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Xenon shortage in air explained: differential capture of noble gases in Earth may be cause.

As detective stories go, the Mystery of the Missing Xenon may not have the catchiest title. But scientists in Germany say they might have cracked this longstanding enigma.

The reason there's less xenon in Earth's atmosphere than expected, the researchers say, is that there was never much xenon dissolved in the planet's depths to begin with. Had there been, it would have made its way over billions of years toward the surface, there to spew into the atmosphere.

"This model is enough to explain the whole xenon deficiency," says Svyatoslav Shcheka, a geochemist at the University of Bayreuth in Germany. He and Hans Keppler, also of Bayreuth, report the finding in the Oct. 25 Nature.

Shcheka and Keppler had been studying materials from the lower part of Earth's mantle where intense pressure creates minerals like perovskite, rich in magnesium, silicon and oxygen. In high-pressure lab experiments, perovskite had been behaving unexpectedly, and the team discovered this was because the mineral's crystal structure doesn't always contain an oxygen atom in the space where an oxygen atom could be. Rather, something else fills that space-possibly a noble gas, a class of elements that includes argon, krypton and xenon.

The scientists decided to see if they could dissolve noble gases in perovskite under pressure to find the missing ingredient. They found that argon dissolved easily, to the point that it made up just over 1 percent of the mineral. Krypton dissolved less readily, and xenon dissolved hardly at all--making up only about 0.03 percent of the perovskite.

That's probably because xenon atoms are too big to slot easily into the spaces in perovskite left by missing oxygen, Shcheka says.

Billions of years ago, the infant Earth was completely molten, with gases trapped within. As it cooled, the new theory goes, minerals began to crystallize out and trap those gases. Perovskite trapped mostly argon and some krypton, but little xenon, the scientists propose.

Meanwhile, the primordial atmosphere and its xenon got mostly stripped away, perhaps blasted off by radiation or incoming meteors. Once the planet cooled enough, it began to churn internally. Like a pot bubbling on the stove, this churning brought materials from deep within the planet to the surface, where they released their contents into the atmosphere. This journey would have involved perovskite rich in argon and poor in xenon.

Other scientists aren't so sure of this scenario. Chrystele Sanloup, a geochemist at the University of Edinburgh, has studied other, shallower places in the Earth where xenon might be locked up. She says the new paper can't explain several aspects of xenon geochemistry, including how Mars could also be lacking xenon in its atmosphere when it has very little perovskite in its depths.
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Title Annotation:Earth
Author:Witze, Alexandra
Publication:Science News
Geographic Code:1USA
Date:Nov 17, 2012
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