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| March
31, 2001 |
| CO2
May Have Carved Martian Gullies |
By Agnieszka
Przychodzen - University of Arizona News
 Liquid
carbon dioxide breakouts rather than water probably created the martian gullies
discovered last summer in high-resolution images from the Mars Global Surveyor
orbiter camera. Donald S. Musselwhite, Timothy D. Swindle, and Jonathan I. Lunine
of the University of Arizona Lunar and Planetary Laboratory publish their hypothesis
in the April 1 issue of Geophysical Research Letters.
Last June scientists
announced that gullies seen on some martian cliffs and crater walls suggest that
liquid water has seeped down the slopes in the geologically recent past. Researchers
found small channels on slopes facing away from mid-day sunlight, with most channels
occurring at high latitudes, near Mars' south pole. The scientists concluded that
the relationship between sunlight and latitude may indicate that ice plays a role
in protecting the liquid water from evaporation until enough pressure builds for
it to be released catastrophically into the surface. If channels are forming today,
liquid water may exist in some regions of Mars barely 500 meters beneath the surface,
they suggest.
Now UA researchers propose an alternative explanation involving
carbon dioxide erosion. They point to several reasons why CO2 is a better candidate
than water in gully formation. One reason is that most gullies are found in the
southern highlands, the oldest and coldest part of the planet, a place where liquid
water is least likely to be stable.
"That's high altitude in a region
of low geological activity. It is difficult to invoke some hydrothermal action
there," Musselwhite said. "The surface is old but the gullies are new."
Another
reason is that the southern hemisphere has more extreme temperature variations
throughout the year than does the northern hemisphere, a result of the fact that
Mars is closer to the sun during southern summer and farther away during southern
winter, Musselwhite said. The gullies are generally on pole-facing sopes where
they receive very little or no sunlight for most of the year.
However,
Musselwhite said, the most compelling fact is that gullies always start about
100 meters below the top of the cliff. At that depth, the pressure of the rock
overhead is just enough for liquid CO2 to be stable, if the temperature is low
enough.
"There are many interesting ideas about how to liquid water might
carve these things. Still, if the process works in these very special locations
where at least during wintertime it is extremely cold, why don't we see the gullies
in other places? If you have water cutting these gullies, you should see that
everywhere, not just at these specific locations. And where is the water coming
from? There is not much of it in the martian atmosphere or on the surface," he
said.
It's not liquid carbon dioxide flowing in the gullies. "What's coming
out is liquid CO2 that suddenly vaporizes," Musselwhite said. "As it comes out,
it expands very quickly, cools, and actually produces CO2 snow. The snow is suspended
in CO2 gas that hasn't solidified yet. Together with rock debris, it forms slurry.
Geologists call it a 'suspended flow.' Suspended flow acts like a liquid. It doesn't
take very much liquid each time to add to gully formation."
There are
analogs on Earth to this process. Martian gullies look almost identical to terrestrial
ones found in polar regions and also on cliff walls, where gullies are carved
by snow pack. Such channels can also be found on the flanks of Earth volcanoes,
carved by a suspended flow of ashes entrained in volcanic gas. And trapped mud
and sediment caught in turbidity currents on the ocean floor cut deep channels
into the continental shelves, Musselwhite noted.
How do Martian gullies
form? The planet's atmosphere is mostly composed of CO2. Under some atmospheric
pressure, CO2 condenses from the atmosphere and into Mars' surface. Mars has been
pummeled by impacts, so its surface is typically porous, spongy gravel. Gas seeps
through the surface and condenses in the pores of rock.
"In wintertime
the cliff surface gets so cold that its temperature falls below the freezing point
of CO2, which at low pressure goes directly to solid. As the cold wave moves from
the surface, the pore space is completely filled in. When spring comes, dry ice
warms up and expands. Since all the rock pore space is filled, pressure builds
until the ice turns to liquid. Liquid CO2 takes up more volume than dry ice, so
pressure continues to build."
At the same time, the dry ice dam evaporates
and thins as temperature rises. At one point the barrier becomes too thin, and
the liquid under pressure bursts out. It breaks through the surface into the atmosphere,
where it evaporates very quickly given the sudden drop in pressure. As carbon
dioxide vaporizes rapidly, it also cools and entrains the CO2 snow, creating the
suspended flow.
Some researchers claim that the gullies are very young
and may be currently forming on Mars. They tie gully locations to oscillations
in the martian climate caused by varying tilt of the planet's rotation axis, called
obliquity. When the obliquity is low. Mars' axis is almost straight up and the
surface near the poles gets less heating all year around. At high obliquity in
winter more of the surface would be shaded, but in the summer time it would get
much more sunlight than usual.
"If this explanation is correct, gullies
are forming today around the south pole," Musslewhite said. "The ones that are
farther from the poles are then older. You might expect these to form close to
the equator in the period of high obliquity, when the axis is more tilted over.
Some may be forming now on a yearly basis."
This idea is supported by
evidence that some researchers say suggests that gullies are forming today near
the south pole but not closer to the equator. Multiple images of the same gullies
are needed to prove that, Musselwhite added.
Contacts:
Don Musselwhite
520-626-2750, donm@lpl.arizona.edu
Timothy Swindle
520-626-5741, tswindle@lpl.arizona.edu
Jonathan I. Lunine
520-621-2789, jlunine@lpl.arizona.edu |
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