The theoretical plausibility of central pit crater formation via melt drainage

Central pit craters are seen in large craters on some icy satellites and on Mars. We investigate the hypothesis that central pits form when impact melt drains into fractures beneath the impact crater. For this process to occur, the volume of melt generated during the impact, the volume of void space in fractures beneath the impact crater, and the volume of melt able to drain before the fractures freeze shut all must exceed the volume of the observed central pits. We estimate the volume of melt generated using results from previous numerical modeling studies. The fracture volume is estimated using gravity anomalies over terrestrial craters. To estimate the amount of melt able to drain before freezing, we consider flow through plane parallel fractures. These calculations all suggest that enough liquid water could drain into fractured ice beneath a crater on Ganymede to form a central pit. On Earth and the Moon, silicate impact melt will freeze before a large volume is able to drain, so we do not expect to see central pits in impact craters in targets with no ice. In summary, we find our calculations are consistent with observed central pits in craters on Ganymede and the lack of central pits in craters on Earth and the Moon.