A. M. Vickery

Size-velocity distribution of large ejecta fragments

Abstract The size-range distributions of secondary craters around three large primary craters on the Moon and Mars were measured. Ejection velocities of the fragments that produced the secondaries were estimated from the ranges of the secondaries from the primary. These velocities were then used in the Schmidt-Holsapple scaling relations to estimate the size of the ejecta fragments. Since it is unclear whether the secondary cratering events lie in the strength-dominated regime or in the gravity-dominated regime (or in the transition between the two), the data were reduced separately for both regimes in order to bracket the “true” answer. Furthermore, Schmidt and Holsapple have defined two scaling relations for the gravity regime, one appropriate for porous rock or regolith, and the other for nonporous target rocks. Since the nature of the target rock is unknown, both gravity scaling relations were used to reduce the data. The inferred size-velocity distributions of ejecta are compared to the predictions of the Melosh spallation model for 10 and 20 km/sec impacts of andesite onto andesite. The maximum inferred ejecta sizes are much greater than the maximum predicted spall sizes. This is consistent with the corresponding secondary craters having been produced by clustered impactors, with the inferred impactor diameter approximating the effective diameter of the cluster (Schultz and Gault, 1985) . The inferred velocities show cutoffs at ⪅1 km sec . Whether this cut-off is real or is due to the inability to recognize secondary craters at greater ranges is unknown.

The Large Crater Origin of SNC Meteorites

A large body of evidence strongly suggests that the shergottite, nakhlite, and Chassigny (SNC) meteorites are from Mars. Various mechanisms for the ejection of large rocks at martian escape velocity (5 kilometers per second) have been investigated, but none has proved wholly satisfactory. This article examines a number of possible ejection and cosmic-ray exposure histories to determine which is most plausible. For each possible history, the Melosh spallation model is used to estimate the size of the crater required to produce ejecta fragments of the required size with velocities ≥5 kilometers per second and to produce a total mass of solid ejecta consistent with the observed mass flux of SNC meteorites. Estimates of crater production rates on Mars are then used to evaluate the probability that sufficiently large craters have formed during the available time. The results indicate that the SNC meteorites were probably ejected from a very large crater (> 100 kilometers in diameter) about 200 million years ago, and that cosmic-ray exposure of the recovered meteorites was initiated after collisional fragmentation of the original ejecta in space at much later times (0.5 to 10 million years ago).