Callisto

Identity and discovery

Callisto is Jupiter’s natural satellite, the planet’s second-largest moon and the third-largest in the Solar System. It is close to Mercury in size. NASA attributes its discovery to Galileo Galilei on January 7, 1610. Its identifier is NAIF 504; 599 identifies Jupiter, while MOON-ATLAS is an internal catalog key, not an official astronomical identifier.

Size and physical parameters

JPL gives a mean radius of 2410.30 ± 1.50 km, yielding a calculated equivalent diameter of 4820.60 ± 3.00 km. The tabulated mean density is 1.8340 ± 0.0034 g/cm³ and GM is 7179.28340 ± 0.00324 km³/s². GM is the gravitational parameter, not mass in kilograms; density is derived from GM and mean radius. This profile uses that JPL dataset without mixing in older diameter or density values from other summaries.

Orbit and rotation

The JPL JUP365 mean elements give a semimajor axis of 1882700 km and a period of 16.690440 days, in the Laplace plane at epoch 2000-01-01.5 TDB. The semimajor axis is measured from Jupiter’s center, not from its clouds. These elements are not precise ephemerides. NASA describes an orbit of about 17 days, listing 16.689 days on its public overview; this profile explicitly retains the JPL table’s convention and value. Rotation is synchronous, so Callisto keeps the same side toward Jupiter. It does not take part in the Io–Europa–Ganymede resonance.

Surface and formation

The rocky, icy surface is heavily cratered. NASA estimates a surface age of about 4 billion years and relates crater preservation to low geological activity. Craters have various sizes, some with multiple-ring structures; bright spots are interpreted as peaks covered in water ice. Formation from the disk of leftover material around Jupiter is a scientific scenario, not a directly witnessed event.

Interior and possible ocean

Galileo data and models motivate the hypothesis of a subsurface saltwater ocean. NASA describes an estimated depth of about 250 km, but cautions that the ocean could be deeper or might not exist at all. That depth is therefore not presented as an exact measurement, nor is the ocean’s presence treated as proven. Possible interaction between water and rock would matter for habitability, not establish the detection of life. An interior containing ice mixed with rock and metal is also considered.

Exosphere and exploration

Galileo detected a very thin carbon dioxide exosphere during 1997 observations, with the finding announced in 1999. NASA also describes later evidence for oxygen and hydrogen. It is incorrect to describe Callisto as a body with no atmospheric information available. Galileo’s 1990s data remain important for interpreting the interior. Visible craters and the exosphere are different kinds of evidence; neither alone proves an ocean or biosphere. This profile separates tabulated physical data from interpretations of the interior, without presenting them as new discoveries from October 2026.