[meteorite-list] Study Shows Mars in 4-Billion-Year Freeze

[Ron Baalke]

http://web.mit.edu/newsoffice/2005/mars.html

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Massachusetts Institute of Technology
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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/mars-alh-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