GEOLOGY OF 243-IDA

The surface of 243 Ida is dominated by the effects of impacts. No comp lex crater morphologies are observed. A complete range of crater degra dation states is present, which also reveals optical maturation of the surface (darkening and reddening of materials with increasing exposur e age), Regions of bright material associated with the freshest crater s might be ballistically emplaced deposits or the result of seismic di sturbance of loosely-bound surface materials. Diameter/depth ratios fo r fresh craters on Ida are similar to 1:6.5, similar to Gaspra results , but greater than the 1:5 ratios common on other rocky bodies. Contri buting causes include rim degradation by whole-body ''ringing,'' relat ively thin ejecta blankets around crater rims, or an extended strength gradient in near-surface materials due to low gravitational self-pack ing, Grooves probably represent expressions in surface debris of react ivated fractures in the deeper interior, Isolated positive relief feat ures as large as 150 m are probably ejecta blocks related to large imp acts. Evidence for the presence of debris on the surface includes reso lved ejecta blocks, mass-wasting scars, contrasts in color and albedo of fresh crater materials, and albedo streaks oriented down local slop es. Color data indicate relatively uniform calcium abundance in pyroxe nes and constant pyroxene/olivine ratio. A large, relatively blue unit across the northern polar area is probably related to regolith proces ses involving ejecta from Azzurra rather than representing internal co mpositional heterogeneity. A small number of bluer, brighter craters a re randomly distributed across the surface, unlike on Gaspra where the se features are concentrated along ridges. This implies that debris on Ida is less mobile and/or consistently thicker than on Gaspra. Estima tes of the average depth of mobile materials derived from chute depths (20-60 m), grooves (greater than or equal to 30 m), and shallowing of the largest degraded craters (20-50 m minimum, similar to 100 m maxim um) suggest a thickness of potentially mobile materials of similar to 50 m, and a typical thickness for the debris layer of 50-100 m. (C) 19 96 Academic Press, Inc.