[FoRK] Pack ice / frozen sea discovered on Mars?
jbone at place.org
Mon Feb 21 13:13:42 PST 2005
'Pack ice' suggests frozen sea on Mars
11:48 21 February 2005
NewScientist.com news service
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.
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
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
"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."
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.
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
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.
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.
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
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.
"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,
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.
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
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
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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