" pose real challenges for people trying to figure out the inside of Io," says Laszlo Kestay, who studies astrogeology with the U.S. With a full layer of magma beneath its surface, how Io supports those mountains isn't entirely clear. One reason that scientists entertained the idea of pockets of magma rather than a global layer was that this view helped to explain how the moon's structure could support its 100 or so mountains-the most impressive of which reaches 10 miles high (so tall it would tower over Mount Everest). Meanwhile, Io still poses more questions. Volcanic activity on Io and likely reservoirs of magma on Jupiter's other moons could also be a potential source of energy for future explorations or even habitation of the solar system, McEwen says. These insights on the inner workings of Io and its neighboring moons-where tidal heating melts ice-could help scientists better gauge how much liquid water to expect on Europa, where scientists place hope that life could exist. "This is what we should have probably expected because the same type of result came from icy satellites, which have salty water oceans," says Alfred McEwen, a planetary geologist at the University of Arizona who has studied Io. Like molten rock, salt water has high conductivity, which responds to Jupiter's electromagnetic radiation in a similar manner. The same kind of electromagnetic analysis was used to demonstrate the likely existence of oceans of liquid water on Jupiter's moons Europa, Ganymede and Callisto. This isn't the first time scientists have used these slight electromagnetic changes to see inside a moon. Play icon The triangle icon that indicates to play His calculations indicate the magma is at least 20 percent liquid and more than 30 miles thick. "A liquid layer is required somewhere in Io that causes the electromagnetic response we measured," Khurana says. In addition, the researchers modeled the magnetic effects they would expect from a global shell of underground magma, and it matched their measurements from Io. Once Khurana's team worked out the tiny variations in qualities such as amplitude that should come from Jupiter's field passing through Io, they realized they could be explained only by a moon-wide pool of conductive material-in this case, partially molten rock. But because Galileo didn't orbit Io-the craft only flew by it- it was difficult for scientists to distinguish the signals that passed through the moon from the ones that came directly from Jupiter. Jupiter produces a strong electromagnetic field that passes through Io's interior, creating the signature that Galileo recorded. Their analysis-based on a new understanding of how to interpret changes in magnetic fields-captured the distinct signature of the sea of scalding magma lying below Io's surface, the first time direct measurements have determined its existence. Khurana's team revisited data from a magnetometer aboard the Galileo spacecraft, which flew by Jupiter and its moons a decade ago. Research published today in the journal Science may put that theory to rest. Some thought it might have come from isolated underground pockets or wellsprings, as is the case on Earth, rather than a global layer of magma. Previously, however, scientists weren't sure exactly how that magma was distributed. The heat melts rock to create magma, which spews out of the 400 or so volcanoes on Io's crust. Io's center is in constant flux: Jupiter's enormous gravity pulls on the moon's "slushie-like interior," UCLA geophysicist Krishan Khurana says, and all that friction produces an enormous amount of thermal energy, through what's called tidal heating. But the source of all this volcanism was hidden from scientists, until now-a new study reveals the vast ocean of semimolten magma that lies beneath the moon's volcano-ridden surface. In fact, it's the most volcanically active body in the solar system, producing 30 times more heat than Earth despite being 70 times less massive. Jupiter's moon Io is the only place in the solar system besides Earth with volcanoes that spew molten rock.
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