To help solve the controversy, Holt's team directed the Mars orbiter to send radar echoes into some of the features in the eastern Hellas region in the planet's southern hemisphere.
The radar signal passed through the apron of material and reflected off the deeper surface below without losing much strength—just what's expected to happen in thick ice.
What's more, radio waves also passed though the material at a speed that would be expected for travel through ice.
Holt and colleagues describe their findings in this week's issue of the journal Science.
Ice Age Remnants?
Baerbel Lucchitta, a longtime astrogeologist with the U. S. Geological Survey in Flagstaff, Arizona, famously suggested in 2001 that ice—more so than liquid water—formed much of the geology on Mars.
Lucchitta, who was not involved with the new study, has long believed the slopes are made of ice.
"So I am gratified that the world is coming around to my view," she said. "Yes, I do believe there is evidence for these blankets being mostly water ice, even based on image analysis without fancy modern methods."
In past eras Mars's spin axis has tilted so that the mid-latitudes, not the poles, were the coldest parts of the planet, study co-author Holt noted.
(Related: "Snowmelt Carved Mars Gullies Later Than Thought" [August 25, 2008].)
"This is certainly the most dramatic evidence for that," he said, referring to the newly discovered glaciers.
The landforms in both the northern and southern hemispheres lie at about the same latitudes, and scientists speculate that the glaciers could be holdovers from an ice sheet that covered Mars's middle during a past ice age.
The rocky debris covering the ice may have preserved the glaciers even though the mid-latitudes are now warmer.
Joe Michalski, an astrophysicist at the Université Paris-Sud in France, said the new finding is "a very strong piece of support for the existing models of climate change on Mars.
"This is a huge amount of ice that's been transported through the atmosphere from the poles," he said.
Water Supply
Michalski pointed out that more work is needed to determine the thickness of the rocky debris that blankets the mid-latitude glaciers.
"It may be thin enough that [the ice] could be accessed for a future mission," he said.
Human missions to Mars would likely occur on 20-year timescales, he added, so having water supplies that wouldn't need to be brought from Earth would be a crucial advantage.
"If we ever send humans" to Mars, Michalski said, "we want to have resources to extract water ice for various purposes."
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