Dione is the 15th largest moon in the Solar System, and is more massive than all known moons smaller than itself combined. It is composed primarily of water ice, but as the third densest of Saturn’s moons (after Enceladus and Titan, whose density is increased by gravitational compression) it must have a considerable fraction (~ 46%) of denser material like silicate rock in its interior. So it is much like a frozen snowball, inert and dead. Thanks to close-up images of a 500-mile-long (800-kilometer-long) mountain on the moon from NASA’s Cassini spacecraft, scientists have found more evidence for the idea that Dione was likely geologically active in the past. It could still be active now.