'Pack ice' suggests frozen sea on Mars

From: George (george_at_wtfiswrongwithyou.com)
Date: 02/22/05 

Date: Tue, 22 Feb 2005 12:42:06 GMT

http://www.newscientist.com/channel/space/dn7039

Here is a first glimpse at the embargoed papers related to this article,
including images and data:

ftp://www.lpi.usra.edu/pub/outgoing/lpsc2005/full97.pdf

a.. 11:48 21 February 2005
a.. NewScientist.com news service
a.. Kelly Young

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 of scientists.

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.

"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 million years.

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, US.

Lava flow
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 researchers write.

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.

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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