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Eavesdropping on Europa

October 25, 2003

Sounding Europa On the Cheap: Eavesdropping On Ice

Geological Society of America — Forget drilling. A simpler and cheaper way to search for an ocean under Europa’s glacial surface is to land a solitary electronic ear on the Jovian moon, and listen to the echoes of cracking ice.

By applying a technique already tested on Arctic Sea ice, a single “geophone” listening device could reveal how the icy moon’s surface flexes, cracks and quakes with tidal forces. Just how the resulting vibrations bounce around inside the Moon-sized world could reveal the depth of the ice and extent of the potentially life-sustaining liquid ocean underneath.

Makris will present the advantages of putting an ear to Europan ice at the annual meeting of the Geological Society of America on Wednesday, October 30, in Denver, CO.

“In a way, it is an elegant approach,” says Nick Makris, an acoustical oceanographer and associate professor at the Massachusetts Institute of Technology. And with the funding for a US Europa lander mission currently in limbo, a simpler, lower-cost approach may stand a better chance of surviving budgetary cuts and actually reaching the mysterious ice world in the foreseeable future.

The principle behind the proposal is the same as that employed by ships equipped with echo-sounding bathometers, explains Makris. Bathometers have a single source of sound and then listen with a single “ear” for that sound’s echoes. By analyzing the echoes according to what’s known about the speed of sound through various materials, depths can be determined.

On Europa the sounds and seismic vibrations will not be generated by the geophone, but by the natural cracking and snapping of the ice every few days as the moon reaches the most elongated part of its oblong, 3.5-day orbit around Jupiter.

Like the Earth’s Moon, Europa keeps the same side facing its planet. But during the extreme portion of its orbit there is a tendency for Europa to shimmy a bit from side to side, causing tidal stress within the bulging ice crust that faces the giant planet.

“One scientist has described Europa as creaking like a ship,” said Makris. Exactly how much creaking and cracking goes on is unknown, he says, and it will be the first task of a geophone to find out.

Models of Europa predict that many of the cracks now seen on its surface were probably created by the tidal forces and so are probably still being created, says Makris. Although no changes in cracks have been spotted in either Voyager or Galileo imagery, neither spacecraft had the visual resolution to detect the smaller cracks that probably grow and change on a daily basis, he says.

If there is too much noise, in fact, a lone geophone could be less useful, says Makris. Constant groaning and popping would make it hard determine which snap is related to which echoes, and reveal little about the moon’s interior.

What would be ideal are a few really loud explosive cracking noises or a few meteor impacts every few days. Those would be easy for a geophone to detect, along with the seismic reflections as vibrations bounce revealingly off features within the planet.

Current rough estimates put Europa’s icy crust at about 20 kilometers thick with an ocean beneath that is at least six kilometers deep. That’s about twice as deep as Earth’s open ocean depths (not counting deep sea trenches).

The solitary geophone technique has already been tested on Arctic ice, Makris says, but the ice depth there is just a small fraction of what may separate Europa’s ocean from its surface. Also, it is winds and ocean currents that shift the Arctic ice and generate the natural noises, not tidal forces.

To perform a better field test of the geophone technique, Makris and his colleagues hope to collaborate with NASA and venture to the Antarctic. There the frozen Lake Vostok and other Antarctic deep ice sheets provide more Europa-like conditions, he says.

Despite its promise of detecting the structure of Europa, one thing a geophone cannot do is look for evidence of life under the ice, Makris points out. That will still require the far more complicated and inevitably more expensive drilling technologies that are being studied and developed by other researchers.

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