IMPLIED AQUIFER DEPTH FROM IMPACT CRATERS IN THE MARTIAN NORTH POLAR REGION

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. This study examines the properties and distribution of these possible pingo features in an effort to constrain potential local aquifer depths. 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: pingo-like fill craters, enigmatic fill craters, and polar craters. We find that those craters with pingo-like or enigmatic 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 pingo-like central features. The moderate-sized complex craters have fewer pingo-like attributes while pingo-like deposits are not found in the smaller, simple craters. Pingo-like attributes are more common at high latitudes and at lower elevations. 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.