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. The Orbiters often observed ice clouds early in the morning around and within the Valles Marineris, a system of canyons some 4000 kilometres long and over 6 kilometres deep in places. Water ice is found at the Martian polar regions, together with frozen carbon dioxide (dry ice).
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