Callisto

Callisto, about the size of Mercury, has the most heavily cratered and oldest surface in the solar system. NASA’s Galileo mission (1989-2003) observed Callisto, revealing evidence for a subsurface saltwater ocean—the same sort of evidence for the ocean scientists think exists on Europa. Can life emerge and thrive in such cold, lightless oceans? If there’s life in one of these two oceans, it is more likely to be Europa’s. The ocean there receives more heat because of Europa’s stronger gravitational relationship with Jupiter. Callisto’s ocean, if it exists, is so deep that dense, high-pressure phases of ice will form at the ocean bottom, possibly blocking liquid water and rock from interacting directly to create conditions for life.