Researchers Eliminate One Theory in the Mystery of the Mission Xenon

Scientists looking into the "mystery of the missing xenon" have found strong evidence against one leading theory and, along the way, discovered new information about the behavior of the element. The findings were published in the August 15, 1997 issue of Science magazine.

A team of investigators headed by professors Steven Louie of the University of California, Berkeley, and Lawrence Berkeley National Laboratory and Raymond Jeanloz, also from UC Berkeley, used both experimental and computational science to try to determine if xenon, which makes up only 0.000009 percent of Earth's atmosphere, could also be found elsewhere on Earth, such as the planet's core.

Two Berkeley graduate students, Sander Caldwell and Bernd Pfrommer (who is also associated with the Berkeley Lab Materials Sciences Division and NERSC), were key contributors to the project.

According to Caldwell, xenon is more abundant on the other rocky planets (Mars, Venus, and Mercury), and scientists have long thought more of the noble gas should be present on Earth. One theory is that xenon, usually found as a gas, could have bonded with iron in the Earth's core, and it was this theory that Caldwell tested in his lab. Despite subjecting a sample of xenon and iron to pressures up to 70 gigapascals (or 700,000 times atmospheric pressure at sea level), the two elements did not form a compound.

Within the Earth's core, can pressure cause xenon to react with iron? This simulation shows that even under extreme pressure, iron (blue) does not bond with xenon (green).

Using the computational capabilities of the Cray T3E at NERSC and other parallel computer platforms, Pfrommer performed quantum mechanical calculations and reached similar conclusions. "With our calculations, it is much easier to simulate high pressures than in an experiment," Pfrommer said. Even at pressures as high as 500 gigapascals, the calculations showed no sign of a chemical bond between xenon and iron.

Caldwell also aimed an industrial heating laser at his sample of xenon and iron, trying to cause the two elements to bond. While bonding did not occur, comparisons of the samples at different pressures and temperatures did clear up one mystery -- xenon's phase changes.

At low pressure, xenon's structure is face-centered cubic. At higher pressures, above 75 gigapascals, the structure changes to a hexagonal close-packed structure. In between, the thinking went, was a third structural form that was not entirely understood.

However, by using calculations from NERSC and observing samples, Caldwell and his colleagues determined that there is no third structural form. Rather, at those pressures, xenon "can't decide which phase it should be in."

Calculations showed that there was a very small energy difference between the two phases. "In fact, we had to keep crunching the numbers because the difference is so small, it was hard to calculate," Caldwell said.

By heating the sample, Caldwell provided energy for the sample xenon to change from one phase to the next without going through the predicted middle phase. "We cleaned up area in the middle," Caldwell said. "There is no xenon II phase -- it is actually part of the known phases."

Ironically, it was the iron in the sample that absorbed enough energy from the laser to help solve the mystery of the missing phase.

"Although the question of xenon's presence is still up in the air, so to speak," Caldwell said, "we've probably ruled out that it's sequestered in the core of the Earth. Now we need to seek another explanation."

Next Page
Back to Table of Contents