Beyond Mars lie the planets Jupiter, Saturn, Uranus, and Neptune. These giant gas balls have no solid surface and thus no possibility of volcanic activity. Any such action in this part of the solar system would have to be on a planetary moon.

When the Voyager 1 spacecraft flew past the Jupiter system in March 1979, initial photos of the inner moon Io showed no impact craters. Any dead world, and that’s what scientists expected these moons to be, should have been pockmarked with countless craters.

Astronomers solved the mystery a few days later when navigator Linda Morabito spied a volcanic plume rising above the satellite’s limb. The ejecta rose some 170 miles (270km) above the surface and spread over an area that spanned more than 620 miles (1,000km). Scientists later named this volcano Pele, and it was but the first of nine separate eruptions spied by sharp-eyed researchers.

Today, scientists know of more than 160 volcanoes that spew enough material to cover lo in a layer 4 inches (10 centimeters) thick every year. This modest moon is the most volcanically active body in the solar system.

How can this happen?

Jo is about the size of Earth’s Moon, and its store of radioactive elements shouldn’t create any appreciable warming. But Jo orbits close enough to Jupiter that the giant planet raises enormous tides in the same way the Moon creates tides on Earth (and vice versa). The tides have locked Jo so that it rotates on its axis at the same rate it revolves around Jupiter.

The moon also orbits in a resonance with two more-distant satellites, Europa and Ganymede, so Jo follows a slightly elliptical path and changes speed just a little. The combination of these two effects constantly flexes Io’s crust, generating the internal heat. Stanton Peale of the University of California, Santa Barbara, and his colleagues developed this idea and published it just three days before Voyager 1 arrived.

Although the fire still burns within Jo, the moon hasn’t experienced any major outbursts since 2010. Scientists now track Io’s output from Earth by observing how much infrared radiation (heat) it emits and currently see only about a dozen permanent, relatively low-temperature eruptions. No one knows how long this current quiescent phase will last.