Io

Io is the innermost of Jupiter’s four large moons and the most volcanically active world known. Hundreds of volcanoes cover its surface, some throwing plumes of sulfur hundreds of kilometers high.

That heat comes from tides. As Io orbits Jupiter, the gravity of Jupiter and its neighbor moons squeezes and flexes it, warming its interior. Its yellow, orange and red colors come from sulfur compounds.

Io is slightly larger than our Moon. It was discovered by Galileo in 1610, along with Europa, Ganymede and Callisto, and it races around Jupiter in just under two days, so its position changes visibly within hours.

Io’s volcanoes resurface it so quickly that no impact craters have been found on it, and some of its lava is hotter than any erupting on Earth today. Io also feeds a doughnut-shaped ring of charged particles around Jupiter, called the Io plasma torus.

NASA’s Juno spacecraft made close flybys of Io in 2023 and 2024, returning detailed images of lava lakes and mountains. From Earth, watching Io’s shadow cross Jupiter through a telescope is one of the easiest ways to see orbital motion in action.