Potential storage of molecular hydrogen in CaTiO₃ perovskite in the deep interiors of rocky planets

Finding habitable exoplanets has recently drawn interest from not only astrobiologists and astrophysicists but also earth scientists. Studies have shown that evaluating habitability of a planet requires a fully understanding of deep volatile storage and its contributions to surface evolution. Hydrogen storage in the candidate minerals of deep interior can also greatly affect their properties, such as melting behavior and viscosity, and thus the dynamic evolution of planets. Considering elemental abundances, MgSiO₃ bridgmanite is believed to be an abundant mineral in the interiors of Earth-like planets (similar in sizes and masses). We have studied hydrogen storage in CaTiO₃ perovskite, an important structural analog of MgSiO₃ bridgmanite and chemical component in CaSiO₃ perovskite (third most abundant phase in the Earth’s lower mantle. We saturated CaTiO₃ with water at 500 °C and 10 GPa and maintained it for 8 hours in multi-anvil apparatus. Synthesized products were characterized by synchrotron X-ray diffraction, confirming a perovskite structure. Raman spectroscopy shows the sample has vibrational modes at ~3964 and ~4014 cm^-1, indicating storage of hydrogen in a form of molecular H₂ instead of hydroxyl (OH) in the structure of CaTiO₃ pervoskite. This is an unusual form of storage given the fact that so far water storage in the deep mantle minerals is known to take a form of hydroxyl. If further investigation supports this initial finding, such a speciation requires breakdown of water and therefore potential release of oxygen from the rocky interior. The results may have important implications for understanding the role of deep volatile cycle for the atmosphere of rock planets.