Asteroid Geology from Galileo and NEAR Shoemaker Data

The four asteroids visited by spacecraft, Gaspra, Ida, Mathilde, and Eros, have shapes and surface morphologies dominated by the effects of impact cratering. The presence of impactderived regolith has been inferred on S-class asteroids Gaspra, Ida, and Eros from photometry, morphological features, and subtle color variations related to optical maturation of surface materials. Grooves and ridges suggest the presence of throughgoing structures, implying that interiors are partially competent (e.g., fractured but intact) rock materials, not collections of gravitationally bound fragments reassembled after catastrophic disruption. Mathilde’s shape is dominated by impact craters with diameters comparable to the asteroid’s radius that formed without disrupting the asteroid, suggesting a less-competent, highly porous interior. The surfaces of all these objects exhibit evidence of downslope motion of loose, fragmental material. There is no unambiguous spectral evidence for kilometer-scale compositional heterogeneity within any of these bodies. S(IV) class asteroids Ida and Eros have some compositional similarities to ordinary chondrites.