Mars has vast glaciers hidden under aprons of rocky debris near mid-latitude mountains, a new study confirms, pointing to a new and large potential reservoir of life-supporting water on the planet.
These mounds of ice exist at much lower latitudes than any ice previously found on the red planet.
"Altogether, these glaciers almost certainly represent the largest reservoir of water ice on Mars that's not in the polar caps," said John Holt of the University of Texas at Austin and the main author of the study. "Just one of the features we examined is three times larger than the city of Los Angeles and up to one-half-mile thick, and there are many more."
The gently sloping mid-latitude debris flows have puzzled scientists since they were revealed by NASA's Viking orbiters in the 1970s — they looked very different than the fans and cones of debris found near mountains and cliffs in Mars' equatorial regions.
Since their discovery, scientists have been debating how the features formed, with some proposing they were debris flows lubricated by ice that had since evaporated away. But more recent observations suggested that the features "might be more ice than rock," Holt said. In other words, they could be Martian glaciers.
Holt and his colleagues used radar observations of the features, taken by NASA's Mars Reconnaissance Orbiter (MRO), to peer into the features. The findings, detailed in the Nov. 21 issue of the journal Science, suggest that the glacier theory is correct.
Finding huge deposits of ice at the Martian mid-latitudes is a boon to both the study of past potential Martian habitability, as well as future human exploration. Glaciers are huge reservoirs of water once they melt, key to all life as we know it.
The team used MRO's Shallow Radar instrument to penetrate the rocky debris flows that lie in the Hellas Basin region of Mars' southern hemisphere. They examined the radar echoes to see what lay beneath the surface. All signs pointed to ice, and lots of it.
The radar echoes received back by MRO indicated that radio waves passed through the overlying debris material and reflected off a deeper surface below without losing much strength — the expected signal for thick ice covered by a thin layer of debris.
The radar echoes also showed no signs of significant rock debris within the glaciers, suggesting that they are relatively pure water ice.
"These results are the smoking gun pointing to the presence of large amounts of water ice at these latitudes," said Ali Safaeinili, a Shallow Rader team member at NASA's Jet Propulsion Laboratory in Pasadena, Calif.
The sheer amount of ice present in the flows studied was surprising; extrapolating from the Hellas Basin feature to the many others present in both Martian hemispheres, there seems to be a lot of ice hiding under the Martian surface.
The researchers estimate that the amount of ice in these mid-latitude glaciers is about 1 percent of the ice that's in Mars' polar caps — roughly equivalent to the ratio of Earth's non-polar glaciers to its polar ice, Holt told SPACE.com. The glaciers could hold as much as 10 percent of the ice in the polar caps, similar to comparing Greenland's ice sheets to Antarctica, Holt added.
But just how the ice got there is still a mystery.
"You shouldn't have ice of this quantity at these latitudes," Holt said.
The theory is that the ice formed when Mars' orbital tilt was much different than it is now (the axis the planet spins on has considerable "wobble," meaning its angle changes over time) and the planet was much colder, allowing ice to form on the surface.
Ice on the surface of Mars today would immediately sublimate (or change directly into the gas phase). The rocky debris covering the ice is likely what keeps it in place today and has allowed it to survive below the surface for millions of years.
Scientists aren't exactly sure during which past ice age the glaciers may have formed, but by counting the number of impact craters in the overlying debris, they estimate them to be about 100 million years old, said study team member Jim Head of Brown University in Providence, R.I.
These ancient glaciers could hold clues that would shed more light on Mars' past, particularly whether or not it ever harbored life.
"On Earth," Head said, "such buried glacial ice in Antarctica preserves the record of traces of ancient organisms and past climate history."
Ancient ice layers in glaciers on Earth preserve the signature of the current atmosphere at the time that they formed. Head thinks the same could be true of the Martian glaciers. In particular, small bubbles that form as the ice layers are deposited could have "samples of the atmosphere at that time," he said.
A lander capable of drilling down several meters could be able to sample the ice in the glaciers.
"These are quite accessible to landers," Holt said.
They could also be a source of water for any future manned Mars expeditions. (When the researchers travel to Antarctica, for instance, they simply knock off chunks of ice and melt them instead of lugging water with them.)
"It's a lot of ice," Holt said. "You could support a base for a long time."