Super-Earths may be solid deep inside their mantles

Super-Earths, rocky planets ranging in mass between one and ten times that of Earth, may harbor solid cores deep within their mantles due to intense pressures. These pressures can cause familiar minerals to crystallize into forms rarely observed on Earth, such as Mg2SiO4, a key building block of rocky planets. As pressure increases, Mg2SiO4 transforms into different crystal structures.

At extreme pressures within super-Earths, this mineral becomes stable once more in a new crystal structure called post-post-spinel, which is predicted to be a dominant constituent of their deep mantles. Understanding the melting behavior of this ultrahigh-pressure phase is crucial for comprehending the formation and evolution of these planets.

Recreating the extreme temperatures and pressures found in massive rocky planets has proven difficult in laboratory settings, so researchers have employed computational methods to study the melting curve of post-post-spinel Mg2SiO4 at pressures up to 1,300 gigapascals. Their findings reveal that post-post-spinel Mg2SiO4 is an exceptionally refractory mineral, capable of withstanding extreme temperatures before melting.

Its melting point ranges from 9,780 K to 14,897 K, significantly higher than other related minerals like bridgmanite and MgSiO3 postperovskite, which melt at lower temperatures near Earth's core-mantle boundary. Even with the addition of moderate amounts of iron, a common element in many exoplanets, the melting point of post-post-spinel Mg2SiO4 remains above the temperatures estimated for the deep mantles of most rocky planets.

These results suggest that many super-Earths likely possess solid deep mantles, which could have implications for mantle convection and the generation of magnetic fields within these exoplanets.