THE GEOLOGY OF GASPRA

The surface of Gaspra can be divided into several facets separated by ridges. Superimposed on the facets and ridges are two populations of craters. Type 1 craters are crisply defined and constitute a production function of impact origin. Type 2 craters are mostly irregular shallow depressions and probably of various origins. Some of the type 2 craters may have formed by impact and be remnants of a crater population that predates the type 1 population. The number of type 1 craters suggests that they started to accumulate 2 x 10(7) to 3 x 10(8) years ago. The freshest craters have a depth/diameter ratio of 1: 7, as compared with 1: 5 typically observed on other bodies. The craters appear to have become shallower with age at a rate of 10(-6) to 10(-7) m/year. The shallow depth of newly formed craters and the progressive degradation with age are ascribed to downslope movement of poorly coherent surface materials, the movement being aided by seismic shaking as a result of impact. The surface shows subtle color differences. The most prominent differences are observed around craters on ridges, where the surface has a stronger 1-mum absorption than elsewhere. The contrast is not observed around craters on the facets. The color differences are attributed to alteration of the materials brought to the surface. The unaltered material is estimated to be located at least 50 m below the surface on the facets. Only on the ridges is unaltered material at a shallow enough depth to be excavated by the observed craters. The rate of degradation of craters, the rounded form of the ridges, the depth of altered material on the facets, and the presence of old degraded craters all suggest that Gaspra is covered with a regolith a few tens to several tens of meters thick. (C) 1994 Academic Press, Inc.