S Xxvi Geology of Ida

Several lines of evidence from analysis of impact craters and other morphological features on Ida indicate the asteroid surface is covered with a thin layer of regolith that probably grades downward into less well-sorted rubble/megaregolith. This transition with depth is too gradational or occurs too close to the surface to be expressed in crater morphology at available resolution. Gravitational packing of surface debris under Ida surface gravity of 1 cm/sec2 should be less efficient compared with lunar regolith, suggesting that the progressive change in mechanical properties related to gravitational packing in the upper meter of lunar regolith may occur more gradually and over much greater depths on Ida. Regolith at the surface, lightly held gravitationally, should be especially susceptible to jostling and movement from seismic shaking due to impacts. Despite susceptibility of surface material to periodic disturbance and motion, the optical surface layer is stable enough to record space-weathering trends of material darkening and reddening with increasing exposure age.