Astrochemists and geochemists have long been intrigued by the fact that the noble gas xenon is much less abundant in the earth’s crust and atmosphere than it is in the sun or in meteors. One possible explanation for the “missing xenon” mystery is that xenon might be sequestered in chemical compounds under the high-temperature and high-pressure conditions of the earth’s interior. (Although noble gases are generally “inert,” some of them, particularly argon and xenon, can form chemical compounds.)

Lacking the wherewithal to recreate Jules Verne’s “Journey to the Center of the Earth,” geochemists at the University of California, Berkeley, led by Wendel A. Caldwell and Raymond Jeanloz, sought to induce compound formation between xenon and iron at temperatures up to 3000 K and pressures up to 70 GPa (about 10 million pounds per square inch) in a laser-heated diamond-anvil cell. (1, 2) They monitored their results by x-ray diffraction, which, roughly speaking, monitors changes in atomic spacing. Although they saw phase changes of xenon itself that were known to occur under such extreme conditions, they were unable to detect any compound formation between xenon and iron. To investigate further the possibility of Xe-Fe compound formation, these chemists also carried out theoretical calculations on a set of hypothetical compounds composed of iron and xenon; they used methods known to reproduce well the diffraction patterns of xenon itself. They found that any Xe-Fe bonds that might form were so weak that they could not compensate for the energy required to break the strong Fe-Fe bonds.

As to the “missing xenon” mystery, the authors comment that the problem must be resolved by “other mechanisms.” They conclude: “In particular, the observed pattern of noble gas abundances appears to have been set before Earth and the terrestrial planets were fully accreted, rather than having been subsequently modified due to inclusion in the core.”

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