Specs
| What | Data |
|---|---|
| Name | Callisto |
| Class | Galilean moon |
| Parent planet | Jupiter |
| Diameter | 4,821 km |
| Mass | 1.076 × 10²³ kg |
| Distance from Jupiter | 1,882,700 km |
| Orbital period | 16.69 days |
| Rotation period | 16.69 days (tidally locked) |
| Surface gravity | 1.24 m/s² |
| Discovered | 1610 — Galileo Galilei |
The Ancient One
Callisto is the outermost of the four Galilean moons and the most heavily cratered object in the solar system. Its surface has not been significantly reshaped by tectonics or volcanism in billions of years, making it a kind of fossil record of the early solar system. Every impact it has ever suffered is still visible.
Because it sits beyond Jupiter’s main radiation belts, unlike Io, Europa and Ganymede, Callisto is exposed to far less radiation, which makes it a more practical target for a potential future crewed outpost in the Jovian system.
Surface
Callisto’s surface is uniformly dark and saturated with craters, with no sign of internal geological activity. The largest impact structure, Valhalla, is a multi-ring basin roughly 3,800 km across; one of the largest impact features in the solar system. Bright icy material exposed by impacts contrasts with the dark, ancient terrain around it.
Possible Ocean
Despite its geologically dead appearance, there is evidence that Callisto may also have a subsurface liquid water ocean, though likely less extensive than those of Europa or Ganymede. Measurements of its interaction with Jupiter’s magnetic field suggest a conducting layer, consistent with a salty ocean, at depth.