Subsurface oceans may be a common features of the moons of our solar system's gas giants. Evidence has piled up that there's one on Titan, and data from Jupiter's moons Europa and Ganymede that indicate they too may have a watery layer underneath their crusts. Today's issue of Science turns the focus to Jupiter's moon Io, which has the most active volcanism of any body in the solar system. Instead of a watery ocean, however, evidence suggests that Io has a layer of partially molten rock over 50km thick.

Io's volcanic activity was discovered during the Voyager flyby of Jupiter, and further imaging has revealed a number of active volcanoes on the moon's surface—so many, that their total output is about 30 times the averaged activity of Earth's volcanoes. The power for this activity comes from the extreme gravitational pulls Io experiences. It's the closest to Jupiter of the four planet-sized moons that orbit the gas giant, and so it experiences intense strain as it's pulled between the planet and varying orbits of the outer moons.

To infer the moon's interior structure, researchers used the same technique that helped strengthen the case for a subsurface ocean on Europa. Jupiter's rotating magnetic field sweeps through the moons, and their internal properties will induce changes in that field. Molten rock has a different conductance than solid rock, and so it will induce different changes. Four passes of the Galileo orbiter provided data on the magnetic fields in the neighborhood of Io.

The authors built a model of Io's interior, and changed parameters to find the best fit to the magnetic transformations that were observed. The model that seemed to work well had a partially molten asthenosphere, which is the upper-most layer of the mantle. The level of melt is, at a minimum, 20 percent, and the molten area is over 50km thick. The models indicate that the molten material must also be continuous, rather than composed of isolated pockets of molten rock, which wouldn't produce the same magnetic signature.

The surface lava temperatures on Io indicate that the upper mantle must be over 1250°C, which is consistent with this level of melt. The authors also note that, at these temperatures, the mantle of Io will be too hot to effectively allow the core of the moon to get rid of heat, meaning that the deep interior of Io may be completely molten. All that, simply from the force of gravity twisting the interior.

Science, 2011. DOI: 10.1126/science.1201425 (About DOIs).