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.
Radar echoes
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.
How old?
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.
Water source
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."
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