"Many ice bodies on the Earth where they contact with a rock base there is a liquid layer, but the thermal models on Mars argue against that," Plaut said in an interview. " The radar evidence doesn't compel us to come to that conclusion."
For one thing, the radar data shows the bright streaks coming nearly to the planet's surface. "It seems like it would be physically impossible (to have liquid water in that area) but we're not completely ruling it out," he said.
For now, it's one of the many question marks that dot Mars research initiatives. Scientists still can't explain what happened to all the water that must have been needed to carve Mars' surface features.
NASA has been focused on studying the planet's water history in hopes of answering an even more provocative question of whether life ever arose on Mars.
Mars Express mapped Mars' southern polar regions for four months beginning in late 2005. The scientists' findings are based on 1,800 individual measurements made during 60 orbits of the planet.
Additional studies are planned using a higher frequency radar sounder on NASA's Mars Reconnaissance Orbiter, which began science operations in November.
