Planetary Atmospheres

The structure and properties of a planet's atmosphere depend on a number of factors. One is proximity to the Sun. Those planets closest to the Sun are less likely to contain lighter gases that are driven off by the Sun's radiant energy. Mercury illustrates this principle. It is so close to the Sun that it has essentially no atmosphere. Its atmospheric pressure is only 10^-12 millibars, onequadrillionth that of Earth's atmospheric pressure. The major gases found in this planet's very thin atmosphere are helium and sodium, both of which are probably remnants of the Sun's solar wind rather than intrinsic parts of the planet's own structure. Some astronomers believe that contributions come from gases seeping out from the planet's interior.

Another property determining the nature of a planet's atmosphere is cloud cover or other comparable features. Cloud cover has a variety of sometimes contradictory effects on a planet's atmosphere. As sunlight reaches the planet clouds will reflect some portion of that sunlight back into space. The amount that is reflected depends partly on the composition of clouds, with whiter, brighter clouds reflecting more light than darker clouds. Some of the light that does pass through clouds is absorbed by gases in the planet's atmosphere, and the rest reaches the planet's surface. The distribution of solar radiation that is absorbed and reflected will depend on the gases present in the atmosphere. For example, ozone absorbs radiation in the ultraviolet region of the electromagnetic spectrum, protecting life on Earth from this harmful radiation.

Of the solar radiation that reaches a planet's surface, some will be absorbed, causing the surface to heat up. In response, the surface emits infrared radiation which consists of wavelengths significantly longer than that of the incoming radiation. Depending on the composition of the atmosphere, this infrared radiation may be absorbed, trapping heat energy in the atmosphere. Carbon dioxide in a planet's atmosphere will absorb radiation emitted from a planet's surface, although the gas is transparent to the original incoming solar radiation. This process is known as the greenhouse effect and is responsible for the warmer atmospheres on some planets than would be predicted based on their proximity to the Sun.

A planet's rotational patterns also influence its atmospheric properties. One can describe the way gases would flow in an idealized planet atmosphere. Since the equator of any planet is heated more strongly than the poles, gases near the equator would tend to rise upward, drift toward the poles, be cooled, return to the surface of the planet, and then flow back toward the equator along the planet's surface. This flow of atmospheric gases, driven by temperature differences, is called convection. The simplified flow pattern described is named the Hadley cell. In a planet like Venus, where rotation occurs very slowly, a single planet-wide Hadley cell may very well exist. In planets that rotate more rapidly, such as Earth, single Hadley cells cannot exist because the movement of gases is broken up into smaller cells and because Earth's oceans and continents create a complex pattern of temperature variations over the planet's surface.