The presence of liquid water under that ice cover, along with organic molecules, has led some scientists to speculate that Europa's ocean may harbor some of the biochemistry necessary for life. The largest Galilean moon, Ganymede, is likely a mostly rocky core with a largely icy surface. That surface is marked by grooves hundreds of meters deep that run for thousands of kilometers, probably the result of early tectonic activity. Kin to the rest of the Galilean satellites but different in kind, Callisto's surface shows no evidence of any resurfacing since its craters were first formed by impacts some four billion years ago. Its tail spreads out beyond Saturn's orbit, meaning that Saturn finds itself at times within Jupiter's magnetosphere.
Solar winds push the field, causing the obvious asymmetry.