MARTIAN METEORITE CARBONATES 3.9 BILLION YEARS OLD
Oct. 1, 1999
John Ira Petty
Johnson Space Center, TX
(281) 483-5111
Release: J99-42
Martian Meteorite Carbonates 3.9 Billion
Years Old
A new study of the carbonate minerals found in a meteorite from Mars
shows they were formed about 3.9 billion years ago. Scientists
believe the planet had flowing surface water and warmer temperatures
then, making it more Earth-like. Giant meteorites were blasting huge
craters in its surface.
This study doesn’t directly address the possibility that life
once existed on Mars. But “It’s another piece in the
puzzle,” said Larry E. Nyquist of the Planetary Sciences Branch
of Johnson Space Center’s Earth Science and Solar System
Exploration Division. Nyquist, one of the authors of an article in
Science, a weekly publication of the American Association for the
Advancement of Science, was the principal investigator.
Researchers at Johnson Space Center in Houston and the University of
Texas at Austin did the study, using different techniques. Both
produced similar results, establishing the carbonates’ age
within comparatively narrow limits.
The 4.2 pound meteorite is believed to be part of an igneous rock
formation formed about 4.5 billion years ago as Mars solidified from
a molten mass. The meteorite probably was blasted from the planet
when a huge comet or asteroid struck Mars 16 million years ago.
It fell in Antarctica about 13,000 years ago, and was found in 1984 by
an annual expedition sponsored jointly by NASA, the National Science
Foundation, and the Smithsonian Institution. Called ALH84001, after
the Allan Hills in Antarctica where it was found, it was returned to
Johnson Space Center, and has been preserved at the Meteorite
Processing Laboratory there.
It subsequently was recognized as one of more than a dozen meteorites
with unique Martian characteristics.
Just how the carbonates were deposited within this igneous rock is the
topic of lively debate. Some scientists believe water saturated with
carbon dioxide from the atmosphere seeped down to the subsurface site
where the igneous rock formed and created the carbonate deposits. On
Earth, living organisms often play a role in carbonate formation. In
1996 scientists at Johnson Space Center and Stanford University
examined the carbonates in ALH84001 using electron microscopy and
laser mass spectrometry, and reported evidence suggesting primitive
life may have existed in them.
Other scientists believe the carbonates formed when hot,
carbon-dioxide-bearing fluids were forced into cracks in the rocks
when a meteor struck Mars. The 3.9-billion-year age of the carbonates
eliminates neither possibility.
The carbonates themselves are tiny deposits, reddish globules, some
with purplish centers and many surrounded by white borders. The
different colors are due to variations in the compositions of the
carbonates: purplish manganese-bearing calcium carbonate, reddish
iron carbonate, and white magnesium carbonates. The globules were
found along fractures in the meteorite and make up about 1 percent of
its volume.
The JSC-UT team, using a binocular microscope and tools resembling
dental picks, over a period of months painstakingly separated out
enough of the carbonate material for their analyses. After
experimenting with terrestrial calcium, iron, and magnesium
carbonates, they developed a way to selectively dissolve carbonate
material of differing compositions, enabling them to separate
different elements from the carbonate solutions.
The study established the age of the carbonate deposits by measuring
the decay of rubidium to strontium and of uranium to lead. The
techniques are similar to carbon dating, which is used for much
shorter time periods. The investigators used the dual approach
because “we wanted to make sure we had a result we could
believe in and that other people could believe in,” Nyquist
said.
The leading author of the Science article is Lars E. Borg, formerly of
the National Research Council and Johnson Space Center and now at the
University of New Mexico in Albuquerque. Other authors are James N.
Connelly of the University of Texas at Austin, Chi-Yu Shih, Henry
Weismann, and Young Reese, of Lockheed Engineering and Science in
Houston. K. Manser of the University of Texas contributed to the
investigation.
The age of the carbonates, said Everett K. Gibson of Johnson Space
Center and an author of the 1996 study that reported evidence of
microbial life in the carbonates, had been “one of the real
mysteries” of indications of life on Mars. Had the carbonates
been formed more recently, when the planet’s surface was devoid
of water, it would have been unlikely they were associated with
primitive life on Mars. Dating them at 3.9 billion years, when there
apparently was surface water on Mars is, Gibson said, very important,
and could “suggest events were very similar in the inner solar
system” as primitive life arose.
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