Gas doesn’t dissolve well in deep-Earth minerals

By Alexandra Witze

Web edition: October 10, 2012
Print edition: November 17, 2012; Vol.182 #10 (p. 9)

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 long-standing enigma.

The reason there’s less xenon in Earth’s atmosphere than expected, the researchers say, is because 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 online October 10 in Nature.

Compared with meteorites that formed out of primordial solar system stuff, Earth and Mars have far less xenon in their atmospheres. Scientists have proposed many possible explanations, such as minerals that locked up xenon in the upper parts of Earth’s middle layer, the mantle.

Shcheka and Keppler had been studying minerals from far deeper, in the lower part of the mantle where intense pressure creates minerals like perovskite, rich in magnesium, silicon and oxygen. In high-pressure experiments in the lab, 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 must fill that space — possibly a noble gas, a class of gases 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 bigger than argon and krypton atoms, and too big to slot easily into the spaces in the perovskite structure left by missing oxygen, Shcheka says.

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

Meanwhile, the primordial atmosphere got mostly stripped away, perhaps blasted away by radiation or by incoming meteorites. 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.

“Most people have said that, ‘OK, we don’t have this xenon in the atmosphere, so it should be hidden somewhere,’ ” Shcheka says. “What we say is that we will never find xenon, that it was lost already at the beginning of Earth’s evolution.”

Other scientists aren’t so sure. Chrystèle 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.

Shcheka, for his part, says the atmosphere on Mars is thin enough that not much perovskite is needed to throw off its xenon composition.