Martian North Polar Crater Morphology: Implications for an Aquifer

Abstract

Recent studies of martian north polar craters have suggested that they have more cavity fill than the global crater population, and that the morphology of the crater fill is in some cases-similar to pingo structures on Earth. The presence of hydraulic pingos in polar impact craters implies a polar region aquifer or, alternately, the melting of a permafrost layer upon impact. This study examines the properties and distribution of impact craters in the north polar region of mars, which contain anomalous fill patterns similar to terrestrial pingos. 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 characterize the impact craters and their cavity fill materials. We classified the polar craters into three groups: Types I, II, and III. We find that those craters (Type II and III) with anomalous central fill are shallower than other near-polar craters of similar diameter. Within 18� of the Martian North Pole, nearly all of the largest complex craters have large, central fill features. The moderate-sized complex craters have fewer anomalous fill attributes while similar fill are not found in the smaller, simple craters. Anomalous, central features are more common at high latitudes. When we model the possible polar aquifer as having a linearly increasing depth with increasing distance from the pole, we find that polar crater classification correlates strongly with the depth to aquifer. This is consistent with larger impact crater excavation reaching an aquifer and resulting in hydraulic pingo formation. We suggest that smaller crater excavation depths are not deep enough to reach the modeled aquifer and thus have no pingo-like features. However, a difference in morphology between the Type II and Type III craters suggests another process in the north polar region. Some possible explanations towards the modification of Types II or III are differential solar radiation or wind transport. However, it is possible that Type II and Type III craters are formed from two entirely different processes and not by pingo formation. We are left, then to decide whether Type II and Type III craters are formed by the same mechanism (aquifer puncture or permafrost melting) and are subsequently modified differentially or if they are formed by two independent mechanisms.