[FoRK] Pack ice / frozen sea discovered on Mars?

Jeff Bone jbone at place.org
Mon Feb 21 13:13:42 PST 2005


	http://www.newscientist.com/article.ns?id=dn7039

'Pack ice' suggests frozen sea on Mars
	 	 11:48 21 February 2005
	 	 NewScientist.com news service
	 	 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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