Formed sometime between January and September 2008, this fresh crater has dredged up barely buried water ice and splashed it onto the Martian surface. The HiRISE camera aboard NASA's Mars Reconnaissance Orbiter recorded this colour close-up image on 1 November 2008. The scene is about 30 metres across. (Image: NASA/JPL/University of Arizona)
Mars has a layer of ice as shallow as a few tens of centimeters below the surface. If the Viking lander had been able to dig deeper it would have found it in the 1970s. Analysis of recent impact craters show the exposed ice, which then is sublimated into the atmosphere.
The water ice seems to have a high salt content based on the analysis.
BBC news has more coverage
Over tens to hundreds of millions of years, the ice has been transported to lower latitudes. We have found evidence for huge tropical mountain glaciers where the sides of big volcanoes at the equator have these huge deposits - 170,000 sq km - on their north-west flanks that are caused by big changes in Mars' obliquity.
On Earth, obliquity variations actually caused the Ice Ages that we experienced over the last tens of thousands of years. But changes in Mars' obliquity have been significantly greater. So we're seeing evidence for ice having been transported all the way down to the equator.
Some of it has gone to make the polar caps. Some amount, certainly, has left the planet through dissociation in the upper atmosphere. But what we're finding is that a significant amount may have been sequestered in glacial deposits.
Data from the Sharad (SHAllow RADar) instrument (on Mars Reconnaissance Orbiter), document that some of these glacial-like features, which you find at Mars' mid-latitudes, have a significant volume of ice left below a surface of rock debris.
The expected depth to ground ice is close to 84cm while the crater depth is 65-70cm. This particular model uses an average water vapor concentration of 20pr μm and these new data are so far consistent with this value or perhaps one slightly higher. This contrasts with the current observations of average atmospheric water vapor of ~14pr μm or ~10pr μm. Thus the ground ice exposed here is probably in the process of retreat from a previously larger extent perhaps due to recent variations in the argument of perihelion
New Scientist also has coverage.
The disappearing act might also be due in part to a coating of dust blown in from the atmosphere. Either way, notes HiRISE investigator Shane Byrne of the University of Arizona, the icy deposits had to be at least a couple of inches (several centimetres) thick, and they couldn't have been unearthed from more than a foot or two (0.3-0.6 m) down.
Byrne announced these findings on Friday at the Lunar and Planetary Science Conference in The Woodlands, Texas. He points out that prior surveys, particularly one done by the neutron spectrometer aboard NASA's Mars Odyssey orbiter, show that vast reservoirs of ice lay barely buried across most of the planet's polar and mid-latitude regions.