An international team of scientists has found what may be very large amounts of ice just below the surface of Mars. But while data from a radar instrument orbiting Mars suggests water, it's not the only possible explanation.
Ice has been identified previously in polar regions of the Red Planet, but this potential ice discovery is located near the Martian equator.
Lead scientist Tom Watters of the Smithsonian Institution in Washington says the team used data from a radar instrument on the European Mars Express satellite now orbiting Mars to infer the possible presence of ice just under the surface.
"The radar wave in certain cases will not only reflect off the surface of Mars," Waters said, "but if the materials have a certain property, part of the energy will be transmitted through the material, so the [radar] sounder [instrument] literally allows us, in certain places on Mars, to see through the material."
The way the radar waves are scattered and delayed by the material beneath the surface helps the scientists determine some of the properties of the material belowground.
In a paper published online in SciencExpress, Watters and his colleagues are careful to say that what they found is consistent with ice, but they don't rule out the possibility that it may be some other material with similar properties.
"One of the problems with the [radar] sounder is it doesn't exactly allow us to tell definitively between the two possibilities — this very low density, unconsolidated dry material and, on the other hand, this very ice-rich material."
Watters says one reason for thinking that what they have found is ice, not just loosely-packed dirt, is that the formations they studied are up to several kilometers thick, and they likely would have compressed down of their own weight unless there were bits of ice separating the bits of Martian soil.
So would this Martian ice mixture resemble anything on Earth? "It would look probably like a dirty snowball. It would be maybe a mix of ice with as much as 15 or so percent dust or dirt in it," Watters says.
If it is ice, there is a lot of it, and it's in a place (an equatorial rock formation named Medusae-Fossae) that is more accessible to future human visitors than the ice at higher latitudes.
"If the Medusae Fossae deposits are ice-rich, in volume it is approximately equal to the volume of ice at the South Pole. So it would mean we have another source of water, a huge source of water, in the equatorial zone of Mars. And the equatorial zone of Mars is a lot easier to get to than the poles of Mars."
Watters says that they will continue to collect and analyze data to see if they can be more certain about their conclusions.
"The more data we have," he says, "the more we'll be able to refine the interpretation and narrow our possibilities. But ultimately I think the only way we will definitively know is to go there and to sample the material itself."
Tom Watters is a senior scientist at the Smithsonian's Center for Earth and Planetary Studies in Washington.