Object Analysis

Io is the innermost of the four Galilean moons of Jupiter. It is the fourth-largest moon in the Solar System, with a density and size similar to Earth's Moon. However, unlike our geologically dead Moon, Io is the most geologically active object in the entire Solar System.

Io's surface is dotted with hundreds of volcanoes and expansive lava flows. Plumes of sulfur and sulfur dioxide climb as high as 500 km (300 mi) above the surface. The surface is constantly being resurfaced by volcanic activity, erasing any impact craters.

This extreme activity is caused by tidal heating. Io is caught in a gravitational tug-of-war between Jupiter and the other Galilean moons (Europa and Ganymede). This friction generates immense heat within Io's interior, melting rock and driving the ferocious volcanism that paints the moon yellow, red, and black with sulfur compounds.