Scientists have discovered the first building block in the formation of a super-Earth

Magnesium oxide, a key mineral in planetary formation, may be the first to solidify in developing “super-Earths” exoplanets, with its behavior under extreme conditions greatly influencing planetary evolution, a new study reveals.

Scientists have observed for the first time how magnesium oxide atoms transform and melt under extremely extreme conditions, providing new insights into this key mineral found in Earth’s mantle known to influence planetary formation.

High-energy laser experiments — which subjected tiny crystals of the metal to the kind of heat and pressure found deep in a rocky planet’s mantle — suggest that the compound could be the first metal to solidify from oceans of magma to form “super-Earths” exoplanets.

“Magnesium oxide could be the most important solid controlling the thermodynamics of an emerging super-Earth,” said John Weeks, an assistant professor of Earth and planetary sciences at Johns Hopkins University who led the research. “If it has a very high melting temperature, it will be the first solid to crystallize when a hot, rocky planet begins to cool and its interior separates into a core and mantle.”

Implications for young planets

The results were recently published in Advancement of science.

They point out that the way magnesium oxide transitions from one form to another could have important implications for the factors that control whether a young planet will be a snowball or a molten rock, develop water oceans or an atmosphere, or have some combination of these features. .

“In super-Earths, where this material will be a large component of the mantle, its transformation will contribute greatly to how quickly heat moves in the interior, which will control how the interior and the rest of the Earth moves.” “The planet shapes and deforms over time,” Weeks said. “We can think of this as a proxy for the interiors of these planets, because that would be the material that controls their deformation, which is one of the most important building blocks of rocky planets.”

A view of shock-compacted magnesium oxide (MgO) laser experiments inside the chamber in the Laser Energy Laboratory. High-energy lasers are used to compress MgO samples to pressures exceeding those found at the center of the Earth. A secondary X-ray source is used to explore the crystal structure of MgO. The brightest areas glow with plasma emission on nanosecond timescales. Credit: June Weeks/Johns Hopkins University

Bigger than Earth but smaller than the giants like Neptune or UranusSuper-Earths are prime targets Exoplanet Searches because they are commonly found among other solar systems in the galaxy. While the composition of these planets can vary from gas to ice or water, super-rocky planets are expected to contain large amounts of magnesium oxide, which could affect the planet’s magnetic field, volcanism, and other key geophysics as well, Weeks said. On the ground. .

To mimic the extreme conditions this mineral would endure during planet formation, Wick’s team exposed small samples to very high pressures using the Omega-EP laser facility at the University of Rochester’s Laser Energy Laboratory. The scientists also imaged X-rays and recorded how those light rays bounced off the crystals to track how their atoms rearranged in response to increasing pressures, specifically noting the point at which they changed from solid to liquid.

When pressed with extreme force, the atoms of materials such as magnesium oxide change their arrangement to maintain crushing pressures. This is why the mineral changes from a rock salt “phase” that resembles table salt to a different formation like another salt called cesium chloride as pressure increases. This leads to a transformation that can affect the metal’s viscosity and its impact on the planet as it ages, Weeks said.

Stability of magnesium oxide at high pressures

The team’s results show that magnesium oxide can exist in both phases at pressures of 430 to 500 gigaPascals and temperatures of about 9,700 K, nearly twice the temperature of the Sun’s surface. Experiments also show that the highest pressures the metal can withstand before completely melting are up to 600 GPa, about 600 times the pressure one would feel in the deepest ocean trenches.

“Magnesium oxide melts at a much higher temperature than any other substance or mineral. Diamond may be the hardest material, but this is what will melt last,” Weeks said. “When it comes to extreme materials in small planets, it is most likely to be magnesium oxide.” “Solid, while everything else hanging out there in the mantle will turn to liquid.”

Weeks said the study showcases the stability and simplicity of magnesium oxide under extreme pressures and could help scientists develop more accurate theoretical models to explore key questions about the behavior of this and other minerals within rocky worlds like Earth.

“The study is a love letter to magnesium oxide, because surprisingly it has the highest temperature melting point we know of — at pressures beyond the center of the Earth — and still behaves like regular salt,” Weeks said. “It’s just beautiful, simple salt, even at these record pressures and temperatures.”

Reference: “B1 to B2 Transition in Shock-Pressed Magnesium Oxide” by John K. Weeks, Saransh Singh, Marius Mellot, Dane E. Fratandono, Federica Copari, Martin J. Gorman, Zhixuan Yi, J. Ryan Rigg, Anirudh Hari, John H. Eggert, Thomas S. Duffy, and Raymond F. Smith, June 7, 2024, Advancement of science.
doi: 10.1126/sciadv.adk0306

Other authors are Saransh Singh, Marius Mellot, and Dane E. Fratandono, Federica Copari, and Martin J. Gorman, and John H. Eggert, and Raymond F. Smith of Lawrence Livermore National Laboratory; Zixuan Yi and Anirudh Hari of Johns Hopkins University; J. Ryan Rigg of the University of Rochester; and Thomas S. Duffy from Princeton University.

This research was supported by NNSA through the National Laser User Facility Program under Contract No. DE-NA0002154 and DE-NA0002720 and the Laboratory Directed Research and Development Program at LLNL (Project No. 15-ERD-012). This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344. This research was supported by the National Nuclear Security Administration through the National Laser User Facility Program (Contract No. DE-NA0002154 and DE-NA0002720) and the Laboratory Directed Research and Development Program at LLNL (Project No. 15-ERD-014, 17). -ERD-014, and 20-ERD-044).