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Origins of the Solar System Revisited

Six years ago, not far from the Siberian village of Isheyevo, a Russian farmer happened across a strange rock in one of his fields. Little did he know, but the 16-kilogram meteorite he had discovered was a key to understanding the formation of the early solar system.
Researchers working at LMCM1 have discovered that the Isheyevo meteorite carried within it a record of the history of Earth's immediate neighborhood. And though it is impossible to unequivocally identify the specific parent body of this meteorite, it is believed to have broken off from an asteroid, possibly after a giant impact.
Team member Giacomo Briani explains that the meteorite was found to contain several “intrusions.” These are small assemblages of minerals called xenoliths (literally “foreign rock”) that have an origin different from that of the other components of the host meteorite. The Isheyevo xenoliths contain basic organic matter, which includes elements such as hydrogen and nitrogen. “The usual interpretation is that the organic matter formed before the solar system, in the interstellar medium,” says Briani. The Isheyevo xenoliths have unique variations in their nitrogen isotopic composition–in fact, one sample showed the highest values of the 15N/14N ratio ever recorded in a meteorite.2 “Solar system origin models we have at present explain this nitrogen isotopic composition as being caused by reactions between ions and molecules in the interstellar medium,” explains Briani. And according to those models, the high concentrations of heavy nitrogen should be accompanied by high levels of deuterium (a hydrogen isotope). But since the Isheyevo xenolith lacks the levels of deuterium that would seem to be required by its nitrogen levels, those models are now in question: “It is a problem that concerns the evolution of the primordial organic matter,” says Briani.
Though Isheyevo has left a huge impact crater in our understanding of the solar system, Briani says that the stone is billions of years old–and therefore a cache of information about the free-floating interstellar soup that would eventually coalesce into the solar system.

Mark Reynolds

Notes :

1. Laboratoire de minéralogie et cosmochimie du Muséum (CNRS / Muséum National d'Histoire Naturelle).
2. G. Briani et al., “Pristine extraterrestrial material with unprecedented nitrogen isotopic variation,” PNAS, 2009. 106: 10522-7.

Contacts :

Giacomo Briani,
MNHN, Paris.


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