Earth’s atmosphere is missing huge amounts of xenon, and we don’t really know where it went

Meteorites that have struck our planet over the years have brought with them a mystery: Earth’s atmosphere appears to be lacking large amounts of xenon.

Meteorites – some of which are older than Earth – give us insight into the early solar system and our own planet. Rocky planets formed from these smaller bodies that clumped together, so they should give us clues about the chemical composition of our own early planet.

So it was confusing to discover that in carbonaceous chondrites – ancient, carbon-rich meteorites – scientists found that levels of xenon were much higher than we expected relative to other gases. Because the rocks tell us about the gas proportions in the early solar system, they tell us that the amount of xenon in our current atmosphere is about 10 percent of what we would expect. This is especially confusing because xenon reacts so little with other elements.

“Xenon is part of a family of seven elements called the noble gases, some of which, such as helium and neon, are household names,” Elissaios Stavrou, lead author of a 2018 paper on the missing xenon, explained in a statement to the time. “Their name comes from a kind of chemical detachment; they normally do not combine or react with other elements.”

Fellow noble gases argon and krypton are in our atmosphere, and in the proportions we would expect. Where did the missing xenon go? There have been suggestions that xenon could be hiding in minerals, the Earth’s core or even glaciers.

The team behind the 2018 paper found that xenon can form bonds with other elements under extreme pressure.

“Our study provides the first experimental evidence of previously theoretical compounds of iron and xenon existing under the conditions found in the Earth’s core,” explains co-author Alexander Goncharov. “However, it is unlikely that such connections could have been made early in Earth’s history, while the core was still forming and the pressure from the planet’s interior was not as great as it is today.”

It may be that a few processes combine to capture xenon in the mantle before it is absorbed into the core, but that remains to be seen.

Another idea is that the missing xenon left Earth’s atmosphere long ago through outgassing and was ejected into space when meteorites bombarded Earth and sent our original atmosphere skyward. Since the other heavy gases argon and krypton have not disappeared from our atmosphere, if this is true it should explain why only xenon was swept into space while Earth’s atmosphere was thin.

One team, including Stavrou, found evidence to support this idea. In their research, the team attempted to dissolve xenon and argon in perovskite at temperatures and pressures comparable to those in the Earth’s mantle. The idea was that xenon might be hidden in the magnesium silicate perovskite, which makes up a large part of the mantle.

“I was quite sure that it should be possible to put noble gases in perovskite,” co-author Hans Keppler told Nature. “I suspected it had xenon in it.”

However, the researchers found that while argon could dissolve in the perovskite, xenon only dissolved at trace levels. This led the researchers to the idea that a lot of xenon was transported into space, while other noble gases remained on Earth, safely hidden in perovskite.

“This is completely different from what everyone says. They say the xenon is here, but it’s hiding somewhere,” Keppler explained. “We say it’s not there because very early in Earth’s history it had nowhere to hide.”

The team added that the relative abundance of xenon, krypton and argon in our atmosphere is roughly related to how soluble these elements are in perovskite. However, there are also questions about this idea.

If this is the mechanism by which Earth runs out of xenon, this should also apply to Mars. Mars has small amounts of xenon in its thin atmosphere. The question remains, however, whether Mars has enough perovskite to capture enough xenon to explain this. If not, we may have to look again for that missing xenon.

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