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Extra Friday,
March 14
Pictures of snow from the Red
Planet
When Elton John sang that Mars is "cold as hell," he was right. It's
a problem that has thrown a wrench in the works of geologists studying Martian
gullies for the last three years. Mars is incredibly cold. Dry too. Thanks to
low temperatures and an incredibly thin atmosphere, water left exposed on the
Martian surface would immediately freeze and then sublimate directly into water
vapor. It's been this way for a long, long time — perhaps billions of years.
Most water-carved features on Mars are easily explained. They are thought to
date back to an ancient, warmer and wetter Mars. Gullies, though, just aren't
old enough. First discovered in 2000, the gullies seem to have been carved far
more recently (thousands rather than billions of years ago) when Mars looked
pretty much as it does today. Exactly how you carve gullies in this environment
is the million dollar question. Planetary geologist Phil Christensen of Arizona
State University (ASU) proposed that snow is the answer. He described his theory
in Nature, published online Feb. 19 and also in the March 6 print issue
after look
Phil
Christensen says the gullies in the top right-center of this visible-light image,
taken by the thermal emission imaging system on NASA's 2001 Mars Odyssey spacecraft,
appear to emerge from beneath and within a gradually disappearing blanket of
snow. The current snow pack in this crater (located at 43 degrees south, 214
degrees east) appears to remain only on the cold, pole-facing crater wall (top).
On the less-shaded, warmer sides of the crater (left), the snow cover has completely
disappeared, leaving the gullies exposed. Photo credit: NASA/JPL/Arizona State
University.
The dominant models of gulley formation that preceded Christensen's model relied
on the runoff of subsurface aquifers and the melting of ice near the surface
to carve the gullies. No model ever gained universal acceptance. In some key
ways, the models were counterintuitive, Christensen says. They might work on
warm days near the equator, but the gullies occur on isolated surfaces and seem
to prefer Mars' colder mid-latitudes (between 30 and 70 degrees on both hemispheres).
In these regions, groundwater seepage would be unexpected and unsheltered liquid
water would not remain liquid for long.
Snow, argues Christensen, is a different story. A layer of snow can provide
protection — a localized greenhouse effect that allows snow several inches
beneath the surface to melt while the snow above remains intact. Snow will melt
here, even at temperatures well below freezing. After several thousand years
of such melting a gulley would be carved by a trickle of snowmelt equivalent
to a garden hose, and the snowpack removed. A currently accepted model predicts
that up to about 10 meters of water-rich snow do in fact fall on the surface
of Mars every 50,000 to 500,000 years, when the axis of Mars shifts and the
ice from the polar caps is redistributed.
Christensen, head of ASU's Mars Space Flight Facility, says that pictures of
Mars taken with the Mars Orbiter Camera (MOC) onboard NASA's Mars Global Surveyor
spacecraft, and with his own Thermal Emission Imaging System (THEMIS) show remnant
snowpacks that are still present along the edges of gullies and on crater walls.
These snowpacks are essentially fossilized, buried underneath a thin layer of
desiccated dust. Visible gullies in these same crater walls reveal places where
the snow melted prior to burial. When Christensen looks at the two features
side-by-side, he sees a weathering process frozen in time.
This
image, taken by the Mars orbiter camera on NASA's Mars Global Surveyor spacecraft,
shows gullies on martian crater walls that may be carved by liquid water melting
from remnant snow packs. Numerous gullies are seen, with a remnant of the snow
pack (arrow) proposed to be the source of water that eroded the gullies. Photo
credit: NASA/JPL/Malin Space Science Systems/Philip Christensen
The first time he noticed the snow in a MOC image, he realized its importance
immediately. "The image shows a crater about 20 kilometer in diameter that
on the pole-facing side has this 'pasted-on' terrain. As you come around to
the west there are all these gullies. I saw it and said, 'Ah-ha!' It looks for
all the world like these gullies are being exposed as this terrain is being
removed through melting and evaporation."
Planetary geologist Bruce Jakosky of the University of Colorado, Boulder, believes
Christensen's explanation holds a lot of promise. "At present it is the
best hypothesis going, both for the pasted on features and for the mechanism
of supplying water to carve the gullies," Jakosky says. "It is at
the top of the list in terms of plausibility and likelihood." He cautions,
however, that the model is not yet completely proven. "There are still
some substantial concerns I have about [Christensen's] model that can only be
addressed through additional work looking into the physics of this model, as
well as whether or not snowmelt is the only process involved."
Christensen believes that melting is still occurring, provided there are still
locations on Mars favorable to the process. Liquid water could be flowing on
Mars today. Jakosky, while he doesn't disagree on this point, is not hopeful
about the prospect of locating such a rarity. "I think identifying a place
that might have liquid water today is going to be exceedingly difficult, if
not impossible, short of just going to each snowpack and drilling to see if
it is the one," Jakosky says. As of yet, NASA is still considering ways
to study and explore Martian snowpacks.
Matthew Shindell
Geotimes contributing writer
Links:
Nature
An
interview
with Philip Christensen
