NASA’s Mars Odyssey Points to Melting Snow as Cause of Gullies

Images from the visible light camera on NASA’s Mars
Odyssey spacecraft, combined with images from the Mars Global
Surveyor (MGS), suggest melting snow is the likely cause of
the numerous eroded gullies first documented on Mars by the
Mars Orbiting Camera in 2000 by the MGS orbiter.

The now-famous Martian gullies were created by trickling
water from melting snow packs, not underground springs or
pressurized flows, as has been previously suggested, argues
Dr. Philip Christensen, the principal investigator for
Odyssey’s camera system and a Professor at Arizona State
University in Tempe. He proposes gullies are carved by water
melting and flowing beneath snow packs, where it is sheltered
from rapid evaporation in the planet’s thin atmosphere. His
paper is in the electronic February 19 issue of Nature.

Looking at an image of an impact crater in the southern mid-
latitudes of Mars, Christensen noted eroded gullies on the
crater’s cold, pole-facing northern wall and immediately next
to them a section of what he calls “pasted-on terrain.” Such
unique terrain represents a smooth deposit of material that
Mars researchers have concluded is “volatile” (composed of
materials that evaporate in the thin Mars atmosphere),
because it characteristically occurs only in the coldest,
most sheltered areas. The most likely composition of this
slowly evaporating material is snow. Christensen suspected a
special relationship between the gullies and the snow.
“The Odyssey image shows a crater on the pole-facing side has
this ‘pasted-on’ terrain, and as you come around to the west
there are all these gullies,” said Christensen. “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.”

Eroded gullies on Martian crater walls and cliff sides were
first observed in images taken by NASA’s Mars Global Surveyor
spacecraft in 2000. There have been other scientific theories
offered to explain gully formation on Mars, including seeps
of ground water, pressurized flows of ground water (or carbon
dioxide), and mudflows caused by collapsing permafrost
deposits, but no explanation to date has been universally
accepted. The scientific community has remained puzzled, yet
has been eagerly pursuing various possibilities.

“The gullies are very young,” Christensen said. “That’s
always bothered me, because how is it that Mars has
groundwater close enough to the surface to form these
gullies, and yet the water has stuck around for billions of
years? “Second, you have craters with rims that are raised,
and the gullies go almost to the crest of the rim. If it’s a
leaking subsurface aquifer, there’s not much subsurface up
there. And, finally, why do they occur preferentially on the
cold face of the slope at mid-latitudes? If it’s melting
groundwater causing the flow, that’s the coldest place, and
the least likely place for that to happen,” he added.
Christensen points out that finding water erosion under
melting snow deposits answers many of these problems, “Snow
on Mars is most likely to accumulate on the pole-facing
slopes, the coldest areas. It accumulates and drapes the
landscape in these areas during one climate period, and 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.”

Once he started to
think about snow, Christensen began finding a large number of
other images showing a similar relationship between “pasted
on” snow deposits and gullies in the high resolution images
taken by the camera on the Global Surveyor. Yet it was the
unique mid-range resolution of the visual light camera in
Mars Odyssey’s thermal emission imaging system that was
critical for the insight, because of its wide field of view.
“It was almost like finding a Rosetta Stone. The basic idea
comes out of having a regional view, which Odyssey’s camera
system gives. It’s a kind of you-can’t-see-the forest-for-
the-trees problem. An Odyssey image made it all suddenly
click, because the resolution was high enough to identify
these features and yet low enough to show their relationship
to each other in the landscape,” he said.

“Christensen’s new hypothesis was made possible by NASA’s
tandem of science orbiters currently laying the groundwork
for locating the most interesting areas for future surface
exploration by roving laboratories, such as the Mars
Exploration Rovers, scheduled for launch in May and June of
this year”, said Dr. Jim Garvin, NASA’s lead scientist for
Mars Exploration.

The Jet Propulsion Laboratory manages the Mars Exploration
Program for NASA’s Office of Space Science in Washington.
More information about the 2001 Mars Odyssey is available on
the Internet at:
http://mars.jpl.nasa.gov/odyssey/