Mars' ice is pure, not dirty

by Carolyn Gramling
Thursday, January 5, 2012

Blogging on EARTH

Finding signs of water on Mars feels nearly old hat by now. We know Mars had a lot more water in its past, has polar ice deposits and even has a kind of water cycle, driven by the formation of water-ice clouds in the Martian atmosphere. Scientists have also found signs of "dirty ice" — a 50-50 mix of ice and soil — revealed to be lurking beneath the upper layers of Martian soil by the impacts of meteorites. So when the Phoenix lander recently discovered pure water ice in the subsurface, that was exciting — but it was still thought to be an anomaly.

But now it turns out that pure ice may actually be the norm in the subsurface of Mars. In today's issue of Science, a team of scientists from NASA's Mars Reconnaissance Orbiter team announced new data that suggest a subsurface layer of pure ice, just a meter or two below the surface, might extend over as much as half of the planet, from the poles to the mid-latitudes.

The team talked about their finding Thursday at a NASA press conference (right on the heels of an earlier high-profile water-on-the-moon conference). In late 2008, they said, using data from MRO's high-resolution, greyscale Context Camera, the team identified five very fresh impact craters on Mars — from meteorites that had struck in the early part of 2008. Those impacts had exposed the subsurface ice, as expected.

But what wasn't expected, the researchers said, was how quickly the ice sublimated, a process that they were able to watch via MRO's cameras. Ice is unstable when exposed at the surface of Mars, vanishing away into water vapor. But the speed of the sublimation suggested that the water was nearly pure. Other data, from MRO's Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), confirmed the diagnosis: pure water ice.

This happened at all five sites, too, which suggested that the layer of pure ice was fairly extensive, a broad sheet of ice that could extend over much of the planet's surface — possibly a total volume of water comparable to the volume of the Greenland ice sheet.

"We were able to conclude that the ice is a relic of a previously wetter climate," said Shane Byrne, a team member of MRO's High Resolution Imaging Science Experiment (HiRISE). In fact, there might have been as much as twice as much water on Mars only 10,000 years ago, relative to today.

But even that more humid Martian atmosphere wouldn't have been enough to allow liquid water to be stable at Mars' surface — or to support life, Byrne said.

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