There's a deep ocean of magma lurking beneath the crust of Jupiter's moon Io, a new study says.

Only slightly larger than Earth's moon, Io is the most volcanically active body in the solar system. The discovery solves a long-standing debate over how much of the moon's insides must be molten to feed the ongoing eruptions.

"At any time, Io has 400 [and] maybe more active volcanoes," said study leader Krishan Khurana, a planetary physicist at the University of California, Los Angles.

"They are very powerful—they can shoot plumes out into space to a height of about 300 miles [500 kilometers]." Finding an extensive magma ocean means that "now we know why there are so many, and where the lava comes from."

Jupiter Probe Worked Like Airport Scanners

Khurana and colleagues made the find after reexamining readings from the Galileo spacecraft, which orbited Jupiter from 1995 to 2003 and made occasional flybys of the planet's moons, including Io.

The data showed how Io deflects Jupiter's enormous magnetic field via a process called electromagnetic induction—"very similar to the principle used by the metal detectors at the airport," Khurana said.

Magma has high electrical conductivity, he explained, a trait that laboratory studies have demonstrated with molten rocks similar to those expected to lie beneath Io's surface.

As Jupiter's electromagnetic field penetrates Io, it interacts with the magma ocean, and a current forms on the outer edge of the molten rock layer. This current in turn generates its own electromagnetic waves, which deflect Jupiter's field lines, an effect the Galileo probe was able to detect.

The data show that Io's magma exists in an underground layer that lies about 20 to 30 miles (30 to 50 kilometers) beneath the surface, between the crust and the mantle.

The magma layer is at least 30 miles (50 kilometers) thick, and it might be as thick as 200 miles (320 kilometers).

Moon's Magma a Slushy Mix?

Also, the magma probably isn't completely liquid, Khurana said. Rather, it has the consistency of slushy ice, with a mix of molten rock and crystals.

This hot slush is created by tidal forces from the mammoth gravity of Jupiter, which slowly knead the moon as it moves in its orbit, generating enormous amounts of heat as rock crystals rub against each other.

Once formed, the magma ocean would concentrate tidal energy inside itself, keeping the layer toasty: "A slushy magma ocean is a perfect place for heating from friction," Khurana said.

The discovery wasn't made earlier because Io's volcanoes emit large quantities of ionized gases, which also interact with Jupiter's magnetic field, he added.

It took until now for scientists to produce good enough computer models to pierce though this interference and spot the additional effect of the magma ocean.