So cool: Study shows Mars in 4-billion-year freeze (Forwarded)

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July 21, 2005

So cool: Study shows Mars in 4-billion-year freeze

The current mean temperature on the equator of Mars is a blustery -69
degrees Fahrenheit. Scientists have long thought that the Red Planet was
once temperate enough for water to have existed on the surface and
perhaps for life to have evolved there. But a new study by MIT and
Caltech scientists gives this idea the cold shoulder.

In the July 22 issue of the journal Science, MIT Assistant Professor
Benjamin Weiss and California Institute of Technology graduate student
David Shuster report that their studies of martian meteorites
demonstrate that at least several rocks originally located near the
surface of Mars have been freezing cold for 4 billion years.

Their work is a novel approach to extracting information on the past
climate of Mars through the study of martian meteorites.

In fact, the evidence suggests that during the last 4 billion years,
Mars has never been sufficiently warm for liquid water to have flowed on
the surface for extended periods of time. Mars therefore has probably
never had an environment hospitable to the evolution of life -- unless
life got started during the first half-billion years of its existence,
when the planet was probably warmer.

The work involves two of the seven known "nakhlite" meteorites (named
after El Nakhla, Egypt, where the first such meteorite was discovered),
and the celebrated ALH84001 meteorite that some scientists believe shows
evidence of microbial activity on Mars. Using geochemical techniques,
Shuster and Weiss 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," said Weiss, of MIT's Department
of Earth, Atmospheric and Planetary Sciences. "First, we evaluated what
the meteorites could have experienced during ejection from Mars,
11-to-15 million years ago, in order to set an upper limit on the
temperatures in a worst-case scenario for shock heating."

They concluded that ALH84001 could never have been heated to a
temperature higher than 650 degrees Fahrenheit for even a brief period
of time during the last 15 million years. The nakhlites, which show very
little evidence of shock damage, were unlikely to have been above the
boiling point of water during ejection 11 million years ago.

Those temperatures are still rather high, but the researchers also
looked at the rocks' long-term thermal history on Mars. For this, the
scientists estimated the total amount of argon still remaining in the
samples using data previously published by two teams at the University
of Arizona and NASA's Johnson Space Center.

Argon gas is present in the meteorites as well as in many rocks on Earth
as a natural consequence of the radioactive decay of potassium. As a
noble gas, argon is not very chemically reactive, and because the decay
rate is precisely known, geologists for years have dated rocks by
measuring their argon content.

However, argon is also known to "leak" out of rocks at a
temperature-dependent rate. This means that if the argon remaining in
the rocks is measured, an inference can be made about the maximum heat
to which the rock has been subjected since the argon was first made. The
cooler the rock has been, the more argon it will have retained.

Shuster and Weiss's analysis found that only a tiny fraction of the
argon that was originally produced in the meteorite samples has been
lost through the eons. "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. Their calculations suggest that the martian surface has been in
a deep freeze for most of the last 4 billion years.

"The temperature histories of these two planets are truly different. On
Earth, you couldn't find a single rock that has been below even room
temperature for that long," 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.

"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 freestanding water for 4 billion
years.

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

On a positive note for astrobiology, however, Weiss says the new study
does nothing to disprove the theory of "panspermia," which holds that
life can jump from one planet to another by meteorites. While at Caltech
as a graduate student several years ago, Weiss and his supervising
professor, Joseph Kirschvink, showed that microbes could indeed have
traveled from Mars to Earth in the hairline fractures of ALH84001
without being destroyed by heat. In particular, the fact that the
nakhlites have never been heated above about 200 degrees Fahrenheit
means that they were not heat-sterilized during ejection from Mars and
transfer to Earth.

This work was sponsored by NASA and the National Science Foundation.

IMAGE CAPTION:
[http://web.mit.edu/newsoffice/2005/m...h-enlarged.jpg (71KB)]
A section of the ALH84001 Mars meteorite. Evidence from this meteorite
led MIT researchers to the conclusion that Mars has been extremely cold
for billions of years. Photo courtesy: NASA/JPL