Oct. 1, 1999
John Ira Petty
Johnson Space Center, TX
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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