"The instrument is designed with the sensitivity to see as deep as five kilometers (three miles)," says Jeff Plaut, NASA's MARSIS project manager at the Jet Propulsion Laboratory. "The best chance to detect water will be if it lies in the upper one or two kilometers (one mile)."

Scientists used a similar technique in 1995 to glimpse signs of water ice near the moon's south pole. In that instance, they used the radio dish on a moon-orbiting satellite like a flashlight to beam radar into craters that are perpetually shrouded in shadow.

Radio telescopes on Earth then looked for the radar reflections, detecting stronger returns where ice crystals were present in the soil.

The MARSIS experiment will use longer waves of radar, in some cases with wavelengths longer than a football field, to plumb far deeper for water on Mars than was possible on the moon.

Many planetary scientists feel that's the last place substantial reservoirs of water may exist.

Billions of years ago Mars had a thick enough atmosphere to support liquid water, thanks in part to a massive network of active, gas-belching volcanoes. As Mars cooled and the volcanoes became dormant, the planet's feeble gravity, 38 percent that of Earth's, lost most of its atmosphere and ability to hold onto surface water.

Besides water, MARSIS eyes will see subterranean lava beds, sheets of sand, sediments, and debris from impacts - a veritable field trip beneath the surface of Mars for geologists.

The device will also make soundings of Mars' ionosphere, according to Don Gurnett, a University of Iowa physicist who received a $5 million NASA grant to develop the radar's antennas and radio transmitter.

The effort also involves the University of Rome, with the Italian Space Agency underwriting half the project's $20 million total pricetag.