Dust storms such as this are common on Mars, sometimes covering up to half the planet's surface. Two Viking spacecraft were launched towards Mars by NASA in 1975, each carrying a lander spacecraft and an orbiter. Both succesfully landed their probes on Mars to study the Martian environment, soil constituents and to search for simple life forms. No evidence of life was found, but more recent studies of the Martian landscape suggest that in the past abundant surface water may have been present, which could have enabled life to exist. Today Mars has a very thin atmosphere, mainly of carbon dioxide. No liquid surface water appears to exist, but there are permanent polar ice caps, made up of frozen water and carbon dioxide (dry ice).
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