A new study, published in the Nature Geoscience journal report, says that Jupiter’s Ganymede likely endured a double blasting of comets 3.8 billion years ago, explaining its difference from twin moon, Callisto."
More cool stuff from space. You know, when I was a kid Jupiter only had 12 moons. Did it make rest in the intervening 40 years? No. Telescopes and other measuring devices just got better. The down side to it is we lost Pluto.