Interior–atmosphere coupling on TRAPPIST-1f, 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, radially symmetric interior model that includes silicate and ice layers, we identified interior configurations in thermal equilibrium and quantified the partitioning of internal heat among the different layers and rheological parameters through a Monte Carlo analysis. We also estimated cryovolcanic water outgassing rates and assessed their detectability through transmission spectroscopy, by creating synthetic transmission spectra of hydrostatic atmospheres and sputtered exospheres via radiative transfer. We find that, for all three planets, the internal heat budget is dominated by radiogenic heating and tidal dissipation in high-pressure ice layers, with other contributions remaining minor across the explored parameter space. Thermal equilibrium solutions for the inner planets TRAPPIST-1f and g require relatively thin outer ice I shells, implying shallow subsurface oceans, in agreement with previous work. In contrast, TRAPPIST-1h favors thicker ice shells. We show that globally distributed, non-hydrostatic sputtered exospheres require higher energy conversion efficiencies to produce detectable signals with JWST/NIRISS compared to localized plume-like outgassing configurations. Spatial localization of outgassed material in plume-like configurations substantially enhances effective line-of-sight column densities. On TRAPPIST-1f, both globally uniform and localized outgassing scenarios can produce signals above the JWST/NIRISS detectability threshold corresponding to ∼ 20 transits, depending on the adopted modeling assumptions. Our interior model demonstrates that subsurface oceans are sustained over a broad range of interior configurations. Overall, our results place constraints on the observability of cryovolcanic water vapor on the TRAPPIST-1 f, g, h planets and highlight interior heat budgets and the spatial distribution of outgassed material as key factors governing detectability. This framework strengthens the prospects for detecting cryovolcanically generated water vapor beyond the Solar System and motivates transmission studies of Europa-like exoplanets, that is, icy planets with subsurface oceans sustained by internal heating.