Impactor Populations on the Galilean Satellites

C.R. Chapman, W.J. Merline, B. Bierhaus, J. Keller, S. Brooks (Southwest Res. Inst. (Boulder))

Galilean satellite images from Galileo, considered in the context of Shoemaker-Levy 9 impacts and other evidence, raise crucial questions about the nature of impactor populations in the Jovian system. Several observations suggest that the current impactor population (generally believed to be extinct Jupiter-family comets) is relatively deficient in comets smaller than S-L 9, compared with usually adopted power-laws. These include: (a) lack of saturated surfaces at diameters <100 m; (b) predominance of secondary craters in some regions far from sources; and (c) apparent dearth of small crater chains (catenae). None of these indications is yet conclusive (for example, catenae would be predicted to mimic the cometary size distribution only for certain idealized models of S-L 9-like comet break-up). It is also plausible (certain, in the case of Europa) that there are currently active erosional and resurfacing processes on Europa, Ganymede, and Callisto that contribute to erasing small craters. The often adopted assumption that the heavily cratered terrains on Ganymede and Callisto reflect the same Late Heavy Bombardment recorded on the Moon, while conceivably true, is only one of a number of plausible possibilities. We discuss the qualitative roles of different impactor populations.

We thank the Galileo Imaging Team and associates for discussions, and the Galileo Project, NASA, and NSF for support.