Super-Earths, which have masses and radii larger than Earth but smaller than Neptune, are thought to have compositions similar to terrestrial planets in our solar system. Under the extreme pressures and temperatures within their mantles, MgO is expected to transform from its B1 structure to a cesium chloride (B2) structure. This transformation greatly affects MgO's properties, including a decrease in viscosity, impacting the planet's internal dynamics.
To determine the pressure at which this transition happens, the LLNL team and collaborators developed a new experimental platform. This combines laser-shock compression with simultaneous measurements of pressure, crystal structure, temperature, microstructural texture, and density.
Conducting 12 experiments at the Omega-EP laser facility at the University of Rochester's Laboratory for Laser Energetics, scientists compressed MgO to pressures of up to 634 GPa (6.34 million atmospheres) for several nanoseconds. Using a nanosecond X-ray source, they examined the atomic structure of MgO under these conditions. They found that the B1 to B2 phase transition in MgO occurred between 400-430 GPa at a temperature of around 9,700 K. Beyond 470 GPa, B2-liquid coexistence was observed, with complete melting at 634 GPa.
"This study provides the first direct atomic-level and thermodynamic constraints of the pressure-temperature onset of the B1 to B2 phase transformation and represents the highest-temperature X-ray diffraction data ever recorded," said LLNL scientist Ray Smith. "These data are an essential for developing accurate models of super-Earth interior processes."
The B1-B2 transition serves as a model for other structural phase transformations and has been a subject of theoretical research for decades. Using a forward model to simulate X-ray diffraction conditions, the research team clarified the mechanism of the B1-B2 transition in MgO.
"Our X-ray diffraction data provides direct measurements of atomic-level changes in MgO under shock compression and the first determination of a phase transition mechanism at deep mantle pressures of super-Earth exoplanets," said LLNL scientist Saransh Soderlind.
Contributors to the research include LLNL scientists Marius Millot, Dayne Fratanduono, Federica Coppari, Martin Gorman, and Jon Eggert, as well as collaborators from Johns Hopkins University, University of Rochester, Princeton University, and SLAC National Accelerator Laboratory.
Research Report:B1-B2 transition in shock-compressed MgO
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