Super-Earths are worlds generally similar to Earth, but much more massive. Recently, scientists suggested that pressure inside them may reach such extreme levels that rocks remain solid despite enormous temperatures.
A New Crystalline Form of Magnesium Orthosilicate in Planetary Interiors
Deep inside super-Earths—rocky planets with masses 1–10 times that of Earth—high pressure can force familiar minerals to adopt forms that are extremely rare on our planet. A deeper understanding of the temperature and pressure conditions under which these phases emerge could help researchers learn more about how planets evolve, phys.or reports.
One such mineral is magnesium orthosilicate, or Mg₂SiO₄, one of the main building blocks of rocky planets. As pressure increases in a planet’s interior, the atoms in this mineral rearrange into different crystalline structures. One high-pressure form is known as the spinel phase and can be found in Earth’s mantle. At still higher pressures, this phase breaks down into two different minerals: bridgmanite—the most abundant mineral phase on Earth—and ferropericlase, a magnesium-rich oxide.
However, under the far more extreme pressures expected inside massive rocky planets known as super-Earths, Mg₂SiO₄ becomes stable again in an entirely new crystal structure known as “post-post-spinel.” Scientists predict that this ultra-high-pressure phase of Mg₂SiO₄ is one of the dominant minerals in the deep mantles of super-Earths, making its melting behavior important for understanding how these planets form and evolve.
A Highly Refractory Solid Material
Reproducing the high temperatures and extreme pressures found inside massive rocky planets is difficult under laboratory conditions, so Zheng and his team decided to use a computational approach. The team applied a method known as thermodynamic integration to investigate the melting curve of Mg₂SiO₄ in its post-post-spinel state at pressures of up to 1,300 gigapascals. The results were published in the journal AGU Advances.
They found that post-post-spinel Mg₂SiO₄ is an extremely refractory mineral, meaning that it can withstand extraordinarily high temperatures before melting. Depending on pressure, it melts at temperatures ranging from 9,780 K to 14,897 K, significantly higher than related minerals, including bridgmanite and MgSiO₃ post-perovskite—a high-pressure form of bridgmanite that is stable near Earth’s core-mantle boundary.
Even after adding a moderate amount of iron—which probably occurs to some extent on most exoplanets—the melting temperature of post-post-spinel Mg₂SiO₄ remained higher than the temperatures calculated for the deep mantles of most rocky planets, the authors note.
Their findings suggest that many super-Earths probably have solid deep mantles. This has implications for convection in the interiors of rocky exoplanets and for their magnetic fields, which are generated by the circulation of molten metals deep within them.
