Moons · Space Codex
Callisto
An ancient, dark world saturated with craters, outside the resonance that heats its neighbours.
The most heavily cratered object known
At a glance
Key points
Even so, Galileo data hint at a salty ocean far below.
- Most heavily cratered object in the Solar System
- May hide a salty ocean beneath its ancient crust
- Darkest of the four Galilean moons
Surface
Callisto's dark, dusty surface is speckled at every scale with bright craters where impacts exposed cleaner ice. The Valhalla basin is the largest landform: a bright centre ringed by concentric ridges that spread for thousands of kilometres. Chains of craters trace comets that broke apart before they hit.
Interior
Its moment of inertia shows Callisto never fully separated into rock and ice. Galileo's magnetometer detected a varying field induced by Jupiter, which a layer of salty liquid water deep below the surface would explain.
What we still don't know
Whether the ocean is real, and why Callisto stayed so undifferentiated while similar-sized Ganymede did not. JUICE will fly past it many times.
Missions
- 1989–2003
Galileo
NASACompletedFirst Jupiter orbiter (1995–2003) and first probe into a giant planet's atmosphere.
- 2023–
Earth flyby Sep 2026, another in Jan 2029; Jupiter arrival Jul 2031, then orbit Ganymede.
First-hand · our 3D model
How we render it
Callisto is a sphere wrapped in a global map built from USGS and NASA spacecraft mosaics.
It is drawn at about 9.5% of Jupiter's radius, enlarged from its true 3.4% so it stays visible. It orbits in Jupiter's equatorial frame, inclined 0.3°, with a 16.69-day period kept in true ratio to its neighbours, and keeps one face towards Jupiter.
Moon sizes and orbits are compressed so a whole system fits on screen, but never reordered: each moon is drawn at 0.72 × (true size ratio)^0.6 of its planet, capped so no large moon is drawn bigger than it really is, and orbit distances are log-compressed into a band that clears the planet's rings. Orbital periods keep their true ratios, every moon is tidally locked, and on lighter devices only the two largest moons of each planet are drawn.
Related entries
Sources
- JPL Solar System Dynamics: satellite physical parameters (opens in a new tab)
- JPL Solar System Dynamics: satellite mean orbital elements (opens in a new tab)
- NASA NSSDC Jovian Satellite Fact Sheet (opens in a new tab)
- NASA Science: Callisto (opens in a new tab)
Last reviewed against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.