The atmosphere that originally surrounded the earth was probably much different from the air we breathe today. The earth’s first atmosphere (some 4.6 billion years ago) was most likely hydrogen and helium—the two most abundant gases found in the universe—as well as hydrogen compounds, such as methane and ammonia. Most scientists feel that this early atmosphere escaped into space from the earth’s hot surface. A second, more dense atmosphere, however, gradually enveloped the earth as gases from molten rock within its hot interior escaped through volcanoes and steam vents. We assume that volcanoes spewed out the same gases then as they do today: mostly water vapor (about 80 percent), carbon dioxide (about 10 percent), and up to a few percent nitrogen. These gases (mostly water vapor and carbon dioxide) probably created the earth’s second atmosphere. As millions of years passed, the constant outpouring of gases from the hot interior—known as outgassing— provided a rich supply of water vapor, which formed into clouds.* Rain fell upon the earth for many thousands of years, forming the rivers, lakes, and oceans of the world. During this time, large amounts of CO2 were dissolved in the oceans. Through chemical and biological processes, much of the CO2 became locked up in carbonate sedimentary rocks, such as limestone. With much of the water vapor already condensed and the concentration of CO2 dwindling, the atmosphere gradually became rich in nitrogen (N2), which is usually not chemically active. It appears that oxygen (O2), the second most abundant gas in today’s atmosphere, probably began an extremely slow increase in concentration as energetic rays from the sun split water vapor (H2O) into hydrogen and oxygen. The hydrogen, being lighter, probably rose and escaped into space, while the oxygen remained in the atmosphere.This slow increase in oxygen may have provided enough of this gas for primitive plants to evolve, perhaps 2 to 3 billion years ago. Or the plants may have evolved in an almost oxygen-free (anaerobic) environment. At any rate, plant growth greatly enriched our atmosphere with oxygen. The reason for this enrichment is that, during the process of photosynthesis, plants, in the presence of sunlight, combine carbon dioxide and water to produce oxygen.
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