Moon of Jupiter May Conceal an Unusual Ocean

The biggest moon in the solar system, Jupiter’s monstrous Ganymede, may harbor a hidden liquid ocean under its frozen surface, a team of UCLA and JPL scientists announced Saturday.

This is “a new class of ocean if they exist,” said Torrence Johnson, chief scientist for Jet Propulsion Laboratory’s Galileo mission, which is probing Jupiter and its moons. “Wedged between ice, a trapped ocean with ice on both sides, that makes them a different type of geophysical beast.”

In exploring the solar system, many scientists take a “follow-the-water” approach, looking for traces of the precious fluid that is thought necessary to generate and sustain life.

Until a few months ago, no one suspected that Ganymede might join the list of watery satellites.

Earth, of course, holds the warm salty ocean where life on this planet began. There is evidence that two of Jupiter’s other moons, Europa and Callisto, might also have oceans beneath their icy shells.

Far less glamorous than Europa, to the point of being considered boring, the behemoth Ganymede has long been overshadowed by its smaller siblings.

“We thought it was mostly ice and not very interesting. That’s why we looked at it last,” said Thomas McCord, a geophysicist at the University of Hawaii and an expert on Jovian moons.

While the scientists are intrigued about what the findings might reveal about the formation of the moon and its geological history, they said it is unlikely that the ocean--an “ice-water-ice sandwich”--harbors life.

The new views of Ganymede come from a recent fly-by, just 503 miles above the moon’s surface, by JPL’s workhorse Galileo spacecraft, which has been orbiting Jupiter and its moons since 1995. The scientists announced their findings here at the American Geophysical Union, the world’s largest gathering of earth and planetary scientists.

The strongest evidence of an ocean comes from magnetic readings of the moon taken by Galileo and analyzed by Margaret Kivelson, a professor of space physics at UCLA. The complex readings, she said, appear to be affected by electrical currents. Those currents, she said, most likely come from a deep layer of salty water--a fluid that conducts electricity.

“It’s not the silver bullet we would like to have,” she said, “but it looks really good.”

The story of Ganymede is complex because it is the only moon known to have its own internally generated magnetic field. The field is the product, most likely, of a layer of molten rock circulating around a metallic core--the same process that creates Earth’s magnetic field.

Were Ganymede to circle the sun instead of Jupiter, it would easily be called a planet. It is larger than both Mercury and Pluto and is two-thirds the size of Mars.

Kivelson’s findings are supported by work by McCord, who used infrared scans of the moon to show that the surface has deposits of salty minerals, suggesting that salty water might have emerged from below or melted on the surface in the past.

So far, there is no suggestion of life on Ganymede. Life, said Caltech planetary scientist David Stevenson, would more likely be found on a body like Europa, where the water is next to rock and possibly surrounds hydrothermal vents that might provide the heat and energy necessary for life.

Heat would be necessary on Ganymede, where surface temperatures hover around 280 below zero.

In addition, Stevenson said, life is more likely to be found on a body with the ocean close to the surface. An ocean on Europa might lie just 20 miles below the surface. Ganymede’s is likely to lie more than 100 miles down, Kivelson estimated.

Though new evidence this year has suggested that water might once have existed in abundance on the Martian surface, there is no evidence that large amounts of life-supporting water exist there today.

On Saturday, scientists also released the best images ever taken of Ganymede, photographs shot by Galileo in May.

Ganymede is scarred by the impacts of comets and asteroids; its surface is torn and wrinkled into 2,000-foot grooved ridges. In one area, for hundreds of miles, a smooth lane crosses the distorted terrain.

“It looks a lot like a highway,” said James Head III, a professor of planetary science at Brown University. And looks a lot more like Europa than Head and his colleague Robert Pappalardo ever expected.

It appears that colder ice on the surface is being pulled apart and filled in from below with warmer ice, Head said. Such activity suggests that Ganymede is being shaped by powerful tectonic forces. The heat needed to produce those forces could come from radioactive elements deep within Ganymede.