Scientists discover how magnesium oxide shapes super-earths

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Scientists have made an exciting discovery about magnesium oxide, a key mineral found in the Earth’s mantle, which could also play a crucial role in the formation of super-Earths.

By using high-energy laser experiments, researchers have observed how this mineral behaves under extreme conditions, similar to those found deep inside rocky planets.

June Wicks, an assistant professor at Johns Hopkins University, led the research, which is published in Science Advances.

“Magnesium oxide could be the most important solid controlling the thermodynamics of young super-Earths,” Wicks explains.

“It would be the first solid to crystallize when a hot, rocky planet starts to cool down and its interior separates into a core and a mantle.”

Super-Earths are larger than Earth but smaller than planets like Neptune or Uranus. They are common in other solar systems and are key targets for scientists searching for exoplanets.

These rocky super-Earths are expected to contain large amounts of magnesium oxide, which can influence the planet’s magnetic field, volcanism, and other important geophysical features.

To study magnesium oxide, Wicks and her team used the Omega-EP laser facility at the University of Rochester’s Laboratory for Laser Energetics. They subjected tiny crystals of the mineral to ultra-high pressures and temperatures.

This setup mimics the extreme conditions the mineral might experience during the formation of super-Earths. The team also used X-rays to observe how the atoms in the mineral rearranged under these conditions.

The results showed that magnesium oxide can exist in different forms depending on the pressure.

At pressures ranging from 430 to 500 gigapascals and temperatures around 9,700 Kelvin (almost twice as hot as the surface of the sun), the mineral transitions from a rock salt phase to a different arrangement similar to cesium chloride. This transformation affects the mineral’s viscosity and plays a significant role in how a planet’s interior behaves as it forms and evolves.

Interestingly, the experiments revealed that magnesium oxide can withstand pressures up to 600 gigapascals before it melts.

This is about 600 times the pressure found in the deepest parts of Earth’s oceans. “Magnesium oxide melts at a much higher temperature than any other material or mineral,” Wicks says. “Diamonds may be the hardest materials, but magnesium oxide will be the last to melt.”

This discovery highlights the stability and simplicity of magnesium oxide under extreme pressures and temperatures. It helps scientists develop more accurate models to understand the behavior of minerals in rocky planets like Earth.

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

The findings suggest that magnesium oxide could be the earliest mineral to solidify from magma oceans during the formation of super-Earths. This mineral could significantly influence whether a young planet becomes a snowball or a molten rock, develops water oceans or atmospheres, or has a mixture of these features.

In summary, this groundbreaking research provides new insights into how magnesium oxide, a key mineral in Earth’s mantle, behaves under the extreme conditions of planet formation.

These insights are crucial for understanding the formation and evolution of super-Earths and other rocky exoplanets.