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From Nature magazine
The evidence is in every breath of air, but answers are harder to come by. Xenon, the second heaviest of the chemically inert noble gases, has gone missing. Our atmosphere contains far less xenon, relative to the lighter noble gases, than meteorites similiar to the rocky material that formed the Earth.
The missing-xenon paradox is one of science’s great whodunits. Researchers have hypothesized that the element is lurking in glaciers, minerals or Earth’s core, among other places.
“Scientists always said the xenon is not really missing. It’s not in the atmosphere, but it’s hiding somewhere,” says inspector, sorry, professor Hans Keppler, a geophysicist at the University of Bayreuth in Germany. He and his colleague Svyatoslav Shcheka are the latest geoscientists to tackle the case, in a report published today in Nature.
Elementary, my dear Watson
They went looking for answers in minerals. Magnesium silicate perovskite is the major component of Earth’s lower mantle — the layer of molten rock between the crust and the core, which accounts for half the planet’s mass. The sleuthing scientists wondered whether the missing xenon could be squirreled away in pockets in this mineral. “I was quite sure that it must be possible to stuff noble gases into perovskite,” says Keppler. “I suspected xenon may be in there.”
The researchers tried dissolving xenon and argon in perovskite at temperatures exceeding 1,600 ºC and pressures about 250 times those at sea level. Under these extreme conditions — similar to those in the lower mantle — the mineral sopped up argon yet found little room for xenon.
Those results may sound disappointing, but they gave Keppler and Shcheka an idea. What if xenon isn’t hiding at all?
More than 4 billion years ago, Earth was molten. Meteorites bombarded the planet, causing it to lose much of its primordial atmosphere. Keppler and Shcheka suggest that argon and the other noble gases hid in perovskite, but most of the xenon could not dissolve in the mineral, and disappeared into space.
“This is completely different from what everybody else is saying. They are saying the xenon is here but it's hiding somewhere. We are saying it is not here because very early in Earth’s history it had no place to hide,” says Keppler.
When Earth cooled, argon and other noble gases started seeping out of the perovskite and filling the atmosphere. Xenon, dissolved in the mineral at only trace levels, could in turn make up only trace amounts of the atmosphere.
As further support for their hypothesis, the scientists point out that the relative ratios of three noble gases — xenon, krypton and argon — in the atmosphere roughly correspond to their solubility in perovskite.
The theory may also explain why lighter isotopes of xenon are even more depleted from the atmosphere than heavier ones. “Nobody has ever been able to explain this,” says Keppler. He and Shcheka suggest that over billions of years, when xenon was seeping into space, lighter isotopes were most likely to escape.
Case closed?
Ignoble gas
Not so fast, says Chrystele Sanloup, a geoscientist at Pierre and Marie Curie University in Paris: “I don't think this discovery accounts for the missing xenon.” She notes that the theory does not totally explain all of the excess heavy xenon in the atmosphere, nor for additional xenon made from the radioactive decay of uranium and plutonium in rocks.
Besides, any explanation for Earth’s missing xenon should also apply to Mars, where the atmosphere also has a dearth of the noble gas. Keppler and Shcheka suggest that here, too, the ancient xenon escaped into space: the planet’s puny gravitational field prevented it from holding onto the gas. As a result, all xenon currently found on Mars is what little could dissolve in perovskite.
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Reply | Report Abuse | Link to thisInteresting article.
However please exclude words like 'lurking' and hidden' in a 'science' article. No need to embellish physics and chemistry with anthropomorphic jargon. Science is fascinating on its own.
I wouldn't classify hidden as an anthropomorphic term. Lurking is a stretch as well.
Reply | Report Abuse | Link to thisI like to show this kind of articles to my kids, and I've noticed they do like such 'embelishments' and enjoy articles like this. It does not bother me as a scientist (writing styles vary widely after all) and I fully support any attempt to get more people interested in science. There's a difference between science news and the actual technical reports, right?
Reply | Report Abuse | Link to thisWhere I do object is when they 'water down' the science, though. A good science writer should be able to explain sophisticated concepts without dumbing down the whole note - else they're in the wrong business. Not that that's the case here, but it's been known to happen in SA...
I was first impressed with the notion that atmospheric xenon is in lower abundance than say, 100 years ago.
Reply | Report Abuse | Link to thisAfter reading the whole article, it became obvious that atmospheric xenon exists at the same level as always.
The way the title and first paragraph are worded might lend some the notion that human activity has caused the scarcity of xenon.
Please continue to publish articles written in vernacular that non-scientists who read such material for pleasure and to continue learning throughout life can understand and enjoy. I appreciate that this may seem to "dumb down" articles for some of your more articulate readers, but as Acoyauh2 who plans to share this article with his children says, a little jargon can go a long way to attract curious young minds (and some older ones, too) without detrimate to the material.
Reply | Report Abuse | Link to thisI lurk hidden in my den. Does that make me noble too? Perhaps full of gas at that.
Reply | Report Abuse | Link to thisAgree it was a good article. But poor old Mars comes up short once again.
Somewhere there should be a source of information that describes current research results consistent with their actual findings that has not been embellished to appeal to children. That need was once served by Scientific American.
Reply | Report Abuse | Link to thisAlready most of the 'science' programs on TV employ zip & zoom editing, apparently to attract those with attention difficulties, and use 'baby' talk so as not to lose the presumed audience of children.
Even one of the local TV weather reporters stresses that he's got the "easy to understand" weather forecast coming up real soon. I keep wondering whether anyone will be able to adequately explain potentially more challenging weather in the near future...
I should point out that this article was provided by Nature News - my remarks about Scientific American were more general that this one article.
Reply | Report Abuse | Link to thisBTW, if one is willing to wade through the ads, there's an interesting report on this research at:
http://phys.org/news/2012-10-geochemist-duo-explanation-dearth-xenon.html
Maybe I'm asking for the obvious but, is Xenon important for the atmosphere?
Reply | Report Abuse | Link to thisNot really. The somewhat academic puzzle is simply why the proportion of xenon in Earth's & Mars' atmospheres should be different than primordial meteorites, since they formed from roughly the same region of the Solar system's protoplanetary disk. As the article states:
Reply | Report Abuse | Link to this"Our atmosphere contains far less xenon, relative to the lighter noble gases, than meteorites similiar to the rocky material that formed the Earth."
Maybe xenon is not all that nobel. There are conditions in which xenon and fluorine form a compound the hexafluoride I seem to recall. Given the extreme condions of the solulibity experiment one may ask if there is another mineral that could create a bond with xenon long enough to cage the gas when conditions moderate.
Reply | Report Abuse | Link to thisSo where does Xenon "lurk" in the meteorites, if it cannot be held in the crystal structure of the rocks? How can there be more Xenon in meteorites with no gravity than on Earth with pretty respectable gravity?
Reply | Report Abuse | Link to thisI'm just guessing here, but I think the idea is that other nobel gasses essentially dissolve in perovskite. As for the asteroids, I think they idea is that xenon (and other gasses and liquids) were intermixed with the other primordial material of the protoplanetary disk - held in tiny bubbles within the solid materials.
Reply | Report Abuse | Link to thisPerhaps someone more knowledgeable can explain better...
Thumbs up for <celticmoonthumbs>, down for <geojellyroll>.
Reply | Report Abuse | Link to thisOur science-speak is dry and uninteresting to the non-scientific
world, let a bit of plain English liven it up.
Just to clarify, neither uranium nor plutonium (which doesn't exist naturally in rocks) has xenon anywhere in their decay chains. Dr. Sanloup should know better.
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