Bombardment History of the Jovian System

Abstract

The first five Galileo flybys have provided image data up to two orders of magnitude better in resolution than the best Voyager data. Measurements of crater distributions can now reliably be extended down to sizes of about 100 m crater diameter. Crater distributions show steep slopes (cumulative index about −3) at smaller diameters on each satellite and are shallower at larger diameters, as seen on the Moon and the asteroids Gaspra and Ida. Crater densities differ about a factor of 10 (at D = 1km) between average dark terrain of Galileo Regio and youngest bright resurfaced areas on Ganymede. Crater densities on the most heavily cratered regions on both Ganymede and Callisto are fairly comparable. Crater densities on Europa have turned out to be higher than anticipated but are factors of about 10 lower than the youngest bright terrain in the Uruk Sulcus region of Ganymede. The similarity to crater distributions in the inner solar system suggests that the impactor population during early heavy bombardment preferentially was derived from the asteroid belt and that the impact rate on the Galilean satellites may have had a lunar-like decay with time. Making use of the idea of the “marker horizon” of 3.8 b.y. for the end of the heavy bombardment, i.e. formation of the youngest basin, also in the Jovian system, the frequency superimposed on the Gilgamesh basin can be related to the formation age of 3.8 b.y. and thus an impact rate derived for this point in time. In this way, assuming a lunar-like cratering chronology scenario, absolute ages can be determined for the Galilean moons from crater statistics. Thus, the most densely cratered dark terrains on both Ganymede and Callisto have a likely age of about 4.1 – 4.3 b.y. Some areas counted on Europa high-resolution images may be as old as 3 – 3.3 b.y. Other scenarios based on values proposed for the present-day comet impact rate in the Jovian system with non-lunar-like flux decays are conceivable and would result in generally younger ages, possibly as young as 10 m.y. These young ages and impact rates would result in ages for Ganymede and Callisto which are in serious conflict with the geologic evolution of these two moons as we understand it now.