Cratering on Ida

Craters on Ida have a size distribution consistent with equilibrium under conditions of crater saturation over the range from 100 m to 1 km in diameter. The density of craters is high and they follow a -3 differential slope. The craters show a full spectrum of degradational morphologies at both large and small sizes. Craters >2 km in diameter (and especially those >5 km) are more prevalent on one part of Ida; a sub-equilibrium, production population is seen on the other part, which rises steeply and reaches equilibrium for diameters <1 km, consistent with the production population found on Gaspra. We calculate that the visible craters should have contributed appreciable regolith, ∼150 m average depth. If Gaspra and Ida have similar strengths and are subject to impact by the same projectile population, then Ida is ∼10 times older than Gaspra, roughly 2 byr old. Ida could be younger, however, if it underwent early heavy bombardment in the aftermath of the break-up of the Koronis precursor body.