io

Jupiter's auroras are not scaled-up versions of Earth's. They run mostly on plasma from the volcanic moon Io and on the planet's own rapid spin, and they glow in ultraviolet, infrared, and X-rays whether or not the Sun is active.

Io has more than 400 active volcanoes, mountains taller than Everest, lakes of molten sulfur, and a surface that repaints itself every few decades. It is the most volcanically active body in the solar system — and its power source is not radioactivity or solar heat, but the gravitational squeeze of Jupiter and its sibling moons.

Tidal locking happens when a body's rotation period equals its orbital period, so one face always points at its partner. The Moon is locked to Earth, Mercury is caught in a 3:2 resonance instead, and many planets around red dwarf stars are probably locked to their stars.