SPLIT COMETS AND THE ORIGIN OF CRATER CHAINS ON GANYMEDE AND CALLISTO

WHEN the Voyager 1 spacecraft flew through the jovian system in Januar y 1979, it returned images of several prominent chains of impact crate rs on the surface of the moon Callisto (Fig. 1). These impressively st raight chains, or catenae, are composed of between 4 and 25 craters, a nd 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 satisfa ctory 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 th at the impact of previous split comets might be responsible for at lea st 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-fa cing hemisphere, as are an additional three catenae that we have found on Ganymede. We present a simple model of tidal breakup which both re produces the range of observed chain lengths and indicates that the pa rent comets responsible for the Callisto catenae were typically no mor e than about 10 km in diameter.