Martian crater depth/diameter relationships: comparison with the Moon and Mercury.

New Martian topographic data from Mariner 9 ultraviolet spectrometer (UVS) profiles provide depth data for 139 Martian craters of all degrees of degradation, between the diameters of 15 and 201 km. The population of Martian craters, including morphologically fresh examples, is shallower than both lunar and Mercurian fresh crater populations. Because the surface gravities of Mercury and Mars are identical within 5%, these differences in fresh crater depths suggest that factors other than gravity may play important roles in determining initial crater depths (e.g., differences in impact velocity, substrate variations, and Martian atmospheric effects during the crater-forming event). Degraded Martian craters are, on the average, no shallower than lunar pre-Imbrian craters of similar sizes. If the early bombardment of Mars was as significant a degradational agent as it was on the moon, then major levels of crater degradation and crater shallowing on Mars were associated with this mechanism. Continued eolian infilling, although locally significant, may be a less significant cause of morphometric degradation of large old Martian craters.