Mars was not always a cold desert

Mars has a cold climate, although in summer the temperature can reach 80°F (27°C) in parts of the Tropics. There is weather on Mars, despite its thin atmosphere. From time to time dust storms blanket vast areas—

Atmospheres of Earth, Venus, and Mars

Venus and Mars are our neighbors in the solar system. Both planets possess atmospheres, but these atmospheres are very different from the atmosphere of Earth and the three planets have very different climates.

Our own atmosphere is 78.08 percent nitrogen by volume, 20.95 percent oxygen, and 0.93 percent argon, with minute traces of a range of other gases, including carbon dioxide, which accounts for about 0.04 percent. There is also a variable amount of water vapor. It is the water vapor that produces most of the features of the Earth's climates. The average atmospheric pressure at the Earth's surface is 14.7 lb. in.-2 (100 megapascals, MPa, or 1 bar) and the average surface temperature is 59°F (I5°C).

Earth is the third planet, counting outward from the Sun. Mercury (with no atmosphere and therefore no climate) is the first and Venus the second. Venus is almost the same size as Earth, but slightly less massive (its mass is 81.5 percent of Earth's mass). Its atmosphere is much more massive than our own and the surface atmospheric pressure is about 1,470 lb. in.-2 (1,000 MPa, 100 bar). The atmos phere is 96.5 percent carbon dioxide and 3.5 percent nitrogen, with small traces of carbon monoxide, sulfur dioxide, water vapor, argon, and helium.

Venus is permanently shrouded in cloud, extending from about 28 miles (45 km) above the surface to a height of nearly 43 miles (70 km), with layers of haze both below and above the clouds. The clouds themselves are composed mainly of sul-furic acid. The dense atmosphere and clouds combine to produce a very strong greenhouse effect. The surface temperature is about 850°F (454°C).

Mars is the fourth planet and so farther from the Sun than Earth. It is also smaller—about half the size of Earth. Its atmosphere is very thin, producing a surface pressure of about 0.9 lb. in.-2 (600 pascals, Pa; 6 millibars, mb). Martian air is 95.3 percent carbon dioxide, 2.7 percent nitrogen, I.6 percent argon, 0.I3 percent oxygen, and there are traces of water vapor, neon, krypton, and xenon.

The average surface temperature is -67°F (-55°C). Temperatures at the surface range from -207° F (-I33°C) high in the polar mountains in winter to 80°F (27°C) in dark regions in the Tropics in summer.

sometimes they cover the entire planet. Then the sky is pink. More often, though, the sky is dark blue and cloudy. The clouds are made from crystals of water ice. The surface temperature rises during dust storms and falls when the dust settles and the sky is clear.

This is the present climate, however, and it was not always so cold and dry. Surface features closely resemble riverbeds and shorelines, and scientists suspect that is what they are. If so, there were times in the past when the climate of Mars was warmer than it is today. Rivers flowed, there were lakes and shallow seas, and perhaps rain fell from the sky.

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