Gaspra's Cratering Record.

Abstract

The images of the asteroid 951 Gaspra obtained by Galileo show impact craters down to the resolution limit of 50m/pixel. The observed crater population has characteristics which very much resemble those seen on the inner solar system planets. The remarkable finding is that the distribution is steep. The cumulative size-frequency distribution can be approximated by a power law with an exponent (population index) of between -3.3 and -3.7 in the size range 200m to 800m. This is very close to the distribution characteristics found on the Earth's moon, on Mars, and on Mercury with population indices (cumulative) around -3.5 in that size range. At larger sizes (D&#611km) there appears to be a slight flattening similar to what is found for distributions on the inner solar system objects. The lunarcurve(standard distribution, cf. Neukum et al., 1975) can be approximates very well to the Gaspra distribution taking into account the differences in the impact velocity and surface gravity conditions. A steep curve was expected for the asteroid belt cratering record (cf. Neukum 1983) if the cratering record in the inner solar system is directly linked to this region in terms of a source region. The findings on Gaspra support the view long held by the first author that (1) the small craters(D<1km) on the inner solar system bodies are essentially primary impact craters and not caused by "background secondaries", and (2) the inner solar system cratering is due to the bombardement by one and the same family of bodies derived from the asteroid belt. The time dependence of the cratering ratein the asteroid belt and in the inner solar system may have been quite different. If Gaspra is between 200 and 500 m.y. old as estimated in combining the crater frequency measurement with impact probability data derives from astronomical observations of large-size asteroids, then it appears impossible that the time.