- A frozen sea, surviving as blocks of pack ice, may lie
just beneath the surface of Mars, suggest observations from Europe's Mars
Express spacecraft. The sea is just 5 north of the Martian equator and
would be the first discovery of a large body of water beyond the planet's
polar ice caps.
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- Images from the High Resolution Stereo Camera on Mars
Express show raft-like ground structures - dubbed "plates" -
that look similar to ice formations near Earth's poles, according to an
international team of scientists.
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- But the site of the plates, near the equator, means that
sunlight should have melted any ice there. So the team suggests that a
layer of volcanic ash, perhaps a few centimetres thick, may protect the
structures.
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- "I think it's fairly plausible," says Michael
Carr, an expert on Martian water at the US Geological Survey in Menlo Park,
California, who was not part of the team. He says scientists had previously
suspected there was a past water source north of the Elysium plates. "We
know where the water came from," Carr told New Scientist. "You
can trace the valleys carved by water down to this area."
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- He says the evidence is "compelling" for past
flooding near the plates. "Maybe the ice is still there in the ground,
protected by a volcanic cover, as they suggest," he says.
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- There is abundant evidence for the past presence of water
on Mars but today it appears relatively dry, with water ice confined to
the planet's polar caps. Remote observations of hydrogen atoms by NASA's
Odyssey spacecraft in 2002 hinted that ice might be locked in the top metre
of soil at lower latitudes. But the evidence was inconclusive as the signal
could have come from minerals exposed to water in the past. 45 metres deep
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- The team of researchers, led by John Murray at the Open
University, UK, estimates the submerged ice sea is about 800 by 900 kilometres
in size and averages 45 metres deep. Images of the pack-ice-like plates
can be seen in this PDF document, which was not embargoed when New Scientist
first viewed it on 15 February.
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- The paper is for a presentation to be made at the Lunar
and Planetary Science Conference in Texas on March 18. A talk with the
same title is scheduled to be given by Murray at the 1st Mars Express Science
Conference in Noordwijk, the Netherlands, today.
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- The team arrived at the depth estimate by studying craters
in the plates. They say the craters appear too shallow for their diameters
- suggesting ice is filling them up. Moreover, the surface appears unusually
level - as if ice were beneath it. This evidence suggests the plates are
not just imprints left by ice that has now completely vanished. Crater
counts indicate the age of the plates is about 5 million years.
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- In their paper, the researchers trace a possible history
for the underground ice. It begins with huge masses of ice floating in
water on Mars. The ice was later covered with volcanic ash, preventing
it from sublimating away into the thin atmosphere. Then, the ice broke
up and drifted before the remaining liquid water froze. All of the ice
not protected by ash sublimated away, leaving the pack ice plates behind.
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- "If the reported hypothesis is true, then this would
be a prime candidate landing site to search for possible extant life on
Mars," says Brian Hynek, a research scientist at the Laboratory for
Atmospheric and Space Physics at the University of Colorado at Boulder,
US. Lava flow
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- One problem with this proposed frozen sea is that there
is very little water vapour in the Martian atmosphere today. Carr says
that if there had been relatively recent sublimation, as the scientists
propose, some traces of water should remain in the atmosphere.
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- Also, similar plate formations have been seen on Mars
before but attributed to solidified lava. But Murray's team says a lava
flow does not fit their observations. These plates are up to two times
larger than known lava plates on Earth, and they leave behind smooth, straight
lanes when they ram into craters and islands. These observations "imply
an extremely mobile fluid, with similar characteristics to water,"
the researchers write.
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- Carr says there are other regions on Mars with similar
plate formations, meaning this might not be the only subterranean water.
But ultimately, it may be difficult to prove whether the frozen sea still
exists today.
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- The MARSIS radar, which will soon be deployed on Mars
Express, should be able to detect underground liquid water but may have
trouble differentiating between ice and rocky soil. And the ice is not
visible directly. "To preserve it, you've got to bury it," Carr
says. "But if you bury it, you can't detect it."
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- http://www.newscientist.com/article.ns?id=dn7039
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