Mercurian impact craters: Implications for polar ground ice

Recent radar observations of Mercury have detected strong depolarized echoes from the north and south polar regions which have been interpreted by some as ice deposits in the floors of permanently shadowed impact craters. We have used the experience from Mars, where subsurface ice lowers the depth-to-diameter ratio (d/D) of impact craters, to test for subsurface ice deposits on Mercury. This analysis determines the d/D ratios for 170 impact craters in the Borealis (north polar), Tolstoj (equatorial), Kuiper (equatorial), and Each (south polar) quadrangles of the planet. Possible effects from sun angle and terrain were eliminated. To test whether d/D differences could be detected at Mariner 10 resolutions (similar to 1 km/pixel), we perform a similar analysis using Mariner 9 images of Mars which have similar resolutions. We demonstrate that d/D differences due to terrain softening can be detected between craters in the martian polar regions and the equatorial regions at the Mariner 9 resolutions. Although our initial results indicate that the south polar Each Quadrangle has a statistically lower d/D than the north polar (Borealis) or two equatorial (Tolstoj and Kuiper) quadrangles, further investigation reveals that this finding is most likely the result of the filtering which was applied to the images of the Each quadrangle by JPL. Thus, no unequivocal evidence exists that the possible ice deposits in craters at Mercury's north and south poles are the exposed portions of more extensive subsurface ice caps. Combined with the temporal constraint imposed by the fact that the proposed ice deposits are found only in USGS Class 4 craters, this suggests a large, rapidly emplaced exogenic source of water to Mercury during the Mansurian period. We suggest that the source was multiple impacts from a fragmented comet or a comet shower. (C) 1999 Academic Press.