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"We have come up with three possible scenarios, and none of the three looks especially conducive to life," said Laurie Leshin, a UCLA geochemist in the department of earth and space sciences, who will discuss her research at the international Lunar and Planetary Science Conference in Houston on Wednesday, March 19. "If you stretch the imagination, you may be able to argue that one of the three scenarios may be consistent with life, but even under the most charitable scenario, you have to stretch the imagination pretty far."
UCLA's ion microprobe enables scientists to learn the exact composition of samples. The microprobe shoots a beam of ions -- charged atoms -- at a sample, releasing from the sample its own ions that are analyzed in a mass spectrometer. Scientists can aim the beam of ions at specific microscopic areas of a sample and analyze them. The microprobe was used in recent months to determine that life on Earth began at least 3.85 billion years ago and that Mount Everest and the Himalayas evolved as the highest mountain peaks in the world some 15 million years later than scientists had believed.
Supporters of ancient life on Mars argue that evidence of primitive life is associated with crystallized carbonate globules in the meteorite.
Studying bulk samples of the meteorite, advocates concluded the carbonates could have formed at temperatures cool enough to sustain life. Other scientists have argued, based on the mineral chemistry of the carbonates, that a higher temperature could not support life.
"We carefully correlated the chemical composition of the carbonates with their isotope composition, which cannot be done in bulk samples where they are mixed together," said Leshin, a Rubey faculty fellow at UCLA.
Leshin and her colleagues -- Kevin McKeegan, a UCLA research geochemist; and Ralph Harvey, a research scientist at Case Western Reserve University -- are the first scientists to individually pinpoint a wide range of carbonate compositions from the meteorite and analyze their oxygen isotopes.
"What we found," Leshin said, "is that these two seemingly unrelated data sets -- the chemistry of the carbonates and their isotope composition -- are in fact related. Any theory that explains the carbonate formations must also explain the variation in isotopes -- oxygen-18 to oxygen-16, and the calcium content.
"When we placed the samples in the ion microprobe, we found strong evidence that the first formed calcium-rich carbonates contain the lowest ratios of oxygen-18 to oxygen-16," she added.
Leshin said the findings provided hints to the conditions that prevailed when the carbonates formed billions of years ago. The scientists have produced three theories, which will be tested over the next several months, to explain the findings.
The first theory -- which would explain the isotope variations detected by the ion microprobe -- shows that the environment where the rock was located on Mars when the carbonates formed contained a very limited amount of fluid, which consisted largely of carbon monoxide rather than water. If this theory proves to be correct, it virtually rules out the possibility that the meteorite contains any signs of ancient life because water is necessary to support life, Leshin said.
Under a second theory, which also seems to be plausible, the environment from which the carbonates were formed on Mars contained a substantial amount of fluid that interacted with the meteorite. If that is correct, then the variation that the ion microprobe detected in the isotope ratio of oxygen-16 and oxygen-18 would most likely be explained by temperatures that were variable, rising above 200 degrees Celsius -- far higher than could support life, Leshin said.
"Under this scenario, even if the carbonates were at 0 degrees when they neared final crystallization, the temperature was boiling when they started forming," she said.
Under the third theory, the fluid on Mars that interacted with the rock was largely carbon dioxide when the carbonates started forming, and largely water when they were fully crystallized. This theory does not seem conducive to life either, but makes it more difficult to exclude the possibility entirely, said Leshin, adding that under this theory, the argument for ancient life is "conceivable, but not persuasive."
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