Pwyll secondaries and other small craters on Europa

We examine a population of Pwyll secondary craters, as well as several smal l 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 Chao s region. The Pwyll impact deposited a high-density secondary crater popula tion in this area. The two terrains in Conamara Chaos show very different c rater densities with one terrain containing anywhere from a few to 10 times higher density than the other. Surface activity within the one terrain, wh ich degraded and erased part of that terrain's crater population relative t o the other terrain, best explains the density difference. Because Pwyll is a young impact crater, 18 million years old or younger, subsequent endogen ic surface activity is consistent with Europa remaining active today. The s teepness of the average differential power-law slope (-4.2) of the small-cr ater size distribution suggests that many of the small craters are secondar ies. 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) 20 01 Academic Press.