Phobos: Regolith and ejecta blocks investigated with Mars Orbiter Camera images

Four flybys of Phobos by the Mars Global Surveyor spacecraft in 1998 allowed imaging at resolutions (similar to 2-7 m) better than obtained for any other satellite or asteroid except Earth's Moon. The images show the interior and vicinity of the large crater Stickney. There are similar to 2000 identifiable ejecta blocks, the largest about 85 m across. The great majority of these blocks come from Stickney as part of low-velocity ejecta spread eastward by the influence of Phobos's rapid rotation. They appear to have experienced only a few meters, at most, of burial or modification since emplacement. The images also show materials of different albedos within Stickney that mark downslope motion of regolith. The number of craters on the steep slopes of Stickney that are subject to the downslope motion suggests that <10 m depth of material has moved since Stickney formed. Depths of regolith east of Stickney inferred from crater morphologies and from lengths of groove slopes are greater than can be attributed to ejecta from Stickney or other visible craters. This finding suggests many large craters (>4 km) have been degraded to the point that they are no longer recognizable.