UNITED STATES - MAY 13: This picture, taken by one of the Viking Orbiters, reveals many small impact craters on the Martian surface. Two Viking spacecraft were launched towards Mars by NASA in 1975, each carrying a lander spacecraft and an orbiter. Both successfully 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. In common with all the other planetary bodies in the Solar System, Mars has been subjected to a continuous bombardment from space by meteorites and asteroids of various sizes. (Photo by SSPL/Getty Images)
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