Cratering on Ida

Abstract

Craters on Ida have a size distribution consistent with equilibrium under conditions of crater saturation over the range 100 m to 1 km diameter. The density of craters is high and they follow a -3 differential slope. Craters show a full spectrum of degradational morphologies at both large and small sizes. Craters >5 km diameter are more prevalent on one part of Ida; a sub-equilibrium, production population should have produced appreciable regolith, at least many tens of meters deep, consistent with photogeologic interpretations. If Gaspra and Ida have similar strentghs and are subject to impact by the same projectile population, then Ida is about ten times older than Gaspra, roughly 2 b.y. old. However, Ida's presence in the Koronis family and the existence of Idas's satellite, Dactyl, suggest that an unknown fraction of Ida's craters could have formed early in its history, shortly after the break-up of the Koronis precursor body. In that case, Ida could be considerably younger and only a fraction of its crater would have been produced by general asteroid belt projectiles.