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Water-ice deposits found beneath Martian hills

Apronlike reserves in mid-latitude regions largest outside Mars' poles

1:33pm, November 20, 2008
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There’s ice in them thar hills!

Using radar from an orbiting spacecraft to penetrate the hidden recesses of Mars, planetary prospectors have uncovered vast reserves of water-ice buried beneath rocky debris. The ice resides in hilly sections of the Red Planet’s southern and northern mid-latitudes and amounts to the largest reservoir of frozen water outside of Mars’ polar regions. The ice could be equal to as much as 10 percent of the volume of frozen water in the planet’s polar ice caps.

The concealed deposits, referred to as glaciers because they appear to have inched along the subsurface of the planet in the past, could be a valuable resource for future visitors — supplying drinking water or hydrogen fuel, notes Jack Holt of the University of Texas in Austin. Preserved beneath about 10 meters of dust and rocky debris, the deposits may also provide a pristine record of the Martian climate and atmosphere several hundred million years ago, when these glaciers were likely to have formed.

Holt and his colleagues, including Jim Head of Brown University in Providence, R.I., describe the radar evidence for the buried ice deposits in the Nov. 21 Science and in an upcoming Geophysical Research Letters.

The covered deposits, which resemble gentle, upward-sloping aprons, extend tens of kilometers from the edges of steep hills. Numbering in the hundreds, the buried glaciers could be the remnants of a giant ice sheet that blanketed Mars’ mid-latitudes during a past ice age, when climate models suggest the planet was tipped over so that its poles faced the sun and the mid-latitudes were much colder.

The apronlike landforms have intrigued scientists ever since the structures were spotted by the Viking spacecraft in the 1970s. The aprons are lined with ridges and overlapping wrinkles, signs that the surface was deformed due to a flow of viscous material — something akin to cold molasses. For decades, researchers assumed that the molasses was created by small pockets of ice that had filled the pores of the surface rocks, lubricating the rocky material and causing it to slowly flow in what geologists call a rock glacier.

The alternative, that the structures might be mostly ice with just a covering of dirt, wasn’t on anyone’s radar screen, recalls Head. The aprons were far away from the poles and no one imagined then that the tilt of Mars’ spin axis had varied drastically over the past 10 million years, as studies have now shown. Variations in the tilt mean that the poles were warmer in the past. Researchers hypothesize that during that time some of the frozen water at the poles evaporated into the atmosphere and then settled down onto the then-colder mid-latitude regions.

But for Head, the clincher was his first foray, a decade ago, to study the dry valleys of Antarctica. These debris-covered glaciers, which abut cliffs and mountains, appeared to be dead ringers for the features on Mars. Now, radar studies with the Mars Reconnaissance Orbiter have confirmed the existence of the Martian covered glaciers, although the size and scale of these differ from Earth’s.

Radar echoes detected by the orbiter indicated that the radio waves beamed by the craft had passed unimpeded through a thin veneer and were then reflected back from a much deeper layer without a significant loss in strength. That’s just what one would expect from a thick layer of ice blanketed by debris. Holt and his colleagues estimate the buried glaciers, some of which lie in the Hellas Basin in Mars’ southern hemisphere, are a few hundred meters thick, covered by a roughly 10-meter-deep layer of dust and rock. The team has only studied a few dozen of the several hundred aprons in the regions, Holt says.

The findings “appear to be related to changes in the planet's tilt and orbital parameters in recent epochs, that last a few tens to a few hundreds of millions of years,” says planetary scientist Jim Bell of Cornell University in Ithaca, N.Y.

The team suggests that the overlying debris accumulated from rock piles falling off of nearby cliffs. Wind-blown dust may also have added to the layer atop the glacier. In addition, bits of rock carried in the ice became more concentrated as some of the frozen water evaporated. Without the protective covering of dirt, all of the glacier ice would probably have sublimated into Mars’ thin atmosphere, erasing this record of past climate change, says Holt.

“It could very well be the case that these subsurface regions need to get added to the list of ‘potentially habitable regions’ on Mars,” says Bell. Possible sources of localized heat, such as volcanic activity or the impact of space rocks, could melt some of the ice, he notes. “Whether there is anything living down there in places like that either now or in the past … will no doubt be the subject of intense future debate and exploration,” he says.

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