Callisto, 2,400 mi (4,800 km) in diameter, is the outermost and, apparently, the least geologically active of Jupiter's four major satellites. Its density is less than twice that of water. Callisto has the oldest body and most cratered face of any body yet observed in the solar system. Like Ganymede, it seems to have a rocky core surrounded by ice. Unlike Ganymede, the surface of Callisto is completely covered with scars left by tens of thousands of meteoric impacts. Scientists estimate that it would take several billion years to accumulate the number of craters found there. So Callisto is believed to be inactive for at least that long. Although it is the darkest of the Galilean satellites, it is twice as bright as Earth's Moon.
Data from the Galileo spacecraft in 1998 suggest that Callisto has a salty ocean beneath its crust, similar to Europa's.