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 surface water may have been present, which could have enabled life to exist. Towards th top of this picture, Ascraeus Mons, one of the planet's large extinct volcanoes, can be seen swathed in clouds of ice crystals. Clouds of water ice have been observed in the thin Martian atmosphere, and ice caps of frozen water and carbon dioxide (dry ice) exist on the surface at the poles.
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