Meteorite sheds light on birth of the solar system


15 June 2009

French and Italian scientists have analysed a meteorite and discovered that it contains a unique and primordial rock fragment that is thought to have remained largely unaltered since the solar system formed around 4.6 billion years ago. The scientists believe the xenolith, which shows unprecedented isotopic variations of nitrogen, may offer insight into the solar system's formation and say it poses serious problems for current models of light element isotopic fractionation.  

Light element isotopic ratios are the result of formation mechanisms and particular physical and chemical conditions. Understanding them can therefore help determine whether extraterrestrial materials formed in the solar nebula - the giant molecular cloud that is thought to have given rise to the solar system - or whether they predate the solar nebula and were formed in other environments. 

Giacomo Briani and colleagues measured the light element isotopic ratios of hydrogen, carbon and nitrogen in the xenolith, called PX-18. 'This inclusion [material trapped inside a mineral during its formation] is really a very primordial inclusion and it has conserved some memory of the primordial nebula. It's by studying this object that we can understand our solar system forming,' suggests Briani at the Laboratory of Mineralogy and Astrochemistry in the French Museum of Natural History, Paris.

The team detected isotopic variations of nitrogen beyond anything so far observed in other solar system material. 'The extreme values of the N15 to N14 that we measured can only be explained by a model that concerns chemical reactions that occur at very low temperatures, namely ion molecule reactions,' explains Briani.

Cosmic conundrum

With this explanation, however, comes a problem. Current models predict that hydrogen isotopic ratios should show extreme enrichment following cold interstellar reactions too. But this was not observed by the team. 'We have only observed the nitrogen fractionation and not the hydrogen one. And that's the problem,' says Briani.

'This is a beautiful example of mapping out the isotopic anomalies in extraterrestrial materials,' says Max Bernstein, an astrochemist at NASA Ames, California. 'They don't really tell us why they're seeing what they seeing. But I don't hold it against them because nobody knows why!'

The team think that PX-18 could have originated in the Kuiper Belt beyond the planets on the outer reaches of the solar system. However, Bernstein thinks this is too simplistic. Indeed, astrochemist George Cody at the Carnegie Institute of Washington in the US, agrees. 'Invoking a Kuiper Belt origin for the clast [fragment of rock] is inconsistent with the author's own Raman data that indicates that some of the clast's constituents had experienced very high temperatures at some point,' says Cody.

'This discovery will make us reevaluate models of nitrogen fractionation in the early solar system,' says Jamie Elsila, an astrochemist at NASA's Goddard Space Flight Center.  She comments that the decoupling of nitrogen and hydrogen enrichments has been noted before, but this is another piece of evidence pushing for an explanation. 'The presence of this extremely primitive material mixed into this meteorite has implications for understanding the mixing of material between the outer and inner solar system during formation,' she adds.

James Urquhart

 

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References

G Briani et alPNAS, 2009. DOI: 10.1073_pnas.0901546106

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