Finding an Ocean of Fire on Jupiter's Moon Io

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A high-resolution image of part of Prometheus, an active volcano on Jupiter's volcanic moon Io, from Oct. 10, 1999

Old spacecraft never die. Well, actually, they do die, and sometimes in spectacular fashion — burning up as they plunge through a planet's atmosphere or into the fires of the sun. The venerable Galileo probe, which was launched in 1989 and arrived at Jupiter in 1995, was sent on just such a suicide dive into the planet when its work was done in 2003 — the better to avoid even the tiny risk that it would crash onto one of the Jovian moons and contaminate it with Earthly bacteria.

But if spacecraft are mortal, their data stream lives forever, and scientists with new theories and new analytic methods often pore over the old records, looking for insights that may have slipped past earlier investigators. A collaborative team from UCLA, the University of Michigan and the University of California, Santa Cruz, recently did exactly that with some of the data Galileo collected about the Jovian moon Io and found that bubbling just beneath the surface of the little world is a massive ocean of magma — a feature unlike any other found anywhere else in the modern solar system.

It was in 1979 when astronomers learned how hot and geologically active Io is, when the Voyager I spacecraft returned images of a massive volcanic plume rising 160 miles (257 km) into space. On Earth, the same column of fire would climb 30 times as high as the peak of Mount Everest. Io comes by all that heat rightly. Jupiter's moon census changes all the time, as more and more of the smallest ones are detected. The current count is 63, but it's the four biggest ones — Io, Europa, Ganymede and Callisto — that are the most complex.

Io, as the innermost of the four, has the hardest life. Held tight in the gravity fist of Jupiter, it orbits the planet close and fast, repeatedly outpacing its slower, more distant sister. Every time Io laps another moon, however, that body's gravity briefly grabs it, causing Io to flex slightly. Do that again and again over the course of 4.5 billion years and the interior heats up the way a wire hanger becomes almost hot enough to burn your fingers when you bend it back and forth rapidly.

As Galileo observed, that constant heating causes volcanoes to erupt all over Io all the time, and astronomers estimate that the moon produces 100 times as much lava in a single year as do all of Earth's volcanoes combined. What was unknown was the size of the magma reservoir that feeds the constant sky fire.

To find out, UCLA's Krishan Khurana led a study reanalyzing a quirky bit of data Galileo picked up in Jupiter's magnetic field — something the investigators called a sort of "sounding signal." The anomaly would be coming not from Jupiter but from its interactions with one of its satellites. That was as far as the analysis went in the past, but much newer work in mineral physics — specifically concerning what are known as ultramafic rocks — has changed all that.

Ultramafics are igneous rocks — formed volcanically — that become capable of carrying electrical current when they are melted back down into magma. Every ultramafic has a different composition, though all are high in magnesium and iron. The precise nature of the current the rock conducts is determined by the rock's precise makeup and quantity. The signal buried in the Galileo data indicated that it came from a type of rock similar to lherzolite — a coarse-grained, relatively loosely compressed rock — within Io, and quite a lot of it.

On Earth, lherzolite is never found below certain depths since the pressure would convert into a different, denser type of rock. The same should be true on Io, and indeed, the reanalysis of the Galileo data suggests that the moon's main magma deposits are as little as 20 miles (30 km) below the surface, forming a vast ocean at least 30 miles (50 km) deep. The temperature of the ocean is a searing 2200°F (1200°C).

Why do we care? Well, there are a few reasons. First, is a world like Io cool or what? Seriously. As scientists (and science geeks) everywhere can tell you, one of the reasons we go looking for new findings is that what we discover can often flat-out amaze us — particularly when the answer was in our data cache all along, just waiting for us to get smart enough to dig it out.

Closer to home, the nature of Io may tell us a lot about the nature of Earth and our own now cold moon. "It has been suggested that both the Earth and the moon may have had similar magma oceans billions of years ago," says Torrence Johnson, a former Galileo scientist not involved in the current study. "Io's volcanism informs us how volcanoes work and provides a window in time."

Finally, the more we learn about the gravitational flexing that keeps Io's flames burning, the better we can understand how similar processes play out on its sister moon, the ice world Europa. There, the subsurface ocean that's thought to exist is made not of magma but of ordinary water — rich in minerals and hydrocarbons, with billions of years of history and enough natural heat to lead, theoretically, to life. It's Io that caught scientists' eyes first, but it's Europa — with its potentially living innards — that may be home to the true Jovian wonders.