8. Mars: the red planet
The Martian atmosphere
A carbon-dioxide atmosphere
Astronomers have long known that Mars has an atmosphere. It is required for the formation and support of the clouds that have been observed telescopically since the 19th century. The seasonal waxing and waning of the polar caps also suggests the presence of an atmosphere on Mars. Gases are released into the Martian atmosphere when a polar cap warms up during the local summer and the cap becomes smaller; gases are extracted from the atmosphere during the winter growth of the cap.
The Viking 1 and 2 landers made detailed measurements of the composition of the Martian atmosphere. Carbon dioxide is indeed the principal constituent, amounting to 95.32 percent of the atmosphere at ground level, followed by nitrogen (2.7 percent) and argon (1.6 percent). When compared with Earth, there is much more argon in the Martian air.
Small amounts of oxygen, ozone and water vapor
Oxygen molecules are present in the Martian atmosphere, but in a miniscule amount of just 0.13 percent. In contrast, the Earthís atmosphere is filled with breathable oxygen, amounting to 21 percent of our air. Detection of significant amounts of oxygen would have argued for the possibility of plant life on Mars, since oxygen is unstable in a planetary atmosphere and plants are needed to continuously supply it. The small amount of free oxygen that is now present on Mars is the by-product of the destruction of carbon dioxide by energetic sunlight. This process also results in the production of exceedingly small amounts of ozone.
Since there is so little ozone in the Martian atmosphere, it has no ozone layer, and the planetís surface is exposed to the full intensity of the Sunís ultraviolet radiation. The lethal rays would destroy any exposed microorganisms, so there might not be any live ones left on Mars. By way of comparison, the Earth has a thick ozone layer high in its atmosphere, which absorbs most of the dangerous ultraviolet sunlight and keeps it from reaching the ground.
There is now very little water vapor in the Martian atmosphere, about 0.03 percent near the surface. And that is about as much of the vapor that the atmosphere can hold. It is practically saturated with water vapor. When the temperature drops, water can condense and freeze out of the saturated air, forming low-lying mists or ground fogs in canyons and frosts on the surface.
The thin, cold Martian air
The surface pressure on Mars was first accurately determined when Mariner 4 passed behind the planet, and its radio signal penetrated the Martian atmosphere in order to reach Earth. From the manner in which the signal was altered, a surface pressure of about 0.005 bars was determined, compared to 1.000 bars at sea level on Earth. Warmed only by direct sunlight, without any pronounced greenhouse effect, the surface temperature on Mars averages 210 degrees kelvin, well below the freezing point of water at 273 degrees kelvin.
Under present conditions on Mars, liquid water is unstable and cannot stay on the surface of Mars. Because the temperature and pressure are so low, water on Mars is now stable only as ice or vapor. Over most of the surface, the temperature is usually below the freezing point of water, and when it warms above freezing the water turns almost directly into vapor. If liquid water was released onto the surface from the warmer interior, that water would survive for just a brief time before freezing into ice or evaporating explosively, turning into water vapor.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University