents as a result of plate tectonics, ocean chemistry became dominated by weathering by-products of the continents. As continents weather, or undergo the chemical and mechanical breakdown of rock material, river runoff carries enormous volumes of these chemicals into the sea, where they can greatly affect ocean chemistry and cause mineralization—such as carbonate formation. Larger continents also, paradoxically enough, meant larger shallow-water regions, for the emergence of continents created the shallow continental shelves as well as large inland seas and lakes. Thus the following sequence unfolded: Large shallow regions were created; nutrient influx from continental regions increased; the amount of plant material on Earth (mainly in the surface regions of the shallow seas and on shallow sea bottoms) skyrocketed; and oxygen production began in earnest. All of these events opened the pathway to the eventual evolution of animals.
The critical question is why, on Earth, the volume of water was sufficiently large to buffer global temperatures, but small enough so that shallow seas could be formed by the uplifting of continents. If Earth's ocean volume had been greater, even the formation of continents would not have produced shallow seas. To show that there can be great relative volumes of oceans on a planet, we need only look at Jupiter's moon Europa, where the planet-covering ocean (now frozen) is 100 kilometers thick. No Mt. Everest rising from the sea floor would ever poke through an ocean even half that deep. There would be none of the shallows necessary for limestone formation and no continental weathering.
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. With greater continental area, we might expect temperature swings to have been even greater, and the prospects for continued existence of at least land animals far lower, because large land areas create very high and very low seasonal temperatures. Large land areas also reduce CO2 drawdown, because carbonate formation takes place almost exclusively in oceans. On landdominated worlds, opportunities for life to thrive would thus be reduced.
It appears that Earth got it just right. Without continents there seems a strong likelihood that a planet will become too hot (especially because main-sequence stars such as the sun increase their energy output through time, and planets cannot move away from this increasing heat source). With too much continental area, the opposite is likely to happen, as continental weathering draws down carbon dioxide so much that glaciations ensue. Earth may have been headed down the path toward a global mean temperature so high as to boil away its oceans, or perhaps still cool enough to retain its oceans but yet too warm for complex metazoans to evolve. Animals are not thermophiles.
How much land area is "just right," and how much is too little or too much? The answer probably depends on the given planet's distance from the sun. A planet whose orbit dictates that it receives less energy from its star than Earth does from the sun might need a greater amount of ocean cover (assuming that an increased sea surface creates warmer planetary temperatures because of greater greenhouse effects from CO2 buildup).
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.
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