Getti di anidride carbonica dai poli di Marte

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Guy Webster 818-354-6278
Carolina Martinez 818-354-9382
Jet Propulsion Laboratory, Pasadena, Calif.

Robert Burnham 480-458-8207
Arizona State University, Tempe

News Release: 2006-100 August 16, 2006

NASA Findings Suggest Jets Bursting From Martian Ice Cap

Every spring brings violent eruptions to the south polar ice
cap of Mars, according to researchers interpreting new
observations by NASA’s Mars Odyssey
orbiter.

Jets of carbon dioxide gas erupting from the ice cap as it warms
in the spring carry dark sand and dust high aloft. The dark
material falls back to the surface, creating dark patches
on the ice cap which have long puzzled scientists. Deducing
the eruptions of carbon dioxide gas from under the warming ice
cap solves the riddle of the spots. It also reveals that this
part of Mars is much more dynamically active than had been
expected for any part of the planet.

“If you were there, you’d be standing on a slab of carbon-dioxide
ice,” said Phil Christensen of Arizona State University, Tempe,
principal investigator for Odyssey’s camera. “All around you,
roaring jets of carbon dioxide gas are throwing sand and dust a
couple hundred feet into the air.”

You’d also feel vibration through your spacesuit boots, he said.
“The ice slab you’re standing on is levitated above the ground
by the pressure of gas at the base of the ice.”

The team began its research in an attempt to explain mysterious
dark spots, fan-like markings, and spider-shaped features seen
in images that cameras on Odyssey and on NASA’s Mars Global
Surveyor have observed on the ice cap at the Martian south pole.

The dark spots, typically 15 to 46 meters (50 to 150 feet) wide
and spaced several hundred feet apart, appear every southern
spring as the sun rises over the ice cap. They last for several
months and then vanish – only to reappear the next year, after
winter’s cold has deposited a fresh layer of ice on the cap.
Most spots even seem to recur at the same locations.

An earlier theory proposed that the spots were patches of warm,
bare ground exposed as the ice disappeared. However, the camera
on Odyssey, which sees in both infrared and visible-light
wavelengths, discovered that the spots are nearly as cold
as the carbon dioxide ice, suggesting they were just a thin
layer of dark material lying on top of the ice and kept chilled
by it. To understand how that layer is produced, Christensen’s
team used the camera – the Thermal Emission Imaging System –
to collect more than 200 images of one area of the ice cap from
the end of winter through midsummer.

Some places remained spot-free for more than 100 days, then
developed many spots in a week. Fan-shaped dark markings didn’t
form until days or weeks after the spots appeared, yet some fans
grew to half a mile in length. Even more puzzling was the origin
of the “spiders,” grooves eroded into the surface under the ice.
The grooves converge at points directly beneath a spot.

“The key to figuring out the spiders and the spots was thinking
through a physical model for what was happening,” said
Christensen. The process begins in the sunless polar winter
when carbon dioxide from the atmosphere freezes into a layer
about three feet thick on top of a permanent ice cap of water ice,
with a thin layer of dark sand and dust in between. In spring,
sunlight passing through the slab of carbon dioxide ice reaches
the dark material and warms it enough that the ice touching the
ground sublimates – turns into gas.

Before long, the swelling reservoir of trapped gas lifts the
slab and eventually breaks through at weak spots that become
vents. High-pressure gas roars through at speeds of 161
kilometers per hour (100 miles per hour) or more. Under the
slab, the gas erodes ground as it rushes toward the vents,
snatching up loose particles of sand and carving the spidery
network of grooves.

Christensen, Hugh Kieffer (U.S. Geological Survey, retired)
and Timothy Titus (USGS) report the new interpretation in the
Aug. 17, 2006, issue of the journal “Nature.”

JPL, a division of the California Institute of Technology,
Pasadena, manages Mars Odyssey and Mars Global Surveyor
missions for the NASA 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 .

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