Simple probe could measure Europa's ocean and icy shell. (Echoes of Icequakes).

A football-size space probe could provide a low-cost way to determine whether there's a liquid ocean on the Jovian moon Europa. The ice-sheathed, nearly airless world is just a bit smaller than our own moon, and it stands as one of the solar system's prime candidates for hosting life beyond Earth. Discussions among scientists about small, discount probes to be sent to Europa have intensified since funds earmarked for a full-scale orbiter were dropped from the President's 2003 budget proposal.

The probe would act as an electronic ear, detecting sound waves traveling through Europa's icy crust, says Nicholas C. Makris, an acoustical oceanographer at the Massachusetts Institute of Technology. Those reverberations could be generated by a variety of sources, including the occasional impacts of objects falling from space. However, Makris contends that if there is a liquid ocean within Europa's thick shell of ice, its tidal motions could provide a nearly continuous source of seismic shivers.

Europa orbits Jupiter, its massive parent planet, once every 3.5 days at an average distance of about 671,000 kilometers, or roughly twice the distance from Earth to our moon. Forces that would cause tides on Europa arise because the moon wobbles slightly as it sweeps through its oblong path. Scientists estimate that tides in Europa's underground ocean, if one is present, could heave the moon's icy surface up and down through a range of about 30 meters during each orbit.

All that flexing would generate cracks, and measurements of the seismic waves generated by the fracturing ice would provide a wealth of information. Scientists could calculate the thickness of Europa's crust by using the time lag between the arrival of seismic waves that traveled directly to the instrument and those that first bounced off the lower surface of the crust. The time it takes for subsequent echoes to reach the sensor could provide information about the depth of the moon's liquid ocean and might even enable researchers to map the ocean bottom under the probe. Makris described the techniques this week at a Geological Society of America meeting in Denver.

These depth-sounding methods are similar to ones that oceanographers use to map the seafloor on Earth, says Makris. Geologists also use such seismic analyses to infer Earth's inner structure. Tests on pack ice floating in the Arctic Ocean show that the proposed techniques can be used to map the thickness of ice, determine ocean depth, and even detect layers of different temperature or density within the ocean.

Makris and his colleagues next intend to test their echo-sounding methods on the ice shelves that fringe Antarctica. Those ice masses often stretch dozens of kilometers from the continent's coast and undergo tidal flexing just as Europa's crust does.

A probe like the one Makris envisions could put an end to debates about the thickness of Europa's ice and whether the moon indeed has a liquid ocean, says Paul M. Schenk, a geologist at the Lunar and Planetary Institute in Houston.