| Type | Ionian (EuHephaestian) Subtype |
| Primary, Position | Jupiter, v (all)/i (Galilean only) |
| Distance from Primary | 421,600 km |
| Satellites | none |
| Diameter | 3,630 km |
| Gravity | 0.183 G |
| Length of day | 1.769 days (tidally locked) |
| Atmosphere | mostly sulphur dioxide |
| Surface Temperature | -143° C (average) |
| Surface Composition | Silicates, sulphur, sulphur dioxide |
| Discovery | Jan 8, 1610 AD, by Galileo Galilei |
| Planetology | Holotype for Ionian Subtype of world. This moon, a body larger than Luna, is unlike any other object in Sol System, although a fairly common type in the right celestial environment. Volcanoes, lava-flows and lakes of bright orange molten sulphur dominate the surface of Io. There are extensive regions frosted with sulphur dioxide snow. |