Mars clearly had a watery past, and it's expected that much of the water is still on the planet. Figuring out where the ice is hiding could tell us a lot about the planet's climate history and something about Mars' current water cycle. It could also help direct future landers to sample the planet's water and possibly use it to support human landings.

While we've found plenty of ice near the pole during the Phoenix Lander mission, that's not a very convenient location for future landings (in part because the site ended up frozen over with dry ice during that pole's Martian winter). In today's issue of Science, researchers are reporting the likely presence of ice sheets in more temperate regions. The sheets are at least 100 meters thick and appear to preserve layers that may help us reconstruct how the water ended up frozen there.

MRO data

As with many things Martian, the work relies on data from the Mars Reconnaissance Orbiter. It has a variety of instruments that can probe the chemical composition and subsurface structure of Mars, along with the best camera we've ever sent to another planet. Over the years, MRO has built up a comprehensive catalog of features on the Martian surface, many of them imaged from multiple angles.

Analyzing these features with a filter that accentuates colors, a team of researchers saw something notable for the Red Planet: a number of them had a distinctively blue color. They were found at mid-latitudes (roughly the Martian equivalent of Canada or the UK) and occupied poleward-facing slopes. The slopes appear to be the product of erosion along the edges of a broad, smooth elevated plain. Imaging with a spectrograph provided evidence of water at the Martian surface in the region, reinforcing the idea that this could be ice.

The bluish slopes were rather steep, having a slope that approached 55 degrees in some cases. Their lower reaches were covered in rubble, making it difficult to determine the total thickness of any ice deposits. Still, whatever it is is more than 100 meters thick and probably at least 130 meters. That's thick enough for the orbiting camera to resolve different colored bands within the material. That suggests the bands were deposited over time and trap different periods in Mars' history. A lack of craters indicates that some of that history could be quite recent.

The authors favor the idea that what they've found is indeed ice, probably mixed with dust, and was deposited during a time when Mars experienced snow. "The presence of banding and color variations suggest layers," they argue, "possibly deposited with changes in the proportion of ice and dust under varying climate conditions." Thus, examining the layers could tell us about the history of how Mars' watery past came to a close.

Lots of ice

Currently, the ice sheets appear to be covered by a very shallow layer of dust that's frozen in place—the authors estimate that this is less than two meters thick. The slopes are probably being continuously exposed as the ice sublimates into the Martian atmosphere, likely to cycle up to the poles and end up frozen there. The researchers estimate that this is causing the loss of about a millimeter a year, which suggests that the ice sheets were once considerably larger than they are today.

These visible ice sheets are likely just a small representative of the total water ice on Mars. Radar studies of the subsurface have found features that have been interpreted as dust and rock covered glaciers and some indications of ice sheets in other areas of the Red Planet. But of course it's hard to confirm the identity of the layers seen in radar echoes, and the instrument doesn't have the resolution to figure out how close the ice might be to the surface beyond "less than 20 meters."

So, if these slopes still look like ice after more detailed examination, they seem like a great location to study the history of water on Mars. They could also make for accessible sites to extract water for human use, although that would obviously conflict with studying the ice's layers for clues to the past.

Science, 2017. DOI: 10.1126/science.aao1619 (About DOIs).