October 11, 2012 weblog
Geochemist duo offer new explanation for dearth of xenon in Earth's atmosphere
Scientists believe that the makeup of the Earth's atmosphere should include the same noble gases in roughly the same percentages as ancient meteorites observed today. Most research has shown that to be the case for all noble gases, with the exception of the gas xenon. For some reason, there is a much higher percentage of xenon in chondritic meteorites than there is in the Earth's atmosphere. Some researchers have suggested that the "missing" xenon is hidden somewhere in the Earth's crust, while others have claimed that it's likely hidden in the depths of glaciers or pack ice.
Shcheka and Keppler propose that the xenon gas isn't hiding; it's actually gone, and has been for a very long time. They theorize that because the gas isn't easily dissolved in perovskite (a mineral that makes up a large proportion of the Earth's mantle), it never became trapped. Instead, it remained near the surface and escaped into space during a time when the atmosphere was too thin to hold onto it. The researchers suggest that other gases, such as argon, were trapped in the perovskite and percolated to the surface after the planet cooled, allowing them to be captured in the atmosphere.
To add credence to their theory, the researchers set up an experiment in their lab where they attempted to dissolve xenon and argon in perovskite by exposing a sample to temperatures and pressures similar to those found in the Earth's mantle. They found that while the argon became trapped, the xenon did not, indicating that it could never have been trapped in the Earth's interior in the first place.
The two researchers say their theory also explains the small proportion of xenon found in the Martian atmosphere: the small gravitational field of the planet would not have been strong enough to keep it from escaping into space.
In the atmospheres of Earth and Mars, xenon is strongly depleted relative to argon, when compared to the abundances in chondritic meteorites. The origin of this depletion is poorly understood. Here we show that more than one weight per cent of argon may be dissolved in MgSiO3 perovskite, the most abundant phase of Earth's lower mantle, whereas the xenon solubility in MgSiO3 perovskite is orders of magnitude lower. We therefore suggest that crystallization of perovskite from a magma ocean in the very early stages of Earth's history concentrated argon in the lower mantle. After most of the primordial atmosphere had been lost, degassing of the lower mantle replenished argon and krypton, but not xenon, in the atmosphere. Our model implies that the depletion of xenon relative to argon indicates that perovskite crystallized from a magma ocean in the early history of Earth and perhaps also Mars.
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