The mystery of the disappearing small craters on Callisto has had several proposed solutions. One is devolatilization of the mounds of ice around the crater: basically evaporation into a vacuum. J.M. Moore at NASA Ames Research Center has even suggested that this evaporation creates an electric charge which levitates the left-over debris, and can spread it around evenly on the surface! Still, no one is really happy with these answers, and certainly no one predicted it beforehand. Small craters on Ganymede and Europa don't seem to have undergone the same processes -- what is different about Callisto? Keep reading the story to learn another possibility...