Ejecta Emplacement on the Martian Satellites

Abstract

Ejecta features on the martian satellites are compared to models of ballistic emplacement and downslope motion. The asymmetric distribution of ejecta around the large crater Stickney on Phobos appears most easily explained if the crater were formed when Phobos was only slightly farther from Mars than at present (due to tidal evolution), where rotational and tidal effects allow spreading of low velocity material several kilometers to the east of the crater, but which do not allow a large fraction of the crater's ejecta to be launched into temporary Mars orbit. An additional condition needed to explain the distribution of Stickney's ejecta is that the crater excavation occurred in a gravity-dominated regime. The distribution of Stickney's ejecta is not closely related to the patterns of grooves nor to potential long-distance downslope motion. On Deimos, ejecta from a large impact near the south pole also appear to have been produced by cratering largely in the gravity regime. The morphology of older craters, evidence of ejecta ponding, and crater density data indicate that the ejecta from this large impact have been subject to significant redistribution downslope. This crater is much larger relative to Deimos than Stickney is to Phobos; the resulting greater ejecta cover and possibly much greater seismic shaking on Deimos may help explain the strikingly different appearances of the martian moons. © 1998 Academic Press.