What's Mars Hiding?

1 December 2005 (All day)

ASI/NASA/ESA/University of Rome/JPL/MOLA

What lies beneath.
Red and white arcs trace the outline of a buried impact crater uncovered by the radar instrument of Mars Express.

PARIS--It may not be an underground ocean, but a first peek beneath the Martian surface has revealed some buried treasure, including impact craters and thick ice layers, that could shed new light on the geologic history of the planet. The findings come thanks to the European Mars Express spacecraft's MARSIS radar instrument, which was switched on last summer.

From orbit, MARSIS (short for Mars Advanced Radar for Subsurface and Ionospheric Sounding) probes the upper few kilometers of the martian crust by sending down radar waves and registering reflections from subsurface layers. "No other space mission has done this before," says Mars Express project scientist Agustín Chicarro of the European Space Research and Technology Centre in Noordwijk, the Netherlands.

Near the North Pole of Mars, astronomers detected radar reflections from the base of the layered ice deposits there. The data indicate that the layers are about 1.8 kilometers thick, but that the underlying crust shows almost no deflection, suggesting it is very thick. From the strength of the radar reflections, the MARSIS team deduces that the polar cap ice must be very pure, containing less than 2 percent of dust. Ice-rich material may also fill a large buried crater found at Mars's mid-northern latitudes. According to team member Jeffrey Plaut of NASA's Jet Propulsion Laboratory in Pasadena, California, the 250-kilometer crater is currently the best example of a subsurface impact structure on Mars, but a few less conspicuous ones have been hinted at by the MARSIS data, which so far cover only a tiny fraction of the planet. The new results were presented Wednesday at a press conference in Paris, coinciding with an online publication by Science.

"The find of subsurface craters is in itself not surprising," says planetary geologist Michael H. Carr of the U.S. Geological Survey in Menlo Park, California, "but if they are filled with ice, that would be a very interesting discovery, since we don't know where the water went that was present on Mars in its early history." Future MARSIS observations will hopefully shed more light on this, says Plaut. Also, a firm detection of subsurface liquid water may have to wait until next spring, when Mars Express's orbit is more favorable for detailed radar observations of the planet's low-lying Hellas Basin, where water might be closer to the surface.

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