[meteorite-list] 'Four-Billion-Year Chill' on Mars

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Jul 21 18:59:32 2005
Message-ID: <200507212258.j6LMwfW09962_at_zagami.jpl.nasa.gov>

http://news.bbc.co.uk/1/hi/sci/tech/4703055.stm

'Four-billion-year chill' on Mars
By David Whitehouse
BBC News
July 21, 2005

A chemical study of Martian meteorites implies that the planet has
always been cold and was rarely above freezing.

Writing in Science, researchers have been able to determine the maximum
temperature the rock experienced.

There is no evidence that it was ever warm, they say, as it records near
surface conditions for four billion years.

The water erosional features seen on Mars must have been made during
very brief periods, they conclude.

Thermal history

Although the current average temperature at the Martian equator is about
minus 55 Celsius, many scientists believe that the Red Planet was once
warm enough for water to have existed on its surface, and for life to
possibly have evolved.

There is plentiful evidence that water has flowed on the surface. They
include deep canyons, dried up river beds and many examples of deposits
left behind by running water.

The ALH84001 meteorite couldn't have been above freezing for more than a
million years during the last 3.5 billion years of history
David Shuster, California Institute of Technology

But the recent analysis, by David Shuster of the California Institute of
Technology and Benjamin Weiss of the Berkeley Geochronology Center, of
meteorites blasted from Mars seems to paint a different picture.

The new work involves two of the seven known "nakhlite" meteorites
(named after El Nakhla, Egypt, where the first such meteorite was
found), and the celebrated ALH84001 meteorite that some scientists
believe shows evidence of past microbial activity on Mars.

Using geochemical analysis techniques the researchers reconstructed a
"thermal history" for each of the meteorites to estimate the maximum
long-term average temperatures to which they were subjected.

"We looked at meteorites in two ways," says Weiss. "First, we evaluated
what the meteorites could have experienced during ejection from Mars, 11
to 15 million years ago."

Their conclusions were that ALH84001 could never have been heated to a
temperature higher than 350 Celsius for even a brief period of time
during the last 15 million years.

The nakhlites, which also show very little evidence of shock-damage,
were unlikely to have been above the boiling point of water during their
ejection from Mars 11 million years ago.

'Leaking' Argon

The other part of the research addressed the long-term thermal history
of the rocks while they resided on Mars. The scientists did this by
estimating the total amount of argon still remaining in the samples.

The gas argon is present in the meteorites as well as in many rocks on
Earth as a consequence of the radioactive decay of potassium. A noble
gas, argon is not very chemically reactive, and because the decay rate
is precisely known it can be used to date rocks.

However, argon is also known to leak out of rocks at a
temperature-dependent rate. The cooler the rock has been, the more argon
will have been retained.

The researchers found that only a tiny fraction of the argon that was
originally produced in the meteorite samples has been lost through the
aeons suggesting that the Martian surface has been in deep-freeze for
most of the last four billion years.

"The small amount of argon loss that has apparently taken place in these
meteorites is remarkable. Any way we look at it, these rocks have been
cold for a very long time," says Shuster.

"The ALH84001 meteorite, in fact, couldn't have been above freezing for
more than a million years during the last 3.5 billion years of history."

Water, water, everywhere?

This new line of research is a puzzle given the contrary evidence of
running water on Mars.

"Our research doesn't mean that there weren't pockets of isolated water
in geothermal springs for long periods of time, but suggests instead
that there haven't been large areas of free-standing water for four
billion years," says Shuster.

"Our results seem to imply that surface features indicating the presence
and flow of liquid water formed over relatively short time periods."

In fact, the evidence shows that during the last four billion years,
Mars has likely never been sufficiently warm for liquid water to have
flowed on the surface for extended periods of time.

This implies that Mars has probably never had a hospitable environment
for life to have evolved, unless biology got started started during the
first half-billion years of its existence, when the planet was probably
warmer.

The study is bound to be controversial showing a disparity between those
scientists who look at pictures of Mars to discern its history and those
who study the only pieces of the planet we can examine in detail in the
laboratory.
Received on Thu 21 Jul 2005 06:58:41 PM PDT