2nd Largest Moon of Jupiter - Callisto (8th Moon outwards from Jupiter)

Callisto is the third-largest moon in the Solar System and the second largest in the Jovian system.

Discovery

Callisto's discovery is credited to Galileo Galilei, who was the first to observe it on January 7, 1610.

The satellite's name was soon suggested by astronomer Simon Marius. Callisto is named after one of Zeus's many lovers in Greek mythology. Callisto was a nymph (or, according to some sources, the daughter of Lycaon) who was associated with the goddess of the hunt, Artemis.

Together with Ganymede, Io and Europa, they are collectively known as Galilean satellites after the discover.

Stats

Diameter: 4,821 km

Semi-major axis: 1,882,709 km

Orbital Period: 16.69 days

Orbits

Callisto is the outermost of the four Galilean moons of Jupiter. It orbits at a distance of 1,882,709 km. This is significantly larger than the orbital radius —1,070,412 km— of the next-closest Galilean satellite, Ganymede. As a result of this relatively distant orbit, Callisto does not participate in the mean-motion resonance in which the three inner Galilean satellites are locked.

Callisto rotates synchronously with its orbital period, so the same hemisphere always faces (is tidally locked to) Jupiter. Callisto's surface is less affected by Jupiter's magnetosphere than the other inner satellites because it orbits farther away, and thus does not experience appreciable tidal heating.

Composition

Callisto is composed of approximately equal amounts of rock and ices. Compounds detected spectroscopically on the surface include water ice, carbon dioxide, silicates, and organic compounds. Investigation by the Galileo spacecraft revealed that Callisto may have a small silicate core and possibly a subsurface ocean of liquid water at depths greater than 100 km.

The surface of Callisto is heavily cratered and extremely old. It does not show any signatures of subsurface processes such as plate tectonics or volcanism, and is thought to have evolved predominantly under the influence of impacts.

Atmosphere

Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxide and probably molecular oxygen, as well as by a rather intense ionosphere.

Life?

There is a distinct possibility that Callisto's saltwater ocean could harbour life. If there are thermal vents on the floor of this saltwater ocean as there are on Earth, it is remotely possible that similar organisms to those which live around the vents on Earth could also survive there.

Life has less chance at Callisto than at Europa and Ganymede, because Callisto orbits further away from Jupiter and lack the tidal heat energy.