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This color-coded map indicates the depth to icy layers at a site in
southern Mars. The dense, icy layer retains heat better than the
looser soil above it, so where the icy layer is closer to the
surface, the surface temperature changes more slowly than where the
icy layer is buried deeper. May 02, 2007
Using observations by NASA's Mars Odyssey orbiter, scientists have
discovered that water ice lies at variable depths over small-scale
patches on Mars.
The findings draw a much more detailed picture of underground ice on
Mars than was previously available. They suggest that when NASA's next
Mars mission, the Phoenix Mars Lander, starts digging to icy soil on an
arctic plain in 2008, it might find the depth to the ice differs in
trenches just a few feet apart. The new results appear in the May 3, 2007,
issue of the journal Nature.
"We find the top layer of soil has a huge effect on the water ice in the
ground," said Joshua Bandfield, a research specialist at Arizona State
University, Tempe, and author of the paper. His findings come from data
sent back to Earth by the Thermal Emission Imaging System camera on Mars
Odyssey. The instrument takes images in five visual bands and 10
heat-sensing (infrared) ones.
The new results were made using infrared images of sites on far-northern
and far-southern Mars, where buried water ice within an arm's length of
the surface was found five years ago by the Gamma Ray Spectrometer suite
of instruments on Mars Odyssey. The smallest patches detectable by those
instruments are several hundred times larger than details detectable by
the new method of mapping depth-to-ice, which sees differences over scales
of a few hundred yards or meters.
The new approach uses thermal imaging as a thermometer to measure how fast
the ground changes temperature during local spring, summer and fall. The dense,
icy layer retains heat better than the looser soil above it, so where the icy
layer is closer to the surface, the surface temperature changes more slowly than
where the icy layer is buried deeper.
The resulting maps show that the nature of the surface soil makes a difference
in how close to the surface the ice lies. Areas with many rocks at the surface,
Bandfield explained, "pump a lot of heat into the ground and increase the depth
where you'll find stable ice." In contrast, dusty areas tend to insulate the ice,
allowing it to survive closer to the surface. "These two surface materials -- rock
and dust -- vary widely across the ground, giving underground ice a patchy
distribution," he said.
Computer models helped him interpret the temperature observations, he said.
"They show areas where water ice would be only an inch or so under the soil,
while in other areas ice could lie many feet below the surface."
The results fit long-term climatic models for Mars. These show the planet
has been both warmer and colder in the past, similar to glacial cycles on Earth.
Bandfield said, "The fact that ice is present near the depth of stability in the
current Martian climate shows that the ground ice is responding to climate cycles."
In turn, he added, this implies that water ice in the ground can swap places with
water vapor in the atmosphere as the climate changes.
Philip Christensen of Arizona State University, Tempe, principal investigator
for the Thermal Emission Imaging System, said, "Scientists have known for more
than a decade that water is on Mars, mostly in the form of ice. What's exciting
is finding out where the ice is in detail and how it got there. We've reached the
next level of sophistication in our questions."
NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology,
Pasadena, Calif., manages Mars Odyssey for NASA's Science Mission Directorate. Odyssey's
Thermal Emission Imaging System is operated by Arizona State University. For additional
information about Odyssey and the new findings, visit http://www.nasa.gov/mars and
http://themis.asu.edu .
Media contacts: Guy Webster 818-354-6278
Jet Propulsion Laboratory, Pasadena, Calif.
Robert Burnham 480-458-8207
Arizona State University, Tempe
2007-049
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