[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Dating of Meteorite Mineral Leaves Mars-Life Hypothesis Alive


http://www.space.com/science/solarsystem/mars-meteorite990930.html

Dating of Meteorite Mineral Leaves Mars-Life Hypothesis Alive
By Greg Clark
Sep 30 1999 15:29:19 ET

Scientists who proposed three years ago that a meteorite from Mars holds
evidence of primitive martian life are thrilled by new research that sets
the age of carbonate mineral deposits in the rock to 3.9 billion years old.

The dating research, headed by Lars Borg, a geochemist at the Institute of
Meteoritics at the University of New Mexico, is being lauded as the first to
establish a reliable age for mineral processes that occurred after the
formation of a meteorite. The research is published in this week's issue of
the journal Science.

Borg, who specializes in dating martian meteorites and lunar samples, has
pinned down the age at which mineral deposits called carbonates formed in
the famous Allan Hills 84001 meteorite. That meteorite, which was found in
Antarcitica in 1993, gained international notoriety in 1996 when a group at
NASA's Johnson Space Center announced that carbonate deposits in the rock
contained records left by microbial martian life.

The fact that the mineral processes occurred so long ago, and within about
500 million years of the time the rock was formed by volcanic processes on
Mars, means that the Mars-life argument, while highly controversial, is
still viable.

"The age dating had the potential to essentially eliminate the possibility
for life on Mars," at least in the case of the Allen Hills 84001 meteorite,
Borg said.

"If we got a young age, then it would be very difficult to envision a
process by which life could still be present on Mars, given the fact that
it's very arid and very cold," he said.

"However, we got an age of 3.9 billion years. So that's very consistent with
the life hypothesis, because we know that Mars was relatively warm. We know
that there was a relatively large abundance of water, the atmosphere may
have been significantly different, so the conditions for life were probably
better at 3.9 billion years ago than they are today."

An ancient age of almost 4 billion years for carbonates leaves plenty of
time for a Mars that may have been nurturing to change into the inhospitable
planet it is now, said Everett Gibson, a geochemist at the Johnson Space
Center. With his colleague David McKay, Gibson co-authored the 1996 paper
that announced the meteorite's carbonates held evidence of microbial life.

"We see in the carbonate structures, what we think are the signatures of
some biological activity. If that's the case, it was within 100 million
years of when we know life was evolving on the Earth. Why not another place
in the solar system?" Gibson said.

On Earth carbonates are generally deposited when minerals precipitate out of
a fluid, usually water. Calcium carbonate, which makes up limestone, is the
most common carbonate on Earth. It is also formed by living organisms. Coral
reefs and shells are made of calcium carbonate.

But it is unclear what kind of processes would be required to make
carbonates like those found in the martian meteorite. The carbonates found
in the meteorite are unlike anything found on Earth. They are mostly made of
iron and magnesium-rich carbonates with traces of calcium.

One explanation for the unique composition of the carbonates, Borg said, is
that the minerals may have formed in an environment rich in carbon dioxide.
For instance, such minerals might form at temperatures only slightly above
32 degrees Fahrenheit (0 degrees Celsius) in water that is heavily saturated
with carbon dioxide.

Such conditions may have existed on Mars billions of years ago. Massive
floods shaped the planet's surface geology, and its current atmosphere is
primarily carbon dioxide.

But other explanations have also been proposed, and dating the carbonates
does nothing to rule out those possibilities, Borg said.

One proposed scenario for carbonate precipitation that does not require
water to form suggests that the minerals were deposited by superheated
carbonate gas that might have been vaporized by a tremendous meteorite
impact. The heat of the explosion might force the carbonate vapor to
percolate through a rock and condense in various places throughout the stone
as the rock cools.

Still another explanation might be that only very small amounts of water
transported the carbonate minerals. Allan Treiman is a geologist who studies
meteorites at the Lunar and Planetary Institute in Houston. He said a
meteorite impact might simply melt ice, allowing carbonates to precipitate
into rock during the brief period before the water, or other fluid, froze
once again.

Borg's date of carbonate formation may be best appreciated in what it
answers about overall geologic processes on Mars, Treiman said. While the
debate about life will likely have to wait until martian samples are
returned to Earth, the date of carbonate formation is already answering old
questions about when Mars' surface was more geologically and chemically
active than it is today, he said.

----------
Archives located at:
http://www.meteoritecentral.com/list_best.html

For help, FAQ's and sub. info. visit:
http://www.meteoritecentral.com/mailing_list.html
----------