Structure in the meteorite ALH 84001.The tiny piece of carbon found in meteorites from Mars were formed by the cooling of volcanic emission, were not left there by ancient space germs. This is good news and bad news for astrobiologists.
The discovery in 1996 of carbonate structures in the meteorite ALH 84001, which traveled from Mars to Earth 13,000 years ago, was hailed at the time as evidence that extraterrestrial microbes have ever lived on the Red Planet. However, further studies of both the carbonate structures as tiny bits of macromolecular carbon (MMC) in the meteorite cast doubt on those claims.
To better understand where carbon could have been Mars, Andrew Steele and his colleagues at the Carnegie Institution of Washington in Washington DC, examined 11 samples of Martian meteorites, including the famous ALH 84001.
prior studies used samples of rock powder to analyze their chemical composition. As a result, they could not reveal where he had been originally located carbon in meteorites. Steele and his colleagues used Raman spectroscopy, a technique that disperses a laser light in a substance to identify its structure and chemical composition, to locate the MMC on the rocks with an accuracy of about 360 nanometers.
They found that pieces of carbon are locked up in crystals could only have formed when the rock is initially cooled from magma. Because they are well sealed, these "mineral bottles" filled with carbon exclude the possibility that the MMC came from ancient life.
Paradoxically, in fact, the finding raises the possibility of finding signs of ancient life in Martian rocks. Carbon in the MMC was originally chemically reduced, which means it has extra electrons and is quick to react. This easily accessible and reactive carbon could have joined with other elements to create complex molecules, perhaps even life.
"The presence of organic carbon in or near the Martian surface provides a potential source of nutrients for life," says co-author Francis McCabe.
"Perhaps the formation of periodic chemistry on Mars was as simple as Martian lava cool," says Marc Hirschman, a planetary scientist at the University of Minnesota in Minneapolis and Saint Paul, who was not involved in the investigation. "It reinforces the idea that early Mars may have been conducive to the development of life."
Steele says the findings could help provide a benchmark for carbon measurements taken by the Mars Science Laboratory, scheduled to land on Mars in August this year. "It helps give context to the measurements made to detect life."