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

Four flybys of Phobos by the Mars Global Surveyor spacecraft in 1998 allowe d imaging at resolutions (similar to 2-7 m) better than obtained for any ot her satellite or asteroid except Earth's Moon. The images show the interior and vicinity of the large crater Stickney. There are similar to 2000 ident ifiable ejecta blocks, the largest about 85 m across. The great majority of these blocks come from Stickney as part of low-velocity ejecta spread east ward by the influence of Phobos's rapid rotation. They appear to have exper ienced only a few meters, at most, of burial or modification since emplacem ent. The images also show materials of different albedos within Stickney th at mark downslope motion of regolith. The number of craters on the steep sl opes of Stickney that are subject to the downslope motion suggests that <10 m depth of material has moved since Stickney formed. Depths of regolith ea st 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 bee n degraded to the point that they are no longer recognizable.