Black crystals of perovskite.

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Mysterious missing xenon gas traced to Earth's interior

In answer to one of chemistry's great conundrums, researchers show that vast quantities of 'missing' xenon gas may lie kilometres below our feet.

A research team including Museum analytical chemist Dr Stanislav Strekopytov has shown that xenon can be incorporated into a suite of minerals believed to make up most of the Earth's mantle.

Published in the journal Angewandte Chemie, the study may help to solve a long-standing mystery known as the missing xenon paradox - xenon levels in Earth's atmosphere are less than 10 per cent of their expected value. 

Where's all the xenon?

Xenon has been playing hide-and-seek for years. When scientists analysed primitive meteorite material to study relative gas proportions of the early solar system, they discovered that xenon levels in the current atmosphere are far lower than expected.

Used in lamps, headlights and nuclear energy production, xenon occurs naturally in Earth's atmosphere but is very rare and can only be extracted through a complex distillation process. The volume of xenon in the atmosphere on Earth (and Mars) is particularly low compared to xenon's fellow noble gases argon and krypton.

So where did all the xenon go? Scientists are generally split into two camps: those who believe xenon somehow escaped into space, and those who think it is held somewhere beneath Earth's surface.

Many of the gases present soon after the Earth's formation were expelled into space in what scientists call the 'degassing' of the early atmosphere. But as the heaviest of the noble gases, xenon is unlikely to have escaped at this stage, particularly since the lighter noble gases argon and krypton did not. There are other scenarios in which xenon might have been able to reach space, but so far the evidence is thin.

This latest research, led by Professor Sergey Britvin of Saint-Petersburg State University, provides new evidence that the missing xenon could be hidden beneath our feet, trapped within mantle minerals that have a perovskite crystal structure.

Fitting in

Xenon is a noble gas - one of a group of elements originally thought to be unreactive and therefore incapable of binding with other elements to form compounds.

Prof Britvin's team proved experimentally that xenon is more capable of binding with other elements and forming solid mineral compounds than was previously believed.

'The idea that the missing xenon could be lurking in minerals inside Earth has been around for a while,' says Prof Britvin. 'We have now proven that xenon can be incorporated into perovskite-like minerals that make up most of the Earth's interior. This provides a realistic solution to the missing xenon paradox.'

The researchers created xenon-bearing perovskite in the laboratory, and the resulting compounds were analysed by Dr Strekopytov at the Museum using inductively coupled plasma (ICP) techniques, which can detect trace amounts of substances even in extremely small quantities of material.

'The Museum's expertise in high-precision analysis of very small amounts of material has been developed through many years of research of rare and valuable specimens, such as meteorites. We routinely work with unique samples weighing just a few milligrams,' said Dr Strekopytov.
The atomic structure of xenon-bearing perovskite.

The atomic structure of xenon-bearing perovskite. Xenon (blue) is bound with sodium (brown) in a perovskite structure.


Minerals in the mantle

Earth's mantle makes up around 85 per cent of the planet's volume. It begins beneath the crust around 35-60 kilometres below our feet, and ends 3,000-5,000 kilometres deeper at the outer core.

The lower part of the mantle mainly consists of minerals with the same structure (but not composition) as the mineral perovskite. Even if xenon is only present in small amounts within these minerals, the vast volume of the mantle means that it could feasibly be hiding all of the xenon missing from our atmosphere.

Previous experiments have shown that at temperatures and pressures typical of the upper mantle, xenon is easily absorbed by silicate minerals such as perovskite. This fits with the idea that xenon is able to assimilate into mineral frameworks in the mantle.

The team's research suggests that xenon could take the place of titanium, niobium and tantalum in mineral structures. As a result, they expect to discover more xenon compounds in the future, advancing our understanding of the noble gases and mineral chemistry.