DPS Pasadena Meeting 2000, 23-27 October 2000
Session 38. Europa
Oral, Chairs: W. McKinnon, W. Moore, Thursday, 2000/10/26, 8:30-10:00am, C106

## [38.06] Secondary Cratering on Europa

E.B. Bierhaus (Univ. of Colorado, SwRI), C.R. Chapman, W.J. Merline (SwRI)

An analysis of Galileo imaging data shows that it is possible for secondary craters to dominate Europa's small crater population. Europa's crater density is much lower than that of the Moon at all sizes, with only 15 known craters larger than 10 km, and a small crater population that is well below saturation. Yet, in several localities, segments of the small-crater size-distributions exhibit a steeply-sloping power-law relationship, reminiscent of secondary craters on the Moon and other bodies. Given this size-distribution and the number of crater clusters, we became intrigued by the possibility that the majority of the small craters may be secondaries. At issue is whether or not the mass ejected from the small number of large primary craters is sufficient to generate enough distant secondaries to account for the bulk of the measured small-crater population.

We calculate, from scaling laws, the amount of mass ejected from all the known primary craters on Europa larger than 10~km diameter. This establishes an upper limit to the amount of mass available to produce secondary craters. Then, based on crater densities from six high-resolution image sequences, we estimate a typical spatial density of small craters on Europa. Using relevant scaling relations, we calculate the amount of mass necessary to produce these small craters assuming all of the small craters are secondaries. Although the derived mass has a large range of values due to the uncertainties of the scaling relations, our preliminary results show the lower end of the range falls well within the limits established by the mass ejected from the primary craters. This does not {\em prove} that most or all small craters on Europa are secondaries; however, this work demonstrates that it is {\em possible} that secondaries dominate Europa's small crater population.