Meet Callisto 🌕 The third-largest moon in the solar system—almost as big as the planet Mercury—Callisto was first spotted orbiting Jupiter in 1610 by the astronomer Galileo Galilei. Callisto has been pummeled by comets and asteroids for 4 billion years, shaping its rocky, icy surface. Scientists think that, like fellow moon Europa, Callisto could hold a massive ocean underneath. Much of what we know about Callisto comes from a NASASolarSystem spacecraft named (appropriately) Galileo, which orbited Jupiter and studied its moons from 1995 to 2003. Galileo captured this image of Callisto in May 2001, two years before the spacecraft ended its mission with a dramatic plunge into Jupiter's crushing atmosphere. Image description: The moon of Callisto, a spherical, mottled ball, as seen from space. Its dark, brownish surface is covered with craters of various sizes and white marks, which are thought to be crater peaks capped with ice. The background of the photo is completely black. Credi...