Xenon has almost vanished from Earth's atmosphere. German geoscientists think they know where it went.
The evidence is in every breath of air, but answers are harder to come by. Xenon, the second heaviest of the chemically inert noble gases (after radon), has gone missing. Our atmosphere contains far less xenon, relative to the lighter noble gases, than meteorites similiar to the rocky material that formed the Earth. The missing-xenon paradox is one of science’s great mysteries. Researchers have hypothesized that the element is lurking in glaciers, minerals or Earth’s core, among other places.
Scientists went looking for answers in minerals. Magnesium silicate perovskite is the major component of Earth’s lower mantle — the layer of molten rock between the crust and the core, which accounts for half the planet’s mass. The sleuthing scientists wondered whether the missing xenon could be squirreled away in pockets in this mineral. The researchers tried dissolving xenon and argon in perovskite at temperatures exceeding 1,600 ºC and pressures about 250 times those at sea level. Under these extreme conditions — similar to those in the lower mantle — the mineral sopped up argon yet found little room for xenon.
Those results may sound disappointing, but what if xenon isn’t hiding at all? More than 4 billion years ago, Earth was molten. Meteorites bombarded the planet, causing it to lose much of its primordial atmosphere. Argon and the other noble gases hid in perovskite, but most of the xenon could not dissolve in the mineral, and disappeared into space. As further support for this hypothesis, scientists point out that the relative ratios of three noble gases — xenon, krypton and argon — in the atmosphere roughly correspond to their solubility in perovskite.
However, any explanation for Earth’s missing xenon should also apply to Mars, where the atmosphere also has a dearth of the noble gas xenon. Perhaps there too, the ancient xenon escaped into space: the planet’s puny gravitational field prevented it from holding onto the gas. As a result, all xenon currently found on Mars is what little could dissolve in perovskite.
However, Mars has not enough (if any) perovskite to explain the xenon in its atmosphere. Until the mystery of missing Martian xenon is solved, the jury is still out on where Earth’s xenon really went.