Ganymede And Callisto

Jupiter's two large moons Ganymede and Callisto are similar in size and composition, but different in appearance. Ganymede has few craters on its surface, while Callisto is heavily cratered. Planetary scientists have tried for 30 years to explain the difference. Now, a computer simulation model suggests an answer.

The model postulates that early in the history of the Solar System, when large bodies were regularly being pounded by smaller bodies, the powerful gravity of Jupiter would have funneled asteroids and comets into itself. Ganymede orbits much closer to Jupiter than Callisto does. Therefore, Ganymede would have been struck more often than Callisto, and the striker would have been moving faster because it was deeper in Jupiter's gravity well.

So, why doesn't Ganymede have more craters than Callisto? Because the pounding Ganymede endured drove the heavy metals to the core of the moon, creating a molten center that supported tectonic processes that resurfaced the moon, wiping out old craters. The same process has wiped out the earliest craters on Earth. Callisto, because it was farther away from Jupiter, took fewer hits, developed no molten core or tectonic action, and therefore we see a more ancient, heavily cratered surface there.