This Echo is READ ONLY !   NO Un-Authorized Messages Please!
 ~~~~~~~~~~~~~~~~~~~~~~~~   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

NASA Science News for February 19, 2003

Snow Gullies on Mars
====================

NASA spacecraft may have finally found the mysterious source of
gullies on Mars: melting snow. 

February 19, 2003: When NASA's Mars Global Surveyor spacecraft
photographed what looked like fresh rain gullies on Mars three years
ago, researchers were baffled. The surface of Mars is extraordinarily
dry. What could have carved the curious features? Now, thanks to data
from the 2001 Mars Odyssey spacecraft, there's a new answer: melting
snow. 

The gullies were created by trickling water from melting snow packs,
not underground springs or pressurized flows, as had been previously
suggested, argues Philip Christensen, a professor at Arizona State
University and the principal investigator for Odyssey's camera system. 
The water melts and could flow beneath snow packs where it is
sheltered from rapid evaporation in the planet's thin atmosphere,
Christensen explains in the Feb. 19th electronic issue of Nature. 

Christensen's idea took shape while he was looking at a Mars Odyssey
image of a Martian impact crater. He noted eroded gullies on the
crater's cold 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. 

"I saw it and said 'Ah-ha!'" recalls Christensen. "It looks for all
the world like these gullies are being exposed as this [pasted-on]
terrain is being removed through melting and evaporation." 

Eroded gullies on crater walls and cliff sides were first observed in
images taken by Mars Global Surveyor in 2000. There have been other
scientific theories put forward to explain the gullies, 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 gullies are very young," Christensen said. "That's always
bothered me. How could Mars have 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 slopes 
at mid-latitudes?" That the coldest place and the least likely place 
for melting groundwater. 

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 slopes that face the
north or south poles--that is, 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 Mars Global Surveyor. Yet it was the wide field of view 
of the visible light camera in Mars Odyssey's thermal emission imaging 
system that was critical for the insight. 

"The basic idea [of melting snow] comes from having a regional view,
which Odyssey's camera system gives," he explains. "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." 

The Jet Propulsion Laboratory manages the Mars Exploration Program for 
NASA's Office of Space Science in Washington, D.C. 

Credits & Contacts
Based on a NASA press release
Responsible NASA official: Ron Koczor 
Production Editor: Dr. Tony Phillips 
Curator: Bryan Walls 
Media Relations: Steve Roy

 - End of File -
================

---