Ejecta Blocks on 243 Ida and on Other Asteroids

Seventeen positive relief features ∼45-150 m across are identified as probable blocks in Galileo high-resolution images of Ida. Their presence provides direct evidence for regolith retention on asteroids. The spatial distribution, maximum size, and integrated volume of the blocks are consistent with those of blocks associated with craters on the Earth, the Moon, Phobos, and Deimos. The concordance suggests that the features are impact ejecta fragments and that cratering mechanics on Ida, an object of average diameter ∼32 km, are similar to those applying on previously studied rocky bodies. The blocks that lie within or near the rims of craters Lascaux and Mammoth were likely mobilized in the low-velocity tail portion of the excavation flow that formed those craters. A few blocks located near smaller craters may have migrated some distance away from their source, possibly by impact-induced spallation, hopping, rolling, and/or sliding. Some blocks on Ida could be surviving fragments from the Koronis parent body, accreted after its breakup. The lifetime of 10²-m sized boulders against collisional disruption is estimated to be in the 10⁸-10⁹year range, consistent with ages considered for the largest and oldest craters on Ida. Extrapolation of successful ejecta scaling laws to other asteroids suggests that blocks ∼15 and 70 m across could be present on Dactyl and Gaspra, respectively (in both cases too small to be identified in available Galileo images). Blocks 100 m in size could be present on 433 Eros, and km-sized megablocks on 4 Vesta.