What's torturing Io, the Jupiter moon that holds the unfortunate distinction of the most volcanically active body in the solar system?
An underground ocean of magma, say scientists who cracked a long-standing puzzle posed by NASA's defunct Galileo probe, which orbited giant Jupiter and surveyed its moons from 1995 to 2003.
The Galileo team used the planet's strong magnetic field to look for distinct radio waves bouncing back from the Jovian moons, a technique that previously netted evidence for underground saltwater oceans on Europa, Callisto and Ganymede.
Oddly, Io, which is far too hot for liquid water, showed some distinctive footprints of liquidity as well. However, it wasn't until team members discovered new research about magnetic properties of melted rock that they were able to piece together Io's odd story
Tugged by Jupiter's massive gravity, rock inside Io has melted into a molten slush that fills a cavity at least 50 kilometers (31 miles) deep.
"I think it's exciting to think there are 'oceans' within all four of the Galileo satellites, but this one is a magma ocean. It's just a fascinating process to think about," planetary scientist Bob Pappalardo, with NASA's Jet Propulsion Laboratory in Pasadena, Calif., told Discovery News.
The research also may explain why Io, unlike sibling moons, lacks an internal magnetic field, lead scientist Krishan Khurana tells Discovery News.
The hot magma ocean, estimated to be about 2,400 degrees Fahrenheit (1,315 degrees Celsius), effectively insulates the moon's metal core so that convection is switched off.
"The core is likely molten, but the heat can't get out," said Khurana.
The research also serves as a model for what may have happened during Earth's evolution.
"One of the big questions about the Earth is: When did plate tectonics start? It turns out early Earth most likely had a magma ocean. Most planets in early formation are hot and as a result the upper layers are molten. Magma oceans are very efficient at transferring heat from the interior. As a result, it switches off plate tectonics, so Earth's magma ocean had to switch off before plate tectonics could start," Khurana said.
Scientists hope to collect more information about Io and the other moons of Jupiter with future space probes, though there still may be more insights to come from information already in hand.
"This data had been sitting around for many years before the puzzle fell into place. I can imagine there are many other gold nuggets to be mined from Galileo," Khurana said.
The research appears in this week's Science.