Illustration of Io (a moon of Jupiter) and its largest volcanic depression, Loki Patera. Io is the innermost of the four Galilean moons of Jupiter, and has the least amount of water of any known astronomical object in the Solar System. Loki Patera is 202 kilometres in diameter, and contains an active lava lake. This consists of molten lava covered by a thin solidified crust layer that episodically overturns, and causes a huge increase in thermal emission from the volcano. Waves of resurfacing lava during an overturning episode (pictured) mean that Loki Patera emits up to ten times more heat than when the crust is stable. This is an artistic depiction of Io and the NASA space probe, Juno, which orbits Jupiter.