What's the News: Jupiter's moon Io is more volcanically active than any other object in our solar system, releasing 30 times more heat than Earth through volcanism. It's thought that Jupiter's gravity pulls so hard on the moon and causes so much friction that the resulting thermal energy melts a huge amount of underground rock, feeding Io's 400 active volcanoes. For years, astronomers have debated whether Io's spewing lava comes from isolated pockets of magma or a layer that spans the entire moon. Astronomers have now peered into Io's interior for the first time, discovering that it has a global sea of magma roughly 30 miles thick. "It turns out Io was continually giving off a 'sounding signal' in Jupiter's ... magnetic field that matched what would be expected from molten or partially molten rocks deep beneath the surface," lead researcher Krishan Khurana told Wired.How the Heck:

As NASA's Galileo spacecraft flew by Jupiter nearly a decade ago, it detected distortions in the planet's magnetic field. "Just like the waves beamed from an airport metal detector bounce off metallic coins in your pocket…Jupiter's rotating magnetic field continually bounces off...Io's interior," Khurana told Cosmos .

At the time, scientists couldn't explain the distortions. But since then, researchers have melted ultramafic rocks, which are rich in iron and magnesium and are highly conductive. Their tests have shown that the magnetic signal from Io matches the one you'd expect if there was an ocean of liquid ultramafic rock under its surface.

What's the Context:

In 1979, scientists first discovered volcanoes on Io's surface when NASA's Voyager spacecraft sent back some conclusive images. NASA's Galileo spacecraft launched in 1989 and reached Jupiter in 1995; it first recorded magnetic distortions caused by Io in 1999.

Scientists have used electromagnetic changes to peer into moons before, but it didn't involved magma: They used the changes to argue for the existence of water on Jupiter's other moons, including Europa, Ganymede, and Callisto. "Like molten rock, salt water has high conductivity, which responds to Jupiter's electromagnetic radiation in a similar manner."

Astronomers suspected Io had pockets of magma and not a huge sea because pockets would more easily explain how the moon has volcanoes that are as tall as 10 miles: Much like how magma plumes rise up to create giant volcanoes like those in Hawaii, it was thought that even larger pockets of magma welled up to support Io’s mountains. Now that we know that there's an entire layer of magma in the moon, "how Io supports those mountains isn't entirely clear."

The Future Holds: Astronomers want to take a closer look at Europa again using the same techniques to measure exactly how much there really is on the moon. Reference: Krishan K. Khurana et al. "Evidence of a Global Magma Ocean in Io’s Interior." Science. DOI: 10.1126/science.1201425 Image: The magnetic field lines (blue) of Jupiter allowed astronomers to peer into Io's interior, revealing a 30-mile thick layer of magma. Xianzhe Jia / University of Michigan, Ann Arbor