 There is very strong evidence that comets have previously been fragmented and collided with Jupiter and its satellites . During the Voyager missions to the planet , planetary scientists identified 13 crater chains on Callisto and three on Ganymede , the origin of which was initially a mystery . Crater chains seen on the Moon often radiate from large craters , and are thought to be caused by secondary impacts of the original ejecta , but the chains on the Jovian moons did not lead back to a larger crater . The impact of SL9 strongly implied that the chains were due to trains of disrupted cometary fragments crashing into the satellites . 
