The story of hydrogen and water: understanding the interaction between planet atmospheres and interiors using density functional theory

Studies show that the typical planet forms with a hydrogen atmosphere, resulting in conditions where hydrogen and the planet’s molten or super-critical interior interact for millions to billions of years. The interaction of the atmosphere and interior is thus critical to understanding their formation, evolution, and interior structure. Our understanding of such interactions is, however, lacking, and to address this, we use a novel approach to conduct DFT-based computational experiments to investigate how hydrogen and water — two of the most important planetary constituents — interact at conditions relevant to Uranus, Neptune and water-rich exoplanets. Specifically, we map out the immiscibility curve for the hydrogen-water system. Our results have fascinating implications. For instance, we demonstrate that when planets and exoplanets such as Neptune and Uranus are young, or those that are hot, like many short-period exoplanets, their deep atmosphere or interior, i.e., below pressures of ~100 bars, should be a region of completely mixed super-critical hydrogen and water. However, as the planet cools over time, water and hydrogen can demix from each other leading to a “waterfall” inside these planets. This process should generate substantial heat due to the release of gravitational energy. We find that this is likely the cause behind the paradoxical observation that Neptune’s internal heat flux is much higher than that of Uranus. Our work highlights the importance of better understanding the interaction between the planet atmospheres and interiors, especially as we move into the era of the JWST, the proposed Uranian orbiter, and other next-generation observatories.