A European spacecraft has detected deep deposits of water ice beneath Mars' surface near the planet's north pole. This finding and the hint of ice in buried craters elsewhere on Mars add strong evidence to earlier U.S. space agency findings suggesting water flowed freely in the Red Planet's early days when it was warmer and wetter.
The first radar soundings of the Martian underground were taken by the European Space Agency's Mars Express earlier this year and the results just published in the journal Science.
The spacecraft arrived at Mars in December 2003, but 13 months passed before its radar could be used because of a delay in the instrument's deployment. Although the results are late in coming, they have not been disappointing to the scientists interpreting the data.
The radar bounces sound waves off layers below the surface. The way they reflect can show height variations or provide information about a material's composition and texture.
One of the scientists reading the radar data, Jeffrey Plaut of the U.S. space agency NASA, says they reveal almost pure ice deposits in layers buried between the Martian surface and a lower layer probably composed of sand mixed with ice. He told reporters at European Space Agency headquarters in Paris that this interpretation is based on radar images echoed off the top of the apparent ice and the sandy mixture below it.
"The results of our analysis of the characteristics of these two signals suggests that there is about 1.8 kilometers thickness of water ice between these two reflections and that from the intensity of the lower reflection, that water ice has to be nearly pure," he said. "Our absorption measurement indicates that the dust component has to be very small, on the order of two percent or less."
At the equatorial region of Mars, the Mars Express radar detected a shallow 250-kilometer diameter ring below the surface that might be an impact crater not previously detected by U.S. spacecraft orbiting the planet. The researchers suggest that it might contain a thick layer of material rich in water ice.
Another paper in the journal Nature describes further hints of ancient Martian water. A second Mars Express instrument that measures infrared light emissions found many places containing clays, the result of water mixing with volcanic rock immersed in it for a long time. It also detected compounds called hydrated sulfates that form as deposits from salt water.
Together, the findings support previous evidence from orbiting U.S. satellites and two robotic rovers on the ground suggesting Mars once had surface water that could have supported primitive life forms. However, those instruments could not determine when the water existed.
The Mars Express scientists have combined their data with those from the other instruments to conclude that water covered the Red Planet only in its youth, then dried up.
"We may have to revise some of our previous views," said researcher Gerhard Neukum of the Free University of Berlin. "So the early warm and wet Mars - it wasn't so warm and not so wet, at least during much of its time. It fell dry very early, 3.5 billion years ago on a global scale, and that's remarkable."
Where did the water go? One guess is that during its early days, Mars lost its magnetic field that, like Earth's, shielded its thick atmosphere from the Sun's fierce radiation. Solar particles are thought to have stripped Mars of its carbon dioxide atmosphere, evaporating its oceans and killing whatever simple life forms it might have had, leaving only buried ice.
A Mars Express scientist from the French Institute of Astrophysics, Jean-Pierre Bibring, suggests that future investigations focus on the clays the orbiter detected as a possible source for remnants of such life.
"If ever life started on Mars, that is probably in what we call the clay-rich areas. More than that, the clay minerals themselves could have been the niche in which, if biology really started, it could still be preserved," he said. "If one is in a future mission looking for biologic relics, we should go to the clay minerals to be returned to the Earth."
In the meantime, NASA's Jeffrey Plaut says Mars Express will search in earnest next year for any liquid water that might possibly remain on or near the martian surface.