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Planetary geology
A watery moon?

Early last year, images from NASA’s Cassini spacecraft revealed a huge plume of water-ice spewing from Enceladus, a tiny moon orbiting Saturn. For more than a year, astronomers have been piecing together evidence and they are finding that the plume stems from underground, indicating that Enceladus is one of the few objects in the solar system known to support active volcanism.

This false-color image emphasizes cracks in Enceladus’ surface through which a water-ice plume escapes. The younger surface of the southern hemisphere, made evident by geologic activity and cracks, contrasts the older cratered surface of the northern hemisphere. Image is courtesy of NASA/JPL/Space Science Institute.

The discovery that a moon seven times smaller than Earth’s moon could sustain geologic activity came as a surprise to some astronomers (see Geotimes, February 2006). Initial surprise, however, gave way to the formulation of new models to explain the activity.

Publishing in the March 10 Science, Carolyn Porco, the Cassini imaging team leader at the Space Science Institute in Boulder, Colo., and colleagues describe the physical processes that they think produce the Old Faithful-like geyser at the moon’s southern pole.

Porco first looked at the images from Cassini’s three flybys in 2005 and thought that the plume stemmed from cracks near the moon’s southern pole. But planetary geologists also had not ruled out that the geyser formed from the sun warming the icy surface and producing vapors, which could then condense into a plume in a process similar to the formation of a comet’s tail.

The new models show, however, that the most likely source of the geyser is an underground reservoir of liquid water. They demonstrated that subsurface liquid water erupting at the surface could explain the observed dynamics of particles in the plume, similar to the way Yellowstone’s Old Faithful works.

“Almost undoubtedly,” some mechanism is needed “where you can get liquid water,” says Peter Thomas, a senior research associate at the Cornell Center for Radiophysics and Space Research in Ithaca, N.Y. Cassini measured temperatures on Enceladus’ surface to be well below freezing, “not warm enough to drive the plume,” Thomas says. Below the surface, however, models suggest that warmer temperatures could sustain liquid water. Similar to processes on Earth, that water could boil, and if a crack forms in the ice, it could erupt as a plume of ice and gas.

Still, the model and images turn up new questions about the range of geologic features observed on Enceladus, such as why geologic activity is currently limited to the southern pole. The density of surface craters show that geologic activity has been present on Enceladus for more than 4 billion years, according to Porco, but the surfaces at the south are as young as a half a million years.

Remnants of old craters remain on most of the planet, but the southern pole is “littered” with house-sized ice boulders, “straddled” with 130-kilometer-long fractures and nearly free of craters, Porco and colleagues wrote. One possibility is that a nonuniform interior — created by the moon’s original formation, subsequent tectonics, gravitational forces or some other process — concentrates heating at the southern pole.

To find out, Thomas says that researchers will need to more deeply analyze specific tectonic patterns and associated stresses. But what’s currently apparent, Thomas says, is that Enceladus’ interior is far from a “perfect onion skin model,” and contains “all different types of geology.”

The amount of subsurface liquid water is likely small, Thomas says, but a gravity survey of the moon could help “nail down” where water might exist in its interior. “It’s a whole different way of hunting for water,” Thomas says. “Apparently, there are many ways to hide liquid water.”

Kathryn Hansen

Links:
"Tiny moon, gigantic geyser," Geotimes, February 2006

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