Super-Earths may be solid deep inside their mantles

Super-Earths may be solid deep inside their mantles A recent study suggests that the deep interiors of super-Earths, rocky planets with masses between one and ten times that of Earth, are likely composed of solid material rather than liquid. This finding challenges previous assumptions about planetary evolution and offers new insights into the internal dynamics of these distant worlds. Researchers led by Donghao Zheng discovered that a specific mineral, post-post-spinel magnesium orthosilicate, remains solid under the immense pressures found within super-Earths. Their findings, published in AGU Advances, reveal that this mineral exhibits an unusually high melting point, potentially explaining why the deep mantles of these planets remain solid. The study focused on magnesium orthosilicate, or Mg₂SiO₄, a key component of rocky planets. Under normal Earth-like conditions, this mineral undergoes structural changes as pressure increases, transitioning through several crystalline phases. In Earth’s mantle, it adopts the spinel phase, which eventually breaks down into bridgmanite and ferropericlase at greater depths. However, under the extreme pressures characteristic of super-Earths, Mg₂SiO₄ stabilizes in a novel form known as post-post-spinel. This phase was previously unknown and is believed to dominate the deep mantles of these larger planets. To investigate the properties of post-post-spinel Mg₂SiO₄, the research team employed a computational method called thermodynamic integration. This allowed them to simulate the melting behavior of the mineral at pressures reaching up to 1,300 gigapascals, a level comparable to the conditions found in the cores of super-Earths. They determined that post-post-spinel Mg₂SiO₄ is highly refractory, capable of resisting extreme heat before melting. Its melting range spans from approximately 9,780 Kelvin to 14,897 Kelvin, vastly exceeding the melting points of related minerals such as bridgmanite and MgSiO₃ postperovskite. Even when small quantities of iron were introduced, an element commonly present in exoplanet compositions, the melting point of post-post-spinel Mg₂SiO₄ remained well above the estimated temperatures of the deep mantles of most super-Earths. This indicates that the mineral does not melt under typical internal conditions, suggesting that the deep mantles of these planets may indeed be solid. The implications of this discovery extend beyond mere geological curiosity. A solid deep mantle would influence the mechanisms of convection within super-Earths, potentially altering how heat is distributed throughout their interiors. Additionally, the presence of solid material in the mantle could impact the generation of magnetic fields, which are typically produced by the movement of molten metallic fluids in a planet’s core. These factors are crucial for understanding the long-term stability and habitability of super-Earths. The research team includes scientists from multiple institutions, though specific affiliations were not detailed in the publication. The study was conducted using advanced computational techniques, highlighting the growing role of simulations in planetary science. Future experiments will aim to validate these findings through direct laboratory measurements, although recreating the extreme conditions of super-Earths remains a formidable challenge. As the scientific community continues to explore the composition and behavior of exoplanets, discoveries like this contribute to a broader understanding of planetary formation and evolution. The persistence of solid materials in the deep mantles of super-Earths underscores the complexity of these worlds and opens new avenues for research into their internal processes.