Split comets and the origin of crater chains on Ganymede and Callisto

Abstract

WHEN the Voyager 1 spacecraft flew through the jovian system in January 1979, it returned images of several prominent chains of impact craters on the surface of the moon Callisto (Fig. 1). These impressively straight chains, or catenae, are composed of between 4 and 25 craters, and are up to 620 km long. They were initially thought to be secondary craters produced by debris from a larger primary impact¹, but detailed searches for source craters have been largely unsuccessful: a satisfactory explanation for the crater chains has yet to be found. Inspired by the recent observations of comet Shoemaker–Levy 9, which split into a line of about 20 fragments as it swept past Jupiter², we suggest that the impact of previous split comets might be responsible for at least some of the catenae on Callisto. In support of this hypothesis, we find that nearly all of Callisto's crater chains are on the Jupiter-facing hemisphere, as are an additional three catenae that we have found on Ganymede. We present a simple model of tidal breakup which both reproduces the range of observed chain lengths and indicates that the parent comets responsible for the Callisto catenae were typically no more than about 10 km in diameter.