ASYMMETRIC LUNAR CRATERS - IMPACTS ON SLOPING SURFACES

Impacts are typically thought of as impacting on a level surface and that any asymmetry of the crater itself or the distribution of ejecta results from the projectile's trajectory angle. It is well known that shallow impact angles will produce asymmetric ejecta, often in a butterfly pattern with little or no ejecta uprange; at very shallow angles the crater becomes elongate. A significant number of craters (up to km scale) have asymmetric interior morphology and ejecta distribution are observed on sloping surfaces on the Moon and asteroids. These observations indicating the importance of surface slope on the excavation and deposition processes of impact cratering. When the dimensions of a crater are smaller than the scale of the topography, that topography can significantly influence the crater's morphology. If the crater is much larger than the scale of the topography it has little obvious influence. The asymmetric craters observed on the Moon have a down-slope diameter larger than the cross-slope diameter; a flat floor is often missing or, if it present, it is elongate perpendicular to the slope. In some cases the up slope crater wall simply intects the down slope crater wall forming a trough perpendicular to the slope. The up-slope inner crater wall is smooth and typically lacks boulders; it may have linear or patchy albedo markings extending down the wall. In contrast, the down-slope inner crater wall is hummocky and covered with boulders. Debris or melt material may extend over the down-slope rim onto the down range surface. This observed asymmetry of the crater morphology and ejecta is clearly related to the topography. There is significant downslope movement of material with the upslope wall collapsing and pushing material onto the downslope wall. The extent to which asymmetry develops is a function of the crater size, the slope angle and the strength of ther material.