From: SKY AND TELESCOPE, April, 1977
How has the earth's atmosphere changed in chemical composition during geologic time? How have these changes affected our planet's ability to support life? A glimpse into what might have taken place is provided by a computer simulation of the atmosphere's evolution during the last 4*/2 billion years, conducted by Michael H. Hart of Goddard Space Flight Center. He reported his results at the January meeting of the American Astronomical Society in Honolulu, Hawaii.
Initially he assumes that any primordial atmosphere of the earth must have es¬caped into space quickly after formation, and that the constituents of the present atmosphere came from the outgassing of our planet's interior.
The diagram at right shows schematically the main ways in which constituents can enter the atmosphere and be removed from it. The geochemical processes that determine the course of atmospheric evolution are complex and interact in an
made specific allowance for these factors (among others): the rate of outgassing from the interior, condensation of water vapor into oceans, solution of atmospheric gases in seawater, photodissociation of water vapor in the upper atmosphere, escape of hydrogen into space, chemical reactions between gases, the effects of life, trapping of carbon dioxide in carbonate minerals, and the burial of organic sediments.
The computer simulation was repeated many times, beginning with different pro¬portions of carbon, oxygen, and nitrogen in the gases from the earth's interior. All atmospheric argon was assumed to come from the radioactive decay of potassium -40. For successive intervals of 2,500,000 years, the computer calculated the amount of each element added or lost from the atmosphere and ocean, and at the end of each step determined the mass of the oceans, mass and composition of the atmosphere, and other data.
The diagram at the very top shows the changing composition of the atmosphere, starting with the mix of volatiles that eventually yielded the best match to the present values. Because most of the water vapor quickly condensed to form oceans, the early atmosphere was largely carbon dioxide, which was rapidly depleted by reactions with silicate rocks. Consequently, from about 4.3 to 1.9 billion years ago the main constituents of air were methane and other reduced carbon compounds,
At first, oxygen was released from the photodissociation of water vapor, and beginning some 3.7 billion years ago also by the photosynthesis of plants. Methane was gradually eliminated by combustion with this oxygen, and since then nitrogen has been the principal constituent of the air.
Roughly 420 million years ago, the amounts of oxygen and therefore of ozone became great enough to reduce the ultraviolet radiation reaching the earth's surface to a level tolerable for living organisms. The ensuing expansion of plant life caused a rapid increase in the abundance of atmospheric free oxygen.
Perhaps the most significant result of the computer simulation has been to trace the long-term variations in the earth's effective and mean surface temperatures. The difference between the two temperature curves in the diagram at the bottom of page 266 is due to the greenhouse effect, which was large during the first 2'/2 billion years, because of the abundance of carbon dioxide, water vapor, methane, and ammonia. The high surface temperature caused much water to evaporate, leading to a cloud-veiled Earth.
Then, as methane and other reduced gases oxidized away, the greenhouse effect declined sharply and the mean surface temperature dropped. By two billion years ago, substantial polar ice caps had formed.
"The evolutionary process is very sensitive to the earth-sun distance," says Dr. Hart. "Had the earth been situated slightly closer to the sun, a runaway greenhouse effect would have occurred fairly early in the earth's history (a result obtained by S. I. Rasool and C. de Bergh in 1970). Had the earth's orbit been slightly larger instead, then runaway glaciation would have occurred about two billion years ago."