The Martian atmosphere

The Martian atmosphere is very different from Earth's, but there are some similarities. Mars has a very thin atmosphere composed of 95.3 per cent carbon dioxide, 2.7 per cent nitrogen, 1.6 per cent argon, and less than 0.2 per cent oxygen. The atmosphere is near its saturation point with water vapour (0.03%). In the past the Martian atmosphere may have been denser, but it is now one-hundredth the density of Earth's atmosphere. This low value is partly a consequence of the low gravitation field. Data returned by the Phobos space probe suggested that the solar wind is carrying away the weakly held atmosphere at a rate of 45 000 tonnes per year.

The average air pressure on Mars is about seven-thousandths of Earth's, but it varies with altitude from almost nine-thousandths in the deepest basins to about one-thousandth at the top of Olympus Mons.

The minute traces of water vapour can at times form clouds, particularly in equatorial regions around midday. Early morning fogs also appear in canyons and basins. Temperatures around the poles are often low enough for carbon dioxide to form a thin layer of cloud.

The atmosphere is thick enough to support strong winds and dust storms that sometimes cover large areas of the planet. At times, the dust storms can hide surface features from Earth view. Such storms occur most often during the southern hemisphere's spring and summer. In 1971, Mariner 9's view of Mars was obscured by a dust storm that lasted for two weeks. In 1977, thirty-five dust storms were observed, and two of these developed into global storms. Global storms spread rapidly, eventually enshrouding the whole planet in a haze that can last a few months.

Mars's thin atmosphere produces a greenhouse effect, but it is only enough to raise the surface temperature by 5°, which is much less than increases on Earth and Venus.

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