[meteorite-list] Cold, Dry and Lifeless - A New Take on Mars

[Ron Baalke]

http://www.newscientist.com/article.ns?id=dn7463 

Cold, dry and lifeless - a new take on Mars
Maggie McKee
New Scientist
June 3, 2005

New research on a green mineral that degrades easily in water and is
present over much of the Martian surface is fuelling debates over the
history of water and the current existence of life on the Red planet.

One study reveals that a region rich in the mineral olivine - which
suggests it is has been "dry" for about 3 billion years - is actually
four times larger than previously thought. That adds to a growing body
of evidence suggesting Mars was mostly cold and dry - and not warm and
wet - in the past.

The second study asserts that subsurface reactions of olivine and water
could produce enough methane to account for recent observations of the
gas in the atmosphere, removing the need to invoke living microbes to do
the job.

Olivine forms at very high temperatures and is one of the first minerals
to crystallise out of molten rock. But at lower temperatures and in the
presence of water, it is thermodynamically "unhappy and breaks down
really quickly into other minerals", says Phil Christensen, a geologist
at Arizona State University in Tempe, US.

He and colleague Victoria Hamilton of the University of Hawaii in
Honolulu, US, have used infrared images taken with NASA's Mars Odyssey
spacecraft to show that the olivine-rich rocks on the flank of the
volcano Syrtis Major cover a surface area of 113,000 square kilometres -
about half the size of the UK.

Higher resolution

That is nearly four times larger than the estimate made by NASA's Mars
Global Surveyor (MGS) spacecraft. The difference has been attributed to
Odyssey's ability to resolve details as small as 100 metres across, a
30-fold improvement over MGS. Christensen and Hamilton's study is
published in the journal Geology (vol 33, p 433).

The rocks, at a latitude of 20° north of the planet's equator, appear to
have formed through successive lava eruptions about 3 billion years ago.
"To keep olivine around so long suggests this area of Mars may not have
seen a lot of water or a warm climate," Hamilton told New Scientist.

That finding differs markedly from the recent discoveries made by the
Mars rovers of minerals that form in the presence of water. But
Christensen says such discoveries represent rare flooding events lasting
for weeks or months and that for most of the planet's 4.5 billion-year
history, any water has been locked in ice.

"I am not a proponent of the idea that Mars had oceans in the past,"
says Christensen. He says mineral mapping from orbit reveals most of the
planet is covered in volcanic rocks, which "shows most of Mars hasn't
seen much water". Scientists have failed to find minerals such as
carbonates and clays that form in oceans on Earth, he says.

"I'm moving in the direction of 'cold and dry' more and more," agrees
Hamilton. "But there are other scientists headed in the other direction,
thinking Mars was warmer and wetter. It is an ongoing discussion."
          
Making methane

The discovery of more olivine on the surface of Mars also supports the
argument that underground reserves of the mineral could produce methane,
says geologist Mukul Sharma of Dartmouth College in Hanover, New
Hampshire, US. He and colleague Chris Oze detail their proposal in the
journal Geophysical Research Letters (vol 32, L10203).

Olivine tends to sink when it crystallises from magma, which "implies at
some depth there has to be a lot more olivine than you see on the
surface," says Sharma. The team says there is enough olivine in the top
10 kilometres of the crust to explain the recent detections of methane
in Mars's atmosphere - if there are stores of liquid water underground,
as many scientists suspect.

Water reacts with a common, iron-rich form of olivine by producing
hydrogen gas, which then combines with carbon dioxide to produce
methane. The gas could then leak to the surface through gullies.

"The easiest way to produce all the methane people have observed is by
the reaction of olivine with water," Sharma told New Scientist. Other
researchers have proposed that microbes might be a continuous source of
the gas, which is easily destroyed by sunlight in the atmosphere.