This story was updated at 6:32 p.m. EDT.
Craters
gouged into the ruddy Martian terrain have revealed subsurface water ice closer
to the red planet's equator than would be expected, new orbiter images show.
The ice
also seems to be 99 percent pure, instead of the dirty dust and ice mixture
some scientists expected to see, scientists said today.
And while
numerous surface features on Mars suggest that water
once flowed on the red planet in the past, the new discovery - detailed in
the Sept. 25 issue of the journal Science - adds to the evidence that has been
piling up in recent years that water exists on present-day Mars, in the form of
subsurface ice. It also gives scientists a way to further probe the Martian
surface for signs of water ice.
"We were able to conclude that this ice is a relic of a previously wetter climate," said research team member Shane Byrne of the University of Arizona in a Thursday teleconference.
Because
water is essential to life as we know it, any findings of potentially
once-liquid water has implications for the search for evidence of possible
past Martian life.
The new observations indicate the presence of vast sheets of ice buried beneath the Martian surface left over from when the planet?s ice caps covered more of the planet, researchers said. The ice averages about a meter thick and contains about the same amount of frozen water as the Greenland ice sheet on Earth, science team member Ken Edgett added.
"These buried ice sheets that extend from the poles all the way down to 45 degrees [latitude] or so don't quite cover half of the planet, but come close to covering half the planet," Byrne said. "So we're talking about maybe about a million cubic kilometers of ice in total."
The Mars
ice finding comes just one day after scientists announced new evidence
for water ice on Earth's moon.
Found just in time
In August
2008, members of the Mars Reconnaissance Orbiter's (MRO) Context camera team
examined images of the northern Martian mid-latitudes taken by the camera for
any dark spots or other changes not seen in earlier images. These dark marks
are signs of meteorites that have recently crashed into the dust-covered
Martian terrain.
They found
several, and the following month, members of MRO's HiRISE camera team followed
up by snapping high-resolution images of these suspected impact craters.
"We
saw something very unusual when we followed up on the first of these impact
craters," said Byrne, a HiRISE team member, "and that was this bright
blue material poking up from the bottom of the crater. It looked a lot like
water ice."
A few days
later, MRO's Compact Reconnaissance Imaging Spectrometer (CRISM) was used to
take the spectrum of the material and, sure enough, it found the spectral signature
of water ice.
?It was just crystal clear, no doubt about it, water ice,? said Selby Cull, a CRISM science team member at Washington University in St. Louis, Mo., of one such signal.
The
behavior of the material over the ensuing days also helped clinch its identity:
"When we started monitoring the material, it faded away like you'd expect
water ice to fade, because water ice is unstable on Mars' surface and turns
directly into water vapor in the atmosphere," Byrne explained.
The craters are small, about the size of a small office room, with the exposed ice patches themselves about the size of a desk, researchers said. They?re shallow too.
?So these are quite small, small features,? Byrne told reporters Thursday. ?If you were standing in one of these craters, they would only be about knee deep or so.?
The
relatively quick disappearance of the ice means the MRO teams were fortunate to
have spotted the craters when they did.
"All
of this had to happen very quickly because 200 days after we first saw the ice,
it was gone, it was the color of dirt," Byrne said. "If we had taken
HiRISE images just a few months later, we wouldn't have noticed anything
unusual. This discovery would have just passed us by."
Further evidence
The
evidence of these ice layers exposed by meteorite impacts stacks on top of
other recently uncovered clues, including the excavation of a shallow
water ice layer by NASA's Phoenix Mars Lander last year.
But Phoenix
was at a more northerly locale than the new craters, so this fresh evidence
shows that the subsurface water ice extends further south than previously
thought.
"We
knew there was ice below the surface at high latitudes of Mars, but we find
that it extends far closer to the equator than you would think," Byrne
said.
Also
surprising was how clean the water ice was (something Phoenix also observed,
along with the gradual sublimation of exposed ice).
?There?s not a lot of dirt mixed in,? Cull said. ?It?s pretty much solid ice.?
"The
thinking before was that ice accumulates below the surface between soil grains,
so there would be a 50-50 mix of dirt and ice," Byrne said. "We were
able to figure out, given how long it took that ice to fade from view, that the
mixture is about one percent dirt and 99 percent ice."
The craters
are about 12 feet (, which ranged from 1.5 to 8 feet (about 0.5 to 2.4 meters)
deep, were located at five Martian sites.
Though the
MRO researchers had identified 80 to 90 craters around the Martian globe
before, this was the first time the spotted ice in the bottoms, likely because
most of the others were more southerly and outside of the likely area of
subsurface water ice.
Byrne told
SPACE.com that it was surprising to the team to find the bluish ice, though
"in retrospect maybe it shouldn't have been." Scientists knew of the
existence of underground ice and had been monitoring craters as they formed,
but "I guess we didn't put the two together," he said.
Several of
the craters were also near the landing site of the Viking Lander 2. Viking also
looked for water ice on Mars, but was only able to dig down about 6 inches (15
cm) below the surface ? about 4 inches (10 cm) shy of where Byrne and his
colleagues think the ice table sits.
"It's
a shame that didn't happen," Byrne said. "You might have been having
this conversation 30 years ago."
How the ice got there
There are
several theories as to how such pure ice could form under the Martian surface.
Byrne thinks that one of the most promising explanations is that the ice formed
in the same way that so-called pure ice lenses form on Earth.
"That's
where you have very thin films of liquid water around ice grains and soil
grains and they migrate around to form clear ice lenses on top of the ice
table, even at temperatures well below zero," he explained. "This
process is called 'frost heave' on Earth, and it's considered a nuisance in
most places because it cracks up roads and tilts walls and destroys the
foundations of houses."
However the
water ice got there, it tells scientists something about Mars recent climate.
The ice is essentially "a remnant of a previous climate," Byrne said,
one which likely existed around 10,000 years ago.
As the
climate changes and becomes drier, the ice is expected to retreat, though based
on estimates of its current extent, it hasn't done so quite as quickly as
expected.
"The
climate has changes but the ice is still there," Byrne said. Just why that
is isn't clear yet.
These ice
lenses are likely to be a source of interest to those studying the possibility
of life on Mars as well, though Byrne said he's "not entirely sure if this
is enough water to be interesting to a microbe."
Byrne and
his colleagues suggest that fresh impact craters can be used as a new tool to
probe the depth and extent of Mars' subsurface water ice.
"These
impacts are really very useful," Byrne said.
And this
time around, Byrne and the rest of the MRO team will be ready. Mars' northern
hemisphere is heading into summer, and Byrne hopes to see about 10 more craters
over this a subsequent seasons, building up a map of where known subsurface ice
exists. Of course, the observations depend on MRO's successful reboot out of
its current safe mode, which has temporarily suspended all science operations.
"This
is I hope the start of a promising new method" of looking for water ice,
he said.
- Image
Gallery - Ice on Mars
- Mars Madness: A
Multimedia Adventure!
- SPACE.com
Video Show: What Went Wrong on Mars?
SPACE.com Managing Editor Tariq Malik contributed to this report from New York City.
