Thin H$_2$-dominated Atmospheres as Signposts of Magmatic Outgassing on Tidally-Heated Terrestrial Exoplanets

TL;DR

This work investigates whether tidally heated, rocky exoplanets can sustain thin, H$_2$-dominated atmospheres through magmatic outgassing over gigayear timescales. The authors develop an interior--atmosphere framework that couples melt partitioning, gas-gas equilibria, and graphite saturation to compute outgassing fluxes and compare them to energy-limited escape, introducing an 'outgassing zone' (OZ) where H$_2$ production exceeds loss. A key result is that long-lived H$_2$ atmospheres require a water-rich basal magma ocean and reduced melts, with conditions that favor H$_2$ production while suppressing heavier outgassed species; detection of a thin H$_2$-dominated atmosphere would thus signal active volcanism and constrain interior properties such as volatile inventory and fO$_2$. The paper also shows JWST is capable of detecting such atmospheres in favorable targets (e.g., certain L 98-59 and TRAPPIST-1 analogs) and that nondetections can place meaningful constraints on interior state and outgassing efficiency. Overall, the study reframes the search for exoplanet volcanism as a JWST-accessible atmospheric diagnostic and highlights how atmospheric detections or nondetections can illuminate planetary interiors, volatiles, and redox states in tidally heated systems.

Abstract

H$_2$-dominated terrestrial exoplanets are highly accessible to atmospheric characterization via transmission spectroscopy, but such atmospheres are generally thought to be unstable to escape. Here, we propose that close-in, eccentric terrestrial exoplanets can sustain H$_2$-dominated atmospheres due to intense tidally-driven volcanic degassing. We develop an interior-atmosphere framework to assess whether volcanic outgassing can sustain \ch{H2}-dominated atmospheres over geologic timescales ($\geq$1 Gyr). We incorporate interior redox state, tidal heating, volatile inventory, and planetary parameters to compute outgassing fluxes and confront them with energy-limited hydrodynamic escape. We demonstrate that to sustain an H$_2$-dominated atmosphere, a terrestrial exoplanet must have a water-rich basal magma ocean and reduced melts, in addition to high eccentricity. We additionally demonstrate that detection of a specifically thin H$_2$-dominated atmosphere is a sign of current magmatic outgassing. We delineate an "outgassing zone" (OZ) most favorable to the existence of such planets, and identify the most observationally compelling targets. We propose combining precise mass-radius-eccentricity measurements with JWST constraints on atmospheric mean molecular mass $μ$ to search for thin H$_2$-dominated atmospheres. Inversely, we argue that robust atmospheric non-detections on OZ exoplanets can constrain the planetary interior, including melt redox state, mantle melt fraction and volatile inventory, and tidal heat flux.

Figures (12)

• Figure 1: Orbital dependence of hydrogen (H2) outgassing and atmospheric escape for planets L 98-59b (blue) and L 98-59d (orange). Solid lines represent energy-limited escape rates, while dashed lines show estimated H2 outgassing rates driven by tidal heating. Shaded regions highlight the "outgassing zones"—orbital ranges where volcanic H2 outgassing exceed atmospheric escape, enabling the potential buildup of a secondary H2-dominated atmosphere. The grey vertical band marks the conservative habitable zone based on the Kopparapu2013 criteria. Points indicate the present-day semi-major axes of L 98-59b (circle) and d (square), showing that both planets reside inside their respective outgassing zones. The inner limit of the outgassing zone is not shown in the figure as it lies inner to 0.01 AU.
• Figure 2: Comparison of outgassing and atmospheric escape fluxes across a range of semi-major axes and eccentricities for different stellar types. Each panel corresponds to a specific orbital eccentricity. Colored bands represent the habitable zones (HZ) for M, K, and G-type stars Kopparapu2013. Solid and dashed lines indicate escape and outgassing fluxes, respectively, for different stellar masses. The shaded purple regions highlight where outgassing exceeds escape within the respective HZs, indicating favorable conditions for atmospheric retention.
• Figure 3: The Figure shows a comparison of spectral simulation of L 98-59 d with two bulk compositions, 100% N2 (blue) and 30 % N2 + 70 % H2 (red). The JWST simulated observations including noise (3 transits, R (wavelength resolution) = 20) for the two respective cases are show in orange (30 % N2 + 70 % H2) and green (100% N2). The low and high $\mu$ atmospheres are detected at 6 $\sigma$ and 3 $\sigma$ confidence using 3 transits.
• Figure 4: Timescale at which planets having higher outgassing rate then escape rate, will loose all the volatile inventory for 2 extreme cases of mass fraction of H2O in the magma (3.3 $\times$ 10$^{-2}$ in solid lines and $1\times10^{-3}$ in dashed lines). The figure presents this result as a function outgassing efficiency varying between 10$^{-4}$ - 1.
• Figure 5: Number of models meeting given $\frac{\ch{H2}}{\ch{S}}$ outgassing flux ratio as a function of fO$_2$ at surface temperature of 1573 K. All other parameters (Table \ref{['tab:parameterraneg']}) are marginalized over. Bulk redox states of Mercuryzolotov2013, Earth 2017aeil.book.....C and Mars Doyle_2020 are marked by dashed lines and earth H2/S outgassing ratio is shown bu solid line Hu2023.