LIFE IN AN OCEAN OF GAS
Our atmosphere is the breath of life to everything we do. It is an ocean of gas in which we live as fish live in an ocean of water. It is an integral part of our planet, and we cannot understand Earth's history without also understanding its atmosphere. In seeking to do this, the wide variation from the airless Moon to the exotic and turbulent atmosphere of Jupiter provides an invaluable context and perspective. As it happens, the solar system has several planetary atmospheres between these extremes.Earth's atmosphere lies intermediate in surface pressure between that of Venus and Mars, Venus' atmosphere being about 100 times that of Earth, and Mars' about one one-hundredth. The composition of the atmospheres of Venus and Mars is predominantly carbon dioxide, while Earth's today is predominantly nitrogen and oxygen. On the other hand, the amount of carbon dioxide tied up in the carbonate of the Earth's crust is comparable to what we see free in the atmosphere of Venus today. Presumably on Earth carbon dioxide exhaled from volcanoes, in the presence of the abundant water, was soon taken up into the crustal rocks. Life on Earth doubtless accounts for the presence of so much oxygen in the atmosphere, most of which was probably generated by photosynthesis. From the Mariner observations of Mercury, it appears that Mercury is closer to the Moon than to Mars as far as atmosphere is concerned.
Much patient work remains to bring out the full significance of Apollo observations. Nevertheless it is already clear that Apollo has made a great contribution to the development of the new field of comparative planetary studies, a field that over the years will provide us priceless insights into our own planet. In days of deep concern over the Earth's environment and finite resources, comparative planetology is a new science of incalculable importance, and Apollo led the way.
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