On the Trail of the Planet's Missing Xenon
By Paul Preuss, paul_preuss@lbl.govand Jon Bashor, jbashor@lbl.gov
November 14, 1997
Earth has a lower proportion of the element xenon than any other rocky planet -- a lower proportion than the solar system as a whole. Two teams of scientists -- one at Berkeley Lab's Materials Sciences Division (MSD) and the National Energy Research Scientific Computing Center (NERSC), and the other at UC Berkeley's Department of Geology and Geophysics -- recently cooperated in an attempt to track the missing xenon to the Earth's core. One group used a computer to simulate the behavior of xenon under extremes of pressure and heat. The other used a diamond-anvil cell and a laser beam to apply real pressure and heat in the laboratory.
Within the Earth's core, can pressure cause xenon to react with iron? This illustration shows that even under extreme pressure, iron (blue) does not bond with xenon (green)
"Several lines of evidence led people to suspect the Earth might be missing xenon," says Sander Caldwell, a member of the team led by Berkeley geophysics professor Raymond Jeanloz. "If you look at the sun and adjust for the fact that it is mostly helium and hydrogen, the elements that are left -- oxygen, silicon, aluminum, iron, and so on -- are in roughly the same proportion as on Earth. It's a crude model, but most of the terrestrial planets follow it. The exception is, there's practically no xenon on Earth."
Most of the miscellany that makes up our solar system condensed from a primitive nebula of dust and gas. Estimates of xenon abundance from telescope studies of Mars, Venus and Mercury, which roughly match counts of the elements in primitive meteorites thought to have formed before the planets condensed, confirm that the Earth has "an order of magnitude less xenon than the other terrestrial planets," according to Caldwell.
There is no easy explanation. Xenon does not react readily with other elements, and it is not locked up in chemical compounds in the Earth's crust. With an atomic mass of 131, many times that of hydrogen, helium and most atmospheric gases, xenon is too heavy to reach escape velocity in a low gravitational field or blow off in solar flares.
Steven Louie of the UC Berkeley Department of Physics and the Lab's MSD calculated that xenon can become a metal when subjected to pressures of around 150 gigapascals (GPa), almost a million and a half times atmospheric pressure at sea level. A metallic form of xenon was observed in the late 1980s. Louie says one hypothesis, discussed in the geophysical community for over a decade, was "that during the formation of the Earth, metallic xenon could have reacted with iron in the Earth's core--that the core is a reservoir of primordial xenon."
Raymond Jeanloz has often used diamond-anvil cells to recreate conditions in the Earth's mantle and core. Diamond-anvil cells squeeze two diamonds together in a vise-like mechanism; between them, test samples contained in a metal gasket are subjected to enormous pressures. By shining a laser through the transparent diamond, the trapped sample can be heated to high temperatures. Jeanloz's group set out to determine if metallic xenon could react with iron under such extremes.
Sander Caldwell holds a diamond-anvil cell used to recreate the pressures found deep within the Earth
The first challenge was to contain xenon gas inside the gasket. "We cooled the diamond-anvil cell with liquid nitrogen," Sander Caldwell explains, "and with a small hose let some xenon flow onto the diamonds. Just as water frosts the lawn on a cold night, the xenon frosted the cold diamonds."
Along with iron powder, the frozen xenon was squeezed to 50 GPa. Then the samples were heated to 3,000 degrees Kelvin. X-ray diffraction patterns taken before and after showed unfamiliar changes in crystal phases. Had the xenon reacted with the iron?
"I could not attribute the change to any known crystal structure. Science is supposed to be objective, but I was hoping to see evidence of an iron compound," Caldwell admits. "It would have been really big news if a noble gas could form a metal alloy."
The truth proved to be less revolutionary. "We knew that xenon alone--at room temperature--shifts from the face-centered cubic crystal phase to hexagonal close-packed with increasing pressure. In our work, heat was facilitating this phase change. What we were seeing in the diffraction patterns was not a compound but a mixture of the two xenon phases, plus unreacted iron," Caldwell says.
Steven Louie has long used computers to model the behavior of novel materials ab initio--that is, from quantum-mechanical first principles; he has been particularly interested in understanding the phase changes of materials under increasing pressure. Louie's student Bernd Pfrommer tackled the xenon question. He was able to run the smaller calculations using Silicon Graphics machines at the National Center for Supercomputer Applications in Illinois, but to complete the main calculations within a reasonable time, he needed NERSC's highly parallel supercomputer, the Cray T3E.
"With our calculations it is much easier to simulate high pressures than it is to produce them experimentally," says Pfrommer. Diamond-anvil cells have achieved pressures of about 360 GPa -- approximating the pressure at the surface of the Earth's inner core -- but Pfrommer subjected a set of xenon and iron compounds to simulated pressures of up to 500 GPa.
The calculations showed no sign of chemical bonds between xenon and iron. They indicated that such bonds were virtually impossible, even at pressures greater than those at the center of the Earth.
Where is Earth's missing xenon? Not in the iron core. "It's very significant work," Steven Louie comments, "because for a long while people thought this was a real possibility."
Yet a minor mystery has been solved--that of elusive "xenon II," a novel form of xenon proposed by earlier researchers. Computer calculations show that xenon can start to change from the face-centered cubic structure to the hexagonal close-packed structure at as low as five GPa; experiments show the transition is complete only over 70 GPa. This sluggish phase transition produced data that had been interpreted as a third structural form, but Pfrommer's NERSC calculations, together with Caldwell's experiments, showed that the confusion was due to the very small energy difference between the two phases.
"Over a wide range of pressures, xenon can't decide which phase it should be in," Caldwell notes. "But there is no such thing as xenon II. It's a mix."
Through a collaboration of calculation and experiment, one false trail has been eliminated. The search for the missing xenon goes on.
