Mariner IV: Analysis of Preliminary Photographs.

Comparison of the distributions of Martian and lunar crater diameters indicates that the visible surface of Mars is 2.2 to 3 x 10(9) years old. This result implies that in the early history of Mars large-scale subaerial erosion occurred. Of 69 Martian craters with diameters greater than 10 kilometers, 13 percent have central peaks. This compares favorably with the frequency (11.7 percent) of central peaks among lunar craters and may indicate that the central peaks are a direct result of the impact mechanism rather than post-impact volcanism. A well-defined system of lineaments is shown in the Mariner photographs. The presence of these lineaments may indicate that Mars has lost appreciable angular momentum during its history.