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

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

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


Don Musselwhite


Timothy Swindle


Jonathan I. Lunine