Earth’s Atmosphere Is Missing Huge Amounts Of Xenon, And We Don’t Really Know Where It Went

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

Meteorites – some of which are older than Earth – give us insights into the early Solar System and our own planet. Rocky planets formed from these smaller bodies clumping together, so they should give us clues about the chemical makeup of our own early planet. 

So it was puzzling to find that in carbonaceous chondrites – old, carbon-rich meteorites – scientists found levels of xenon were much higher than we expected in proportion to other gases. Since the rocks tell us about gas proportions in the early Solar System, it tells us that the amount of xenon in our current atmosphere is around 10 percent of what we’d expect it to be. This is especially puzzling because of how little xenon reacts with other elements.

“Xenon is one 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 on a 2018 paper investigating the missing xenon, explained in a statement at the time. “Their name comes from a kind of chemical aloofness; 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’d expect. So, where did the missing xenon go? There have been suggestions that xenon could be hiding in minerals, Earth’s core, or even in glaciers

The 2018 paper’s team found that under extreme pressure, xenon could form compounds with other elements.

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

It could be that a few processes combined to trap xenon in the mantle before being incorporated into the core, but that remains to be seen.

Another idea is that the missing xenon left Earth’s atmosphere long ago through degassing, being carried off into space as meteorites bombarded Earth and sent our primordial atmosphere flying. Since fellow heavy gases argon and krypton did not disappear from our atmosphere, if this is correct, it would need to be explained why only xenon was swept off into space while Earth’s atmosphere was thin. 

One team, including Stavrou, did find evidence to support this idea. In their study, the team attempted to dissolve xenon and argon in perovskite at temperatures and pressures similar to those found in the Earth’s mantle. The idea was that maybe xenon could be hidden in the magnesium silicate perovskite which makes up a lot of the mantle. 

“I was quite sure that it must be possible to stuff noble gases into perovskite,” co-author Hans Keppler told Nature. “I suspected xenon may be in there.”

However, the researchers found that while argon was able to dissolve into the perovskite, xenon only dissolved in trace levels. This gave the researchers the idea that a lot of xenon was carried off into space while other noble gases remained on Earth, safely hidden away in perovskite.

“This is completely different from what everybody else is saying. They are saying the xenon is here but it’s hiding somewhere,” Keppler explained. “We are saying it is not here because very early in Earth’s history it had no place to hide.”

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

If this is the mechanism that saw Earth depleted of its xenon, it would have to apply to Mars too. Mars does have small amounts of xenon in its thin atmosphere. However, the question remains whether Mars has enough perovskite to trap enough xenon to explain this. If it doesn’t, we may need to look again for that missing xenon.

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