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Chapter 20: An Envelope of Gas: Earth’s Atmosphere and Climate
Feature Articles
The Rest of the Story: Atmospheres of Other Planets
by Stephen MarshakThe composition of Earth's atmosphere reflects the interaction between the atmosphere, the hydrosphere, and life (the biosphere). This makes our atmosphere markedly different from that of other planets, because Earth is the only planet with an ocean and abundant life. The composition of another planet's atmosphere depends on various factors, such as the planet's size and distance from the sun.
- Mercury, which is fairly small andlies close to the Sun, has virtually no atmosphere. Its gravity is too weak to hold on to one, and high temperatures boil away any gas present.
- Venus's atmosphere is composed primarily of carbon dioxide: We can't even see the planet's surface because the atmosphere is so dense. In fact, gas pressure at the surface is seventy times than at the surface of Earth. The atmosphere has become so hot (480°C on the surface, because of its proximity to the Sun and the greenhouse effect, that it has become very dry. Thus, water on Venus can exist only in gaseous form. Much of the water is destroyed by ultraviolet radiation, which breaks it into H2 and O2; the lightweight H2 molecules escape out to space and the O2 reacts with surface rock.
- Earth's Moon, like Mercury, has virtually no atmosphere.
- Mars does have an atmosphere, but it's much less dense than that of Earth-gas pressure is only 1/160 that of gas pressure at sea level on Earth. Perhaps the low density in part reflects Mars's low temperature; water and CO2 probably stay frozen at or near the planet's surface. Also, Mars has not experienced volcanic activity for a long time, so some gases that have leaked out to space have not been replenished.
- The outer planets (such as Jupiter) are so large that their atmospheres consist mostly of gases pulled in from the nebulae out of which the Sun and planets formed in the first place. They contain mostly hydrogen and helium, but also ammonia (NH3), water, and methane in trace amount. The familiar stripes in the outer planets' atmospheres represent slight contrasts in composition caused by contrasts in temperature and pressure. Dark belts are cooler than light belts. And at great depths in the atmosphere, the pressure is so great that the gas turns to liquid.