This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: In the deepest region of the Earth's mantle, just above the core, much of the material consists of crystals of magnesium oxide. In larger rocky exoplanets called super-Earths, the higher pressures in the lower mantle may drive magnesium oxide to transform into a new high-pressure crystal structure or a metallic, electrically conductive liquid that generates a magnetic field.
And in gas giants, magnesium oxide could exist in the cores themselves. To understand interiors across planet types, researchers need to know exactly how magnesium oxide responds to extreme pressures and temperatures. But for years, magnesium oxide experiments have produced results that theories cannot explain.
Now, after nearly 12 years of investigation, a study from Lawrence Livermore National Laboratory (LLNL) and Johns Hopkins University has solved that problem. The work, the third in a series, is published in Physical Review X. "We started with a mystery.
Why didn't our measurements agree with the expected behavior of magnesium oxide?" said Raymond Smith, LLNL physicist and lead author. "The breakthrough came when we questioned an assumption we had taken for granted: that at pressures approaching twice those at Earth's center and temperatures above 10,000 Kelvin, the direction in which you compress the crystal should no longer matter." Magnesium oxide's atoms form a cubic crystal structure. To get the material to the extreme conditions that exist in planetary interiors, researchers hit it with a powerful laser pulse that launches a shock wave through the crystal.
Until this study, the scientific community generally assumed that the direction of the cube's compression was unimportant to the overall material response at extreme levels of pressure and temperature. Testing that expectation required an experimental campaign at the University of Rochester's Laboratory for Laser Energetics, using the Omega EP laser facility. The team launched shocks along three directions: perpendicular to a cube face, along a face diagonal and along the diagonal through the cube.
"What we observed baffled us," Smith said. "None of us expected such a complex response from changing the direction of compression. It took several years, and advances in every part of our analysis, to understand what the measurements were telling us." The researchers combined simultaneous optical measurements of shock speed and temperature with X-ray diffraction, which shows the arrangement of atoms.
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