A process that strongly influenced the ultimate evolution of life on Earth was the creation of the atmosphere, oceans, and land. Events involved in the formation of all three were highly intertwined.
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). 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. 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. Telescopic detection of such peculiar atmospheres is the basis of a strategy for detection of life outside the solar system by the "Terrestrial Planet Finder"; we will discuss it in Chapter 10.
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 ac creted materials, but in Earth's case they are most strongly affected by processes that recycle atmospheric components in and out of the atmosphere.
The oceans are a by-product of outgassing and the formation of the atmosphere. When the atmosphere was very hot, a great deal of it was composed of steam. Gradually, as the early Earth cooled, the steam condensed as water and formed the vast oceans we still see today. Although they were originally fresh, the oceans became salty through chemical interactions with Earth's crust.
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. 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.
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.
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.
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. The next step was to populate it—the subject of the next chapter.
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