A class of meteorites seen as vestiges of the early solar system may have a more complex history than has been recognized, based on a new study of one of its most well-known members.
The 4.56-billion-year-old Allende meteorite is a carbonaceous chondrite, a stony meteorite that contains carbon compounds. Chondrites, which contain large grains of material embedded in a finer-grained matrix, account for more than 80 percent of all meteorites. Carbonaceous chondrites comprise about 6 percent of all chondrites.
Allende has been studied extensively since it landed in Mexico in 1969. It is a particularly interesting specimen because it was collected shortly after it landed, minimizing the possibility of Earth contamination.
Carbonaceous chondrites are generally regarded as direct descendants of the primordial nebula that gave rise to the sun and stars. The nebula's condensation appeared adequate to explain the internal structure and composition of the meteorites.
But new electron microscope observations of Allende tell a different story. Adrian J. Brearley, a planetary scientist at the University of New Mexico, detected mineral traces that appear not to be the result of condensation. His findings are published in the August 27 issue of the journal Science.
As expected, the meteorite's fine-grained matrix is composed of olivines, minerals formed largely of iron, silicon and oxygen. The surprise, however, was that the olivines also included iron nickel sulfide and graphitic carbon -- materials that could not have existed at the high temperatures of the primordial nebula.
"My opinion is that condensation had nothing to do with how the olivines formed," says Brearley in an interview with space.com. Rather, he explains, the meteorite appears to have been subject to water and heat much later, while it was part of an asteroid or other parent body.
One scenario is that the parent body contained ice that was melted during repeated asteroid bombardments. The resulting liquid water could then have reacted with the parent body's minerals to form the substances now observed. "I like the ice mechanism," says Brearley, but he notes that it may not be the final explanation.
Moreover, he expects that many scientists will be displeased at the prospect of revising their theories of how carbonaceous chondrites formed. "A lot of people will hate this paper," he says. "This is not something that people will like very much at all."
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