Callisto is the outermost large moon of Jupiter, a dark, ancient world of ice and rock whose surface is among the most heavily cratered in the Solar System.
At a glance
| radius | 2,410 km (0.378 × Earth) |
|---|---|
| distance from its planet | 1.88 million km |
| one orbit | 16.7 days |
| reflects | 22% of the sunlight it gets |
Read off the piece’s own catalogue, the same values it draws with.
About Callisto
Callisto is almost as large as Mercury, but unlike its neighbours it has barely changed since early in the Solar System’s history. Craters overlap craters across its whole surface, and the largest impact, the basin Valhalla, is ringed by concentric ridges spreading far across the surface. Its dark surface reflects relatively little light.
Unlike Io, Europa and Ganymede, Callisto is not locked into their rhythm of orbits, so Jupiter’s tides barely flex it and it has had little internal heat to drive geology. Measurements of its gravity by the Galileo spacecraft suggested its interior is not fully separated into layers, and magnetic readings hinted at salty water deep inside.
Lying outside the most intense part of Jupiter’s radiation belts, Callisto has often been suggested as the safest place for any future human base in the Jupiter system.
What you see in the piece
Callisto is the darkest and outermost of the four large moons, reflecting a smaller share of sunlight than its brighter siblings. Look for it farthest from Jupiter; from another star it cannot be seen.
Measured or modelled
Both the incompletely layered interior and the possible ocean are inferences from Galileo’s gravity and magnetic data, and both are still debated.
Questions
Why is Callisto so heavily cratered?
Callisto has had almost no geological activity to erase its craters, because it is not tidally heated like the inner moons of Jupiter. Its surface still records impacts from very early in the Solar System’s history.
How far is Callisto from Jupiter?
Callisto orbits 1.88 million km from Jupiter and takes 16.7 days to go round it once.