Buried Mars Glaciers May Be Remnants of Past Ice Age Anne Minard for National Geographic News
November 20, 2008 Low, wide glaciers half a mile thick adorn the middle latitudes of Mars, say scientists who used radar probes to peer into debris-covered formations.
The rounded slopes of material skirting steep ridges have cropped up in numerous satellite images over the years, generating controversy over whether they are mostly made of rock or ice.
Using NASA's Mars Reconnaissance Orbiter, John Holt of the University of Texas at Austin and colleagues found that the landforms appear to be water ice covered by rocky rubble.
The ice exists at much lower latitudes than any other known deposits on the Martian surface, and some experts say the trapped water has the potential to support humans in future missions to Mars.
"Altogether, these glaciers almost certainly represent the largest reservoir of water ice on Mars that's not in the polar caps," Holt said in a statement.
"Just one of the features we examined is three times larger than the city of Los Angeles … and there are many more."
As for the rock-or-ice debate, "I do think it puts to rest the controversy regarding these deposits," Holt added in an email. "But I suppose it's really up to the science community to decide that."
Radar Echoes
Most known concentrations of Martian ice exist at the poles, where temperatures are low enough to sustain large swaths of water ice.
The mysterious aprons of material had therefore puzzled scientists ever since NASA's Viking orbiters revealed them at mid-latitude in both hemispheres in the 1970s.
Although they resemble fan-shaped deposits of debris seen elsewhere on Mars, the formations are larger, steeper, and have features that suggest they were formed by an ancient flow of viscous material.
One theory contended that the landforms are flows of rocky debris lubricated by a little ice, while others suggested they are lumps of ice covered by just enough debris to prevent sublimation (turning directly from a solid to a gas).
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."