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

Researchers have shown through high-energy laser experiments that magnesium oxide is likely the first mineral to solidify in the formation of a super-Earth, which has a crucial influence on the geophysical evolution of these planets.

A new study finds that magnesium oxide, a key mineral in planet formation, may be the first to solidify in the development of ‘super-Earth’ exoplanets, with its behavior under extreme conditions significantly influencing the planet’s development.

Scientists have observed for the first time how atoms in magnesium oxide change and melt under ultra-harsh conditions, providing new insights into this important mineral in Earth’s mantle known to influence planet formation.

High-energy laser experiments – in which tiny crystals of the mineral were subjected to the kind of heat and pressure found deep in a rocky planet’s mantle – suggest that the compound could be the first mineral to solidify from magma oceans to form ‘ super-Earth’ exoplanets. .

“Magnesium oxide could be the key solid controlling the thermodynamics of young super-Earths,” said June Wicks, an assistant professor of Earth and Planetary Sciences at Johns Hopkins University who led the study. “If it has this very high melting temperature, it would be the first solid to crystallize when a hot, rocky planet begins to cool and its interior separates into a core and a mantle.”

Implications for young planets

The findings were recently published in Scientific progress.

They suggest that the way magnesium oxide changes from one form to another could have important implications for the factors that determine whether a young planet will be a snowball or a molten rock, develop water oceans or atmospheres, or have some combination of these features. to have.

“In terrestrial super-Earths, where this material becomes a major part of the mantle, its transformation will contribute significantly to the rate at which heat moves inland, which will determine how the interior and the rest of the Earth will change. planet forms and deforms over time,” Wicks said. “We can think of this as a proxy for the interior of these planets, because it will be the material that controls their deformation, one of the main building blocks of rocky planets.”

Laser-guided experiments with magnesium oxide

View of laser-guided experiments with shock-compressed magnesium oxide (MgO) in the room of the Laboratory of Laser Energetics. High-power laser beams are used to compress MgO samples to pressures beyond those at the center of the Earth. A secondary X-ray source is used to investigate the crystal structure of MgO. Brighter regions are glowing plasma emission over nanosecond timescales. Credit: June Wicks/Johns Hopkins University

Bigger than Earth, but smaller than giants like Neptune or Uranussuper-Earths are prime targets exoplanet searches because they are often found among other solar systems in the Milky Way. While the composition of these planets can range from gas to ice or water, rocky super-Earths are expected to contain significant amounts of magnesium oxide that could influence the planet’s magnetic field, volcanism and other important geophysics, as they do on Earth, Wicks said . .

To simulate the extreme conditions this mineral might experience during planet formation, Wick’s team subjected small samples to ultra-high pressure using the Omega-EP laser facility at the University of Rochester’s Laboratory for Laser Energetics. The scientists also took X-rays and recorded how those light rays bounced off the crystals to track how their atoms rearranged in response to the increasing pressure, specifically noting the point at which they changed from a solid to a liquid.

When squeezed extremely hard, the atoms of materials such as magnesium oxide change arrangement to support the crushing pressure. That’s why the mineral transitions from a rock salt “phase” similar to table salt to another configuration like that of another salt called cesium chloride as the pressure increases. This creates a transformation that can affect a mineral’s viscosity and its impact on a planet as it ages, Wicks said.

Stability of magnesium oxide at high pressure

The team’s results show that magnesium oxide can exist in both phases at pressures ranging from 430 to 500 gigapascals and temperatures of about 9,700 Kelvin, almost twice as hot as the surface of the Sun. The experiments also show that the highest pressure the mineral can withstand before completely melting is more than 600 gigapascals, about 600 times the pressure you would feel in the deepest trenches of the ocean.

“Magnesium oxide melts at a much higher temperature than any other material or mineral. Diamonds may be the hardest materials, but this material is the last to melt,” said Wicks. “When it comes to extreme materials on young planets, magnesium oxide will likely be solid, while whatever else is hanging out down there in the mantle will turn to liquid.”

The study shows the stability and simplicity of magnesium oxide under extreme pressure and could help scientists develop more accurate theoretical models to investigate important questions about the behavior of this and other minerals in rocky worlds like Earth, Wicks said.

“The study is a love letter to magnesium oxide, because it’s amazing that it has the highest melting point known – at pressure outside the center of the Earth – and that it still behaves like a regular salt,” says Wicks. “It’s just a beautiful, simple salt, even at these record pressures and temperatures.”

Reference: “B1-B2 transition in shock-compressed MgO” by June K. Wicks, Saransh Singh, Marius Millot, Dayne E. Fratanduono, Federica Coppari, Martin G. Gorman, Zixuan Ye, J. Ryan Rygg, Anirudh Hari, Jon H . Eggert, Thomas S. Duffy and Raymond F. Smith, June 7, 2024, Scientific progress.
DOI: 10.1126/sciadv.adk0306

Other authors include Saransh Singh, Marius Millot, Dayne E. Fratanduono, Federica Coppari, Martin G. Gorman, Jon H. Eggert, and Raymond F. Smith of Lawrence Livermore National Laboratory; Zixuan Ye and Anirudh Hari of Johns Hopkins University; J. Ryan Rygg of the University of Rochester; and Thomas S. Duffy of Princeton University.

This research was supported by NNSA through the National Laser Users’ Facility Program under Contract Numbers 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 number DE-AC52-07NA27344. The research was supported by the National Nuclear Security Administration through the National Laser Users’ Facility Program (Contract Nos. DE-NA0002154 and DE-NA0002720) and the Laboratory Directed Research and Development Program at LLNL (Project Nos. 15-ERD-014, 17 – ERD-014 and 20-ERD-044).

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