A battered relic of the early Solar System
Callisto
Moon of Jupiter · discovered 1610 by Galileo Galilei
Callisto is the outermost of Jupiter's four Galilean moons and the third-largest moon in the Solar System, nearly as wide as Mercury. Its dark, crater-packed surface is among the oldest landscapes known, a near-perfect record of impacts stretching back around four billion years.
Callisto looks geologically dead, yet it may hide a secret: magnetic measurements suggest a salty ocean deep beneath its ice. And because it orbits beyond Jupiter's most intense radiation, it has even been studied as a possible base for future human explorers.
Key facts
- Diameter
- 4,821 km1.39× Earth's Moon
- Mass
- 1.08 × 10²³ kg
- Surface gravity
- 1.235 m/s²0.126 g
- Density
- 1,834 kg/m³
- Distance from Jupiter
- 1,882,709 km≈ 26.3 Jupiter radii
- Orbital period
- 16.7 daysSame as its rotation (tidally locked)
- Discovered
- 1610Galileo Galilei
- Surface
- Dark ice and rock saturated with impact craters
- Atmosphere
- Very thin carbon dioxide with traces of oxygen
- Surface temperature
- About −140 °C on average
- Notable feature
- Valhalla, a multi-ring impact basin ~3,800 km across
- Named after
- Callisto, a nymph changed into a bear in Greek myth
Values follow NASA/JPL planetary satellite parameters: mean diameter, mean distance (semi-major axis) and sidereal orbital period.
Surface and geology
Callisto is one of the most heavily cratered objects in the Solar System. Its surface is saturated, meaning any new crater tends to erase an old one, and there is little sign of mountains, volcanoes or tectonic activity to break up the pattern.
The most dramatic feature is Valhalla, a multi-ring impact basin whose bright central zone is about 600 kilometres wide, surrounded by concentric rings reaching around 3,800 kilometres across. A smaller cousin, Asgard, spans roughly 1,600 kilometres. Gipul Catena is a chain of craters probably made by a comet that broke apart near Jupiter, much like Shoemaker–Levy 9 did in 1994.
Up close, Galileo images showed something surprising: relatively few small craters, and smooth dark plains dotted with bright, icy knobs. Scientists think frost slowly sublimates from crater rims and slopes, leaving dark dust behind and gradually eroding the smallest features.
Interior and composition
Callisto is made of roughly equal amounts of rock and ice. Gravity data suggest it is only partly differentiated, with rock and ice incompletely separated, rather than the neatly layered structure seen in Ganymede.
Galileo's magnetometer detected a magnetic signature that is best explained by a layer of salty liquid water, perhaps more than 100 kilometres below the surface. Not all scientists agree that the ocean exists, and confirming it is one of the goals of ESA's JUICE mission.
Orbit and rotation
Callisto takes about 16.7 days to orbit Jupiter and is tidally locked. Unlike Io, Europa and Ganymede, it is not part of the Laplace resonance, so it receives very little tidal heating, which helps explain its lack of geological activity.
Its distance from Jupiter also places it outside the planet's harshest radiation belts. Radiation at Callisto's surface is a tiny fraction of that at Europa, which is why a 2003 NASA study considered Callisto as a site for a future crewed outpost.
Origin
Callisto formed in the outer part of Jupiter's circumplanetary disc. Scientists think it accumulated relatively slowly, allowing heat to escape as it grew, so its interior never became hot enough to separate fully into rock and ice layers. That makes it a valuable window onto the raw materials from which the Galilean moons were built.
Exploration
Discovered by Galileo Galilei in January 1610, Callisto has been visited only by flybys so far. JUICE will change that with a long series of close passes.
Voyager 1 and 2 (1979)
The Voyagers imaged about half of Callisto's surface and revealed the enormous Valhalla basin for the first time.
Galileo (1995–2003)
Galileo made several close flybys, mapping the surface in fine detail and detecting the magnetic signal that hints at a hidden ocean.
JUICE (launched 2023)
ESA's Jupiter Icy Moons Explorer is planned to fly past Callisto around 20 times in the early 2030s, using it for gravity assists and probing its interior before moving on to Ganymede.
The moons of Jupiter
Surprising facts about Callisto
- Callisto is almost exactly the same width as Mercury but has only about a third of the planet's mass.
- Its surface may be the oldest landscape of any large moon, essentially unchanged for around four billion years.
- Despite looking dead, Callisto may hold a buried saltwater ocean more than 100 kilometres down.
- The radiation dose on Callisto's surface is low enough that NASA once studied it as a base for human missions to Jupiter.
- Its name, like the moon's lonely orbit, comes from myth: Callisto was turned into a bear and placed in the sky as Ursa Major.