Pwyll secondaries and other small craters on Europa

We examine a population of Pwyll secondary craters, as well as several small crater populations seen in six high-resolution sequences of Europa, taken by the Galileo spacecraft. We conclude that post-Pwyll (the youngest large impact on Europa) endogenic surface activity occurred in the Conamara Chaos region. The Pwyll impact deposited a high-density secondary crater population in this area. The two terrains in Conamara Chaos show very different crater densities with one terrain containing anywhere from a few to 10 times higher density than the other. Surface activity within the one terrain, which degraded and erased part of that terrain's crater population relative to the other terrain, best explains the density difference. Because Pwyll is a young impact crater, 18 million years old or younger, subsequent endogenic surface activity is consistent with Europa remaining active today. The steepness of the average differential power-law slope (-4.2) of the small-crater size distribution suggests that many of the small craters are secondaries. We demonstrate that the amount of mass ejected by Pwyll and the other large craters on Europa is potentially enough to create the majority of the small crater population via the secondary cratering process, allowing the possibility that most small craters on Europa are secondary craters. (C) 2001 Academic Press.