http://www.space.com/scienceastronomy/mars_gullies_030219.html
Trickle Down Theory of Melting Snow May Support Life on Mars
By Leonard David
Senior Space Writer
posted: 02:00 pm ET
19 February 2003
Intriguing and often-examined gullies on Mars might not be created by water
seeping out from underground springs. Rather, they are likely caused by
trickling water from melting snowpacks, an active process that could sustain
biology on the Red Planet.
A leading Mars scientist has proposed a new theory regarding gully formation
on the planet, backed by images taken from NASA's Mars Odyssey spacecraft.
The research bolsters the view that liquid water is sheltered by snow,
preventing the fluid from rapid evaporation in Mars' thin atmosphere.
Images
This image, taken by the Mars Odyssey spacecraft, indicates that gullies on
crater walls may be carved by liquid water melting from remnant snow packs.
The gullies in the top right-center appear to emerge from beneath and within
a gradually disappearing blanket of snow.
This image from the Mars Global Surveyor spacecraft shows more gullies on
crater walls possibly carved in the same manner. Numerous gullies are seen,
with a remnant of the snow pack (arrow) proposed to be the source of water
that eroded the gullies.
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The research was presented today at a space science briefing at NASA
Headquarters in Washington, D.C. and will be detailed in the Feb. 20 issue
of the journal Nature.
Weeping layers
In June 2000, puzzling signs of water seeping into what appear to be young,
freshly-cut gullies and gaps in the martian surface were first reported by
Michael Malin and colleague Kenneth Edgett, both of Malin Space Science
Systems in San Diego, California. The researchers based their observations
on pictures taken by the Mars Global Surveyor.
The surprising detection of recently-formed, weeping layers of rock and
sediment on Mars had planetary experts scratching their heads.
Over the years, a host of scientific theories have been offered to explain
gully formation on Mars, including seeping ground water, pressurized flows
of ground water or carbon dioxide, and mudflows caused by collapsing
permafrost deposits.
No explanation to date has been widely accepted.
Greenhouse on Mars
But today, formation of the martian gullies by springs or pressurized flows
was called into question. Those now-famous gullies are created by trickling
water from melting snowpacks, argues Philip Christensen, a researcher at
Arizona State University and principal investigator for Odyssey's camera
system.
Furthermore, that snow acts as a greenhouse, protecting the water and
allowing it to melt and flow, and not instantly evaporate in the
low-pressure atmosphere.
"This snow would make an unbelievably attractive abode for life,"
Christensen told SPACE.com in an exclusive interview. "You've got sunlight
for photosynthesis. You've got temperatures above freezing. And you've got
liquid water all within a few inches of the surface at mid-latitudes on Mars
over huge areas.
"I would think life, if it exists on Mars, would migrate toward exactly
these environments," Christensen said. "The snow sits there and acts as this
wonderful blanket that allows all of this melting and trickling to go on."
Pasted-on terrain
Christensen points to an image taken by Odyssey of a crater in the southern
mid-latitude Terra Sirenum region of Mars. It shows eroded gullies on the
crater's cold, pole-facing northern wall, he said. But immediately next to
those features is a section he calls "pasted-on terrain."
This smooth deposit of material is thought to be "volatile," composed of
materials that evaporate in Mars' thin atmosphere. This material
characteristically occurs only in the coldest, most sheltered areas.
Christensen reported that the most likely composition of this slowly
evaporating material is water in the form of snow. From this observation,
the Mars scientist suspects a relationship between the gullies and the snow.
Snow on Mars is likely to accumulate most on the pole-facing slopes -- the
coldest areas. It gathers and drapes the landscape in these areas during one
climate period, then it melts during a warmer one. Melting begins first in
the most exposed area right at the crest of the ridge. This explains why
gullies start so high up, Christensen said today.
Once he started to think about snow, Christensen added, he found a large
number of other images showing a similar relationship between "pasted on"
snow deposits and gullies in the higher resolution images taken by the Mars
Global Surveyor.
Nice for life
Christensen told SPACE.com that snowpack melt that forms the gully-carving
streams of water is estimated to equal 12 cubic meters per day per gully. At
that rate you could fill a swimming pool in less than a week with the amount
of water that trickled down each gully. This is water that may occur every
year for hundreds of years before the ice goes away, he said.
Given the commonness of snowpacks on Mars, it's likely that somewhere on the
planet a snowpack is melting right now, Christensen said. "This isn't a dead
process."
"The snow will be back and these gullies will be rejuvenated and
reactivated. What we see today is sort of the fossil remnants of that snow.
But it's by no means a dead, inactive process. It will occur again,"
Christensen said at the NASA press briefing.
The new explanation for gully formation fits well with NASA's Mars
exploration strategy to "follow the water" and to look for life, Christensen
noted. Those melting snowpacks point to places where future robotic craft
should go look, he said.
"If we don't find life in these type of environments," Christensen said, "I
would question, where would you find life?"
Snow algae analog
Lynn Rothschild, a scientist at the Ecosystem Science and Technology Branch
at NASA's Ames Research Center near San Francisco, California, also took
part in today's NASA briefing. The prospect that liquid water might be near
the surface bodes well for life on Mars, she said.
"The real kicker is finding that liquid water…and even very tiny amounts is
good enough," Rothschild said. There are clear analogs on the Earth for
organisms living under the conditions reported by Christensen, she added.
One such community is snow algae, sometimes called watermelon snow,
Rothschild said.
These organisms can survive for long periods, hanging out for much of the
year in a resting stage. Then, when conditions are right and a thin film of
water under the snow is created, the organisms germinate. Now in a mobile
stage, they move up to the surface and photosynthesize, complete their life
cycle, and go back into spore form.
"That's something one could envision surviving" in the type of conditions
that Christensen has proposed, Rothschild said.
Fundamentally new perspective
John Mustard, associate professor at the Department of Geological Sciences
at Brown University in Providence, Rhode Island, called Christensen's new
findings a "fundamentally new perspective" on the origins of the Mars
gullies. It won't close the books on these features, however, he added.
"As scientists we continue to want to probe and discover things," Mustard
said. Future Mars spacecraft, particularly the 2005 launch of NASA's Mars
Reconnaissance Orbiter, will offer new tools to further explore the gullies
and how they are formed, he said.
Also taking part in the NASA briefing, Bruce Jakosky, a Mars expert at the
University of Colorado, Boulder, said the new research "provides a key, new
piece of information" about gully formation on the red planet.
"It's very hard to rule out one idea absolutely, or to rule in one as being
the correct answer," Jakosky said. "In the hierarchy of ideas that work and
ideas that won't, I think this one is at the very top of the list. It is not
a proven idea yet, but it's one that we can explore over the next couple of
years. I think we'll see a lot of attention focused on this."
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