tially unchanged throughout their histories. Now it was clear that bodies the size of the moon, and even smaller ones, whether formed in the molten state or melted after formation, undergo a substantial evolution. This conclusion was borne out further by data from the other planets. Mercury appeared even more cratered than the moon. There was widespread evidence of lava flows on the planet. Large cracks and long scarps were visible in the Mariner 10 pictures. There was no doubt that Mercury underwent a great deal of evolution after its formation. 42

The evidence of activity on Mars was even more striking. After a period of discouragement for scientists when the Mariner 4 pictures appeared to show a dead, moonlike planet, the pictures of Mariner 6 and 7 revived interest, and those of Mariner 9 aroused excitement. Those pictures showed huge volcanoes, one of them-Olympus Mons-twice the diameter of the largest known volcanic structure on Earth, namely, the big island of Hawaii. A great deal of the Martian surface was cratered, indicating an ordeal of bombardment like that experienced by the moon and Mercury. Some of the surface was smooth and featureless, indicating a process of filling in as with blowing, drifting sands. A huge rift, comparable in length to the width of the United States, indicated considerable tectonic activity (fig. 56). Numerous channels hundreds of kilometers long looked as though they might have been produced by flowing water (fig. 57). Consistent with this observation were the frequent formations that looked like alluvial fans produced by the deltas of terrestrial rivers or sedimentary deposits of meandering streams (fig. 58). While the variable frost cover observed in the north and south polar caps was shown to be solid carbon dioxide, a substantial base of frozen water was also found. Various estimates suggested that a considerable amount of water must have outgassed from the planet over time, and if past conditions on the planet were just right there could have been ponds and rivers. But the question of the role of water in the evolution of the planet remained unsolved. 43

It was very clear that Mars is an active planet, by no means dead, as some had prematurely concluded. Some investigators thought they could detect in the marked differences between the cratered highlands of the planet and the volcanic provinces the suggestion of an incipient separation into individual tectonic plates as on Earth, a picture not generally accepted by students of Mars. Paul Lowman, however, was led to conclude that the evidence was piling up that earthlike planetary bodies would follow similar courses of evolution.44 In the larger bodies like Earth, the rates of evolution would be faster and the duration longer than for the smaller planets. Of the inner planets, Earth, still vigorously active, was most advanced in its evolutionary course (fig. 63). Venus might be in a comparable stage, but no life evolved there to convert the carbon dioxide atmosphere to one with large amounts of oxygen. Mars was following the course

Figures 50-51. Photomosaics of Mercury. Fig. 50, left: Eighteen pictures, taken at 42-second intervals, were enhanced by computer at the Jet Propulsion Laboratory and combined into this mosaic. The pictures were taken from Mariner 10 during 13 minutes when the spacecraft was 200 000 kilometers and 6 hours away from Mercury on its approach to the planet, 29 March 1974. About two-thirds of the portion of Mercury seen in this mosaic is in the southern hemisphere. Largest of the craters are about 200 kilometers in diameter. Illumination is from the left.

Fig. 51, right: The semicircle of cratered mountains in the left half of the mosaic forms the boundary for the largest basin on Mercury seen by Mariner 10. The basin is near a subsolar point when the planet is at perihelion, leading investigators to suggest the name Caloris for it. The ring of mountains is 1300 kilometers in diameter and up to 2 kilometers high. The basin floor consists of severely fractured and ridged plains.