WHY IS AN ATMOSPHERE THE WAY IT IS?
We are all aware of the characteristics of the earth's atmosphere and its consequent importance in maintaining life; Mercury and the moon, in contrast, have almost no atmosphere. How did the physical diversity of planetary atmospheres arise?
The nature of an atmosphere results from several factors:
- The planet's distance from the sun along with its size and mass, which influence its ability to retain an atmosphere
- Its chemical composition, which determines what processes go on in the atmosphere
- Geological and chemical evolution of the planet's surface layers
- finally the atmosphere's interaction with biological life, if living organisms are in fact present
The planet's distance and mass are important in the following way: From basic physics we know that, the higher the temperature or the smaller the mass of a molecule (or both), the greater will be the average velocity of gas particles in the atmosphere. Since a particle moving outward might gain enough kinetic energy after colliding with another atmospheric particle to escape into space, can we determine the conditions necessary for the escape of a particular atom or molecule from a planet's gravitational field? Using the planet's temperature, mass, and radius and the masses and thermal velocities of its atmospheric constituents, astronomers have reached the following conclusion about the escape of different constituents:
If a molecule's velocity is near a third of the velocity needed for escape, about half of that chemical species will escape from the atmosphere within weeks. For a planet to preserve various molecular components of its atmosphere indefinitely, the mean velocity of the gases must be less than a tenth of the velocity of escape.
The massive Jovian planets, with their large escape velocities (several tens of kilometers per second), have held their primeval atmospheres of hydrogen and helium, while the less massive terrestrial planets, with smaller escape velocities (several kilometers per second), have lost these light gases. Venus, the earth, and Mars have managed to retain atmospheric water molecules as well as some heavy gases. Mercury and the moon lack any appreciable atmosphere since these two bodies have small masses; moreover Mercury is close to the sun.
In spite of the noble-gas abundance in the sun, the surprising scarcity of neon, argon, krypton, and xenon on the terrestrial planets suggests that they may not have retained their original atmospheres. A secondary atmosphere for Venus, the earth, and Mars may have formed out of gases escaping from their interiors during volcanic eruptions.
The second factor affecting a planet's atmosphere is the atmosphere's chemistry, which for the terrestrial planets is very different from that of the Jovian planets. Because the terrestrial planets formed from rocky materials-such as iron and iron silicates and metal oxides-their early atmospheres should have been largely composed of such gases as carbon dioxide, nitrogen, and some water. Venus and Mars still have that kind of atmosphere. Planets like Jupiter and Saturn are more nearly like the sun in chemical composition than like the terrestrial planets. Depending upon the chemical composition and chemical activity of the planet's atmosphere, the greenhouse effect will be more or less effective in trapping incoming solar radiant energy. This causes a warming of the low atmosphere and the surface, which will influence much of the atmospheric chemistry. Venus is a good example of the long-term consequences of the greenhouse effect. Estimates are that the mean surface temperature of Mars, the earth, and Venus are about 5,35, and 500 K warmer, respectively, than they would be without the greenhouse effect.
The third factor, the ways geological and chemical evolution affect a planet's atmosphere, is important in the case of the earth, Venus, and Mars. Outgassing from these planets' interiors in their early history consisted mainly of water vapor, carbon dioxide, and nitrogen, in approximately the same proportions that we observe in terrestrial volcanic gases today. On earth water vapor condensed to form the oceans, but nitrogen remained in a gaseous state. Most of the carbon dioxide combined with silicate rocks of the crust to forr:n carbonate rocks, such as limestone, a reaction that occurs most efficiently in the presence of liquid water. If it could be released from crustal rocks along with the small amount dissolved in the oceans, carbon dioxide in the earth's atmosphere would equal about one-half of the amount in the dense atmosphere of Venus.
If at some earlier time water vapor condensed on Venus, the high su rface temperature due to the greenhouse effect prevented water from remaining in a liquid form and kept carbon dioxide in a gaseous form. Most of the water vapor apparently dissociated into hydrogen and oxygen by absorbing ultraviolet sunlight. Hydrogen escaped, and the heavier oxygen may have combined with crustal rocks to form oxides. On Mars the outgassing of water vapor, carbon dioxide, and nitrogen was probably less complete than it was on the earth; yet carbon dioxide forms the largest part of the atmosphere of Mars. Mars appears at present to be in a cold phase, and a large amount of water is apparently stored in the polar caps and under the surface of the planet as permafrost.
Finally we need to consider the effect of biological life, if any, on an atmosphere. Living organisms on a planet are .bound to affect its atmosphere if the interaction between its biosphere (the zone in which life exists) and atmosphere is anything like that on earth:
Large expanses of liqu id water will moderate a planet's climate and can provide an environment conducive to the development of life if there is adequate protection from solar ultraviolet radiation; conversely most of the earth's free oxygen, so necessary to animal life, comes from photosynthesis. The oxygen is constantly replenished by green plants, plankton, and some bacteria. When living organisms became able to extract carbon dioxide from the atmosphere, they helped save the earth from the heat death that Venus has apparently experienced.
