After years of debate, NASA researchers have zeroed in on the thickness of the Europa's ice shell. We first discovered that the Jupiter moon's surface looked icy in 1979, when Voyager 2 flew by.

Saturn’s “Death Star”-like moon Mimas may have a boiling ocean under its icy shell. The moon pitted with craters—including a very large one, giving it its nickname—may appear to be geologically dead, ...