Shock a Common Planet Mineral From Different Angles, and Its Melting Behavior Changes
Livermore lasers hit magnesium oxide at 572 gigapascals and found it melts near 13,200 kelvin, but only the direction of compression decides the path it takes there.
A mineral that makes up much of the deep interior of rocky planets behaves differently depending on which way it is squeezed, according to a Lawrence Livermore National Laboratory study that overturns an assumption physicists had held for decades. The result also pins down one of the most extreme melting points ever measured.
The mineral is magnesium oxide, or MgO. It is common in Earth's deepest mantle, just above the core, and it may also be present in super-Earths and in the cores of gas giants. Earlier shock experiments on MgO gave results that existing theories could not explain, and the new work, published in Physical Review X, was designed to find out why. It is the third paper in a series spanning nearly 12 years.
MgO has a cubic crystal structure. The team used powerful laser pulses at the University of Rochester's Laboratory for Laser Energetics, on the Omega EP facility, to send shock waves through crystals. They launched shocks along three directions: perpendicular to a cube face, along a face diagonal and along the cube's diagonal. They combined optical measurements of shock speed and temperature with X-ray diffraction, which shows how atoms are arranged. The lead author is LLNL physicist Raymond Smith, and Johns Hopkins University's June Wicks and doctoral student Zixuan Ye took part.
When the shock ran along the cube's diagonal, MgO behaved as theory predicted: it moved between ordered structures and then melted as pressure rose. In the other directions, a thin, short-lived disordered or liquid layer formed at the shock front, followed by irregular crystal growth behind it. Across the experiments, MgO consistently melted at about 572 gigapascals and 13,200 kelvin. The article describes this as the highest experimentally recorded melting temperature among known materials.
"We started with a mystery," Smith said. "Why didn't our measurements agree with the expected behavior of magnesium oxide?" He added, "None of us expected such a complex response from changing the direction of compression." LLNL's Saransh Soderlind said, "The crystal's orientation determines how the applied compression can drive the atoms into a new arrangement." Marius Millot said the insights "allowed us to explain previous discrepancies between theory and experimental observations."
The melting data now serve as a benchmark for models of planetary interiors. Crystalline MgO may be the first mineral to crystallize out of early Earth's magma oceans, and it could affect how much material vaporizes in a giant impact. Extrapolating the results, the researchers suggest crystalline MgO could persist in parts of Jupiter's and Saturn's cores, while hotter giant planets might hold it as a liquid.
The release also notes that the findings may inform work relevant to stockpile modernization and LLNL's national security mission.
