- 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
- "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
- "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,"
- 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.