Io

Io is the innermost of Jupiter's four Galilean moons and the most geologically violent body known. Trapped in an orbital resonance with Europa and Ganymede, it is squeezed and stretched by tidal forces that pump enough heat into its interior to keep silicate rock molten just below the crust. The result is a sulphur-stained world repaved continuously by lava — its surface so young that almost no impact craters survive long enough to be catalogued.

Unlike its icy Galilean siblings, Io is a rocky world with a probable iron or iron-sulphide core, a silicate mantle that is partly molten, and a thin crust of sulphur-rich basalts. Volatile water was largely cooked off early in its history, leaving sulphur compounds as the dominant volatile species cycling between surface, atmosphere, and plumes.

Io's landscape is dominated by lakes of molten silicate lava, plumes that rise hundreds of kilometres above the surface, and mountains taller than Everest that are not volcanic but tectonic — pushed up by compression as fresh material buries older crust. Sulphur and sulphur dioxide frosts paint the ground in yellows, oranges, reds, and whites, giving Io the look of a pizza. The volcano Loki Patera alone radiates more heat than all of Earth's volcanoes combined.

Voyager 1 captured the first images of an erupting plume in March 1979 — the discovery that Io was volcanically active. The Galileo orbiter (1995-2003) made repeated close passes, mapping hotspots and confirming a partially molten interior. NASA's Juno spacecraft, in extended mission since 2021, has executed the closest Io flybys ever flown — within 1,500 km in late 2023 — returning unprecedented imagery of plumes and lava lakes.