Interior-Atmosphere Coupling on TRAPPIST-1 f, g, and h: Cryovolcanic Water Exospheres and Infrared Detectability
We investigate the interior structures and cryovolcanic observability of the exoplanets TRAPPIST-1f, g, and h. Our aim is to determine which interior configurations can sustain subsurface liquid water oceans in thermal equilibrium and to assess whether the resulting cryovolcanic outgassing could be detectable with current and future observatories. Using a layered interior model with silicate and ice layers, we identify thermal equilibrium configurations and quantify internal heat partitioning through Monte Carlo simulations. We also estimate cryovolcanic water outgassing and assess its detectability using synthetic transmission spectra of atmospheres and exospheres. We find that the internal heat budgets of all three planets are dominated by radiogenic heating and tidal dissipation in high-pressure ice layers. Thermal equilibrium solutions for TRAPPIST-1f and g favor thin outer ice shells and shallow subsurface oceans, whereas TRAPPIST-1h permits thicker ice shells, in agreement with previous work. Localized plume-like outgassing produces stronger JWST/NIRISS transmission signals than globally distributed exospheres. Under favorable assumptions, outgassing on TRAPPIST-1f may be detectable within about 20 transits. Overall, our results show that subsurface oceans can be sustained across a broad range of interior configurations and constrain the detectability of cryovolcanic water vapor on the TRAPPIST-1f, g, and h planets. Interior heat budgets and the spatial distribution of outgassed material emerge as key factors controlling detectability. This framework motivates future transmission studies of Europa-like exoplanets.