Locked away beneath the surface of Mars are vast quantities of water ice. But the properties of that ice—how pure it is, how deep it goes, what shape it takes—remain a mystery to planetary geologists. Those things matter to mission planners, too: Future visitors to Mars, be they short-term sojourners or long-term settlers, will need to understand the planet's subsurface ice reserves if they want to mine it for drinking, growing crops, or converting into hydrogen for fuel, Wired says.
Trouble is, dirt, rocks, and other surface-level contaminants make it hard to study the stuff. Mars landers can dig or drill into the first few centimeters of the planet's surface, and radar can give researchers a sense of what lies tens-of-meters below the surface. But the ice content of the geology in between—the first 20 meters or so—is largely uncharacterized.
Fortunately, land erodes. Forget radar and drilling robots: Locate a spot of land laid bare by time, and you have a direct line of sight on Mars' subterranean layers—and any ice deposited there.
Now, scientists have discovered such a site. In fact, with the help of HiRISE, a powerful camera aboard NASA's Mars Reconnaissance Orbiter, they've found several.
In a recent issue of Science, researchers led by USGS planetary geologist Colin Dundas present detailed observations of eight Martian regions where erosion has uncovered large, steep cross-sections of underlying ice. It’s not just the volume of water they found (it's no mystery that Mars harbors a lot of ice in these particular regions), it’s how mineable it promises to be. The deposits begin at depths as shallow as one meter and extend upwards of 100 meters into the planet. The researchers don't estimate the quantity of ice present, but they do note that the amount of ice near the surface is likely more extensive than the few locations where it's exposed. And what's more, the ice looks pretty damn pure.
NASA calls the use of space-based resources “in-situ resource utilization,” and the agency thinks it will be essential to survival in deep space. Of particular interest to ISRU planners is the depth of the ice, and the ratio of pure ice to that mixed in with bits of Mars regolith. The more pristine the ice, and the closer it is to the surface, the less energy it takes to extract and use.
The ice found this time isn’t crystal clear. Over years, observations showed that the ice is slowly surrendering water to the atmosphere through a process called sublimation, and signs suggest that boulders and sediment are dislodging from the ice as it recedes. But some debris is to be expected. Dundas and his colleagues hypothesize that the ice originated as snow, falling in waves over millions of years. Some rocky material probably found its way in, in between snow events—but the surrounding ice, the researchers think, is relatively clean.