Earth succeeded in acquiring its ark-load of animals and complex 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] is evidence of deep-water, inorganic limestone formation in very old
Earth succeeded in acquiring its ark-load of animals and complex 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] is evidence of deep-water, inorganic limestone formation in very old
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