1610

Callisto

Galileo Galilei (1564–1642)

Galileo’s January 8, 1610, drawing of Jupiter’s four bright satellites, with Callisto’s position on that night highlighted by the red arrow.

The farthest of the Galilean satellites from Jupiter is Callisto, named after another nymph and lover of Zeus from Greek mythology. Like the other major moons of Jupiter, it was discovered in early 1610, when Galileo first trained his astronomical telescope on the giant planet. Callisto is the farthest from Jupiter of the four major satellites, taking almost 17 days to complete one orbit. Perhaps because of this it does not participate in the resonances that characterize the orbits of Io, Europa, and Ganymede.

For more than 350 years after their discovery, it was impossible for astronomers to learn more about Callisto and the other Galilean satellites. Starting in the 1960s, however, it became possible to use spectroscopy from ground-based telescopes to determine their surface compositions. Callisto, Ganymede, and Europa were found to have surfaces dominated by water ice, and Io was discovered to be dry, with colors and spectra dominated by the presence of sulfur. Spectroscopy from more recent space missions has further revealed the presence of ices of carbon dioxide and sulfur dioxide on Callisto and Ganymede, as well as hydrated sulfate salts on all three icy moons.

These missions have also allowed the determination of Callisto’s diameter (2,995 miles [4,820 kilometers]—25 percent larger than our Moon) and density (1.8 grams per cubic centimeter, mostly icy with some rock), and the mapping of its surface features. Callisto is the most heavily cratered of the Galilean satellites, preserving giant impact basins from the Late Heavy Bombardment of the early solar system and suggesting that it has had the least internal activity or resurfacing history of the four. And yet, data from NASA’s Galileo Jupiter Orbiter suggest that there could be a layer of liquid water—an ocean of sorts—deep beneath the scarred, icy crust. Callisto does not get the tidal flexing and heating that the other moons get from their resonant orbital interactions or proximity to Jupiter, however, so the heat source driving Callisto’s possible subsurface ocean is a puzzle waiting to be solved.

NASA’s Galileo Jupiter orbiter spacecraft acquired this “full Callisto” photo of the heavily cratered surface of Jupiter’s fourth Galilean satellite in 2001.