Thick bands of ice (blue) have been spotted in steep cliff faces.


Ice cliffs spotted on Mars

For more than a decade, Colin Dundas, a geologist at the U.S. Geological Survey in Flagstaff, Arizona, has had a daily routine: inspecting a dozen or so high-resolution images beamed back every day from the Mars Reconnaissance Orbiter (MRO). A few years ago, something surprising popped out from the planet’s sea of rust: a pale sliver of blue.

What Dundas saw that day, and subsequently found at seven other sites, are steep cliffs, up to 100 meters tall, that expose what appears to be nearly pure ice. The discovery points to large stores of underground ice buried only a meter or two below the surface at surprisingly low martian latitudes. “This kind of ice is more widespread than previously thought,” says Dundas, who, with his co-authors, describes the cliffs this week in Science. Each cliff seems to be the naked face of a glacier, tantalizing scientists with the promise of a layer-cake record of past martian climates and space enthusiasts with a potential resource for future human bases.

Finding ice on Mars is nothing new. Ice covers the poles, and a radar instrument on the MRO has detected signatures of thick, buried ice across the planet’s belly. Some researchers suggested these deposits could be the remnants of glaciers that existed millions of years ago when the planet’s spin axis and orbit were different. But the depth of the ice and whether it exists as relatively pure sheets or as granules frozen in the pore spaces of martian soil have been uncertain.

A decade ago, researchers using the MRO spotted a related clue: pools of seemingly pure ice in the floors of small craters carved out by fresh meteorite impacts. But it was unclear whether these frozen pools were connected to the buried glaciers or were merely isolated patches. At the ice cliffs, Dundas and his team could see the glaciers in cross section, and they patiently revisited the sites to see how they changed over time.

They found that the ice persisted through the martian summer, when any ephemeral frost would have vaporized. And last year, the MRO caught several boulders tumbling out of one of the cliff faces, suggesting that gradual erosion had released them from a massive ice deposit. Evidently the near-surface ice and the large subsurface deposits are one and the same, says Ali Bramson, a co-author and graduate student at the University of Arizona in Tucson. “This deep, thick, pure ice extends almost all the way up to the surface.”

Banding and subtly varying shades of blue suggest that the slabs of ice are stacked. That implies that the deposits built up over many seasons as layers of snow were compressed in a previous climate cycle, says Susan Conway, a planetary geologist at the University of Nantes in France. Winds then buried the ice sheets in grit. “It’s the only reasonable explanation,” she says.

Drilling a core from one of these deposits and returning it to Earth would offer a treasure trove of information to geologists about the past martian climate, says G. Scott Hubbard, a space scientist at Stanford University in Palo Alto, California. “That preserved record would be of extreme importance to go back to,” he says.

These sites are “very exciting” for potential human bases as well, says Angel Abbud-Madrid, director of the Center for Space Resources at the Colorado School of Mines in Golden, who led a recent NASA study exploring potential landing sites for astronauts. Water is a crucial resource for astronauts, because it could be combined with carbon dioxide, the main ingredient in Mars’s atmosphere, to create oxygen to breathe and methane, a rocket propellant. And although researchers suspected the subsurface glaciers existed, they would only be a useful resource if they were no more than a few meters below the surface. The ice cliffs promise abundant, accessible ice, Abbud-Madrid says.

The cliffs are all found at latitudes about 55° north or south, however, which grow frigid and dark in the martian winter—unpromising latitudes for a solar-powered human base. For this reason, the NASA study was limited to sites to within 50° of the equator. Now, Hubbard wants NASA’s human exploration program to look for similar cliffs closer to the equator. “What’s the cutoff point?” he asks. He hopes the next surprise will be ice closer to the martian tropics.