Alpha Centauri's Universe: Monthly Publication

That air is mostly carbon dioxide, while the pale yellow clouds that shroud the planet are not water vapor but poisonous sulfuric acid. Finally, the "day" on Venus, the time from noon to noon, is 117 of our days. Venus, then, is not an attractive candidate. What about Mars? At first glance, we see major problems. Mars has an atmosphere, but it is as thin as Venus' is dense. The pressure at the surface is only 1 percent of an Earth atmosphere, and the Martian air is mostly carbon dioxide and nitrogen. The polar ice caps of Mars are mainly solid carbon dioxide rather than water ice, good evidence that Mars is cold (solid carbon dioxide turns to gas at minus 79*C). The planet is half as far again from the Sun as Earth and receives less than half as much solar radiation. The Martian lander, Pathfinder, and its mobile robot, Sojourner, reached Mars on July 4, 1997, and recorded a comfortable Sun-warmed temperature of 21*C (70*F) in a thin layer of dust and gravel right at the surface. Nighttime and polar temperatures dip to minus 100*C or colder, however.

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