CRATERING ON IDA

Craters on Ida have a size distribution consistent with equilibrium un der 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 diffe rential slope. The craters show a full spectrum of degradational morph ologies 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-e quilibrium, production population is seen on the other part, which ris es steeply and reaches equilibrium for diameters <1 km, consistent wit h the production population found on Gaspra. We calculate that the vis ible craters should have contributed appreciable regolith, similar to 150 m average depth. If Gaspra and Ida have similar strengths and are subject to impact by the same projectile population, then Ida is simil ar to 10 times older than Gaspra, roughly 2 byr old, Ida could be youn ger, however, if it underwent early heavy bombardment in the aftermath of the breakup of the Koronis precursor body. (C) 1996 Academic Press , Inc.