Degeneracies between signatures of magma oceans, water oceans and clouds in sub-Neptune atmospheres

Although nearly half of the exoplanets in the galaxy are expected to be sub-Neptunes, their atmospheric characterisation efforts with the James Webb Space Telescope (JWST) are being limited by the understanding of interactions between multiple chemical reservoirs, including magma oceans, water oceans and clouds. In this work, we couple interior and atmospheric chemical reservoirs using multi-phase chemical networks, including the effects of equilibrium and disequilibrium chemistry, to account for the partitioning of volatiles between silicate melt, liquid water and condensates. These volatiles include H2O, C-bearing and S-bearing molecules detected in JWST transmission spectra of sub-Neptunes (e.g., K2-18 b, TOI-270 d, TOI-421 b), as well as species predicted by magma-gas, water-gas and condensate-gas reactions. Our results indicate strong degeneracies in atmospheric retrievals between sub-Neptunes hosting magma oceans and water oceans across the near- and mid-infrared, extending known degeneracies between molecular abundances and cloud opacity. Combined with interior constraints from mass-radius data, these degeneracies can be mitigated through simultaneous ultraviolet, visible, and infrared spectroscopy, which helps differentiate between competing compositional scenarios for sub-Neptunes.