New Views of Callisto's Cratering Record from High-Resolution Galileo Imaging
W. J. Merline, C. R. Chapman, B. Bierhaus, J. Keller, S. Brooks (Southwest Research Institute), R. J. Sullivan (Arizona State University)
New images of Callisto's surface, at high resolution, by the Galileo SSI instrument, have revealed a startling lack of craters at small and intermediate sizes (less than 10 km). Based on the cratering record observed at larger sizes by Voyager (Woronow, Strom, and Gurnis 1982, ``Interpreting the Cratering Record: Mercury to Ganymede and Callisto", in ``Satellites of Jupiter", ed. D. Morrison, University of Arizona Press, Tucson), Callisto's surface appeared old and nearly saturated at intermediate sizes. Therefore, we might have expected the surface to be saturated at the smallest sizes, revealing an old, long-dead world.
On the contrary, the crater population becomes successively more depleted (relative to saturation) at increasingly smaller sizes. Instead, we see a pervasive blanketing of the surface by what appears to be a fluffy, possibly mobile material. The blanketing appears to be as prevalent away from steep slopes as near them, and therefore is not likely be be the result of downslope movement of material.
Our crater measurements are complete over the size range 0.1-3 km and complement the Voyager images, which covers 10-300 km in size. The Voyager record showed a dearth of large craters, presumably due to relaxation, but absolute densities comparable to the lunar highlands at sizes of 10 km. Our distribution follows a shallower slope than a production population and is indicative of a process that preferentially destroys small craters that is proportional to the square of the diameter (D). The destruction and crater production must be happening simultaneously due to the spectrum of morphologies. One process, a uniform dusting of material is not the sole explanation because it would vary linearly with D.
Because our high resolution views so far have been only of the Valhalla region, it is not yet clear whether this blanketing is characteristic of the entire surface of Callisto. However, our measurements from various regions, at a variety of sizes, are consistent with one another, and with the Voyager measurements.