Thick sheets of water ice, some barely buried beneath the surface and likely more than 100 meters thick, have been spotted on several Martian cliff faces.

Geologists hoping to study the past climate history of Mars — and visionaries planning future visits by astronauts — got some great news with the discovery that exposures of water ice have been spotted on cliff faces.

The widely scattered outcrops, seven in the southern hemisphere and one in the north, lie at latitudes of 55° to 58° — far from the planet's polar caps of water (and carbon-dioxide) ice.

Colin Dundas (U.S. Geological Survey, Flagstaff) led the team that made the discovery using two instruments aboard NASA's Mars Reconnaissance Orbiter. First, detailed images from the spacecraft's HiRISE camera revealed banded layers in the scarps' steep faces that had a bluer color than their surroundings. Then near-infrared maps from the CRISM spectrometer confirmed that the layers were strongly enriched in water ice.

It quickly became clear that the icy exposures were not merely seasonal coatings of frost but rather distinct layers of ice that persist year-round. They begin within a few meters of the surface and sometimes extend down to more than 100 m.

The team surmises that they started out as dusty snow or frost layers laid down over time that later compacted and recrystallized. The massive exposures are fresh because the scarps are actively eroding and retreating at a rate of a few millimeters each summer. Apparently the exposed ice gradually sublimates (changes directly from frozen to vapor), causing any embedded rocky material to crumble away and fall to the foot of each cliff face. What's left behind are constantly refreshed exposures of previously buried ice.

Planetary scientists have realized for more than a decade that vast deposits of water ice must lie just below the planet's dusty surface. For example, in 2008 the Phoenix lander found both ice-cemented soil and slabs of nearly pure ice covered by a thin veneer of rocky rubble. Radar scans from orbit have revealed that huge glaciers of ice lie within 20 m of the surface — and likely are often much shallower — over roughly a third of the Martian surface.

Deposits of ice on Mars, laid down over millions of years, lie exposed in the planet's polar caps. But these new-found outcrops open an unprecedented window into Martian climatic and geologic history. "It's like having one of those ant farms where you can see through the glass on the side to learn about what's usually hidden beneath the ground," comments co-author Shane Byrne (University of Arizona) in a NASA press release.

Given the planet's thin atmosphere and temperature swings, geophysicists calculate that water ice on Mars at the scarps' locations should be stable at depths of as little as 10 cm (4 inches). Being able to access water so easily would be a huge boon to future human exploration of the planet. Heck, imagine if "Martian" Mark Watney (a.k.a. Matt Damon) merely had to scrape a few inches into the ground to get all the water needed to sustain his potato crop!

The full study, "Exposed subsurface ice sheets in the Martian mid-latitudes," appears in the January 12th issue of Science.