Gaspra's cratering history.

Abstract

The images of the asteroid 951 Gaspra obtained by Galileo show impact craters down to the resolution limit. The observed crater populationresembles very much the size distributions seen on the inner solar system planets. The crater frequencies in the 1-km range on Gaspra can be used to infer the age of Gaspra by relating the present-day large-body (d>_50 km) asteroid impact probability to the large craters in a lunar-like production size-frequency distribution.Cratering rates for 1-km craters on Gaspracan be reliably calculated.The only assumption is that a lunar-like crater size-frequency distribution also holds for the asteroid belt. The resulting age, accounting for all uncertainties in scaling for the impact conditionson Gaspra, ranges between 120 and 800 million years, most likely between 250 and 500 m.Y., when the parent body of the Flora family, a member of which Gaspra is, was shattered. From the new measurementsit can be deduced that very likely the cratering history of the innersolar system planets was dominated by objects removed from the asteroid belt probably by the action of Jupiter. Thus, the high impact rate (Post-accretional bombardement) at the beginning and the dteep subsequent decay in cratering rate was produced. The time dependence of the cratering rate in the asteroid belt, though, cannothave been at the same high level at the beginning as in the solar system.