Like Jupiter (1973) and Alpha Centauri (1976) this presents information about Mars, and how Mars was used as a reference in plotting the solar system, in a solid astronomical context. Explaining such phenomena as planetary orbits, mass, and rotation (and, just as important, how they were established), Asimov goes on to the mapping of Mars, the controversies about its mysterious canals, the discovery and study of its two satellites, the findings of Mariner probes pertaining to Martian atmosphere and surface, and the later Viking experiments revealing soil "which has either some very interesting chemistry going on in it or some very interesting biology." The new information alone, complete with closeup photos, ensures that this will supersede existing titles, and Asimov enlivens the discussion by considering yet unanswered questions: why are the Martian volcanoes so large? is Mars now in a permanent Ice Age? Superior.