MARTIAN NORTH POLAR IMPACT CRATER CAVITY WALL SLOPES AND INTERIOR DEPOSIT PLACEMENT: IMPLICATIONS FOR POLAR PROCESSES

Impact crater morphologies have been used as indicators of variations in target properties and local geologic processes. Recently, a study of a limited subset of martian north polar impact craters suggested that they tend to have more cavity fill than the martian global crater population; other authors suggest that there may be a latitude dependent, north-south wall slope asymmetry. Either a greater cavity fill or a N-S slope asymmetry could have widespread implications for the interpretation of martian polar processes. Therefore, in this study, we evaluate cavity wall slopes and interior deposits for several hundred north polar craters in order to better describe potential variations from the global population. We use high-resolution (256 pixels/degree or about 230 meters/pixel) topographic data from the Mars Orbiter Laser Altimeter (MOLA) instrument on the Mars Global Surveyor Mission to examine the slopes of crater cavity walls and the placement of interior deposits as a function of crater location and diameter. We find that, for several hundred north polar craters, approximately 30% of the craters have significant interior deposits, and the presence or absence of these interior deposits appears independent of crater diameter. We cannot eliminate the possibility of a latitude dependence for cavity interior deposits, but if one exists, it is not a strong dependence. For north polar craters with interior deposits, approximately 60% were notably offset to the East, West, or West-Southwest. We do not see a consistent asymmetry between the northern (equator-facing) and southern (pole-facing) cavity wall slopes. However, we do find a modest dependence of cavity wall slope on latitude coupled with slope orientation. While cavity wall slopes overall tend to be shallower closer to the pole, this tendency is perhaps 4 times larger for north AND south cavity wall slopes than for the east and west slopes for the same crater. Since neither the cavity slope nor the interior deposit asymmetric variations are probable target effects, we suggest that they are likely the result of processes more active in polar regions. Any processes suggested as a cause must account for (1) the interior deposit apparent preference for East or West crater walls and (2) the stronger N and S (and SE, SW) versus E or W crater wall slope latitude dependency.