Habitability of Tidally Heated H$_2$-Dominated Exomoons around Free-Floating Planets

TL;DR

This work addresses the lack of stellar energy input for exomoons around free-floating planets by testing hydrogen-dominated atmospheres for long-term habitability. Using a coupled radiative-transfer and condensation-chemistry model (HELIOS GGchem), the authors explore how H2–H2 collision-induced absorption can trap internal tidal heat and maintain liquid water at the surface across a range of atmospheric compositions. They find that surface liquid water is sustainable for up to about 4.3 Gyr at high surface pressures (e.g., 100 bar), with CH4 as the dominant minor absorber and NH3 capable of increasing surface temperatures when present above the convective boundary; these conditions also open a plausible pathway for RNA polymerisation via wet-dry cycles aided by alkaline NH3. The study expands the habitable parameter space for exomoons around free-floating planets, suggesting a Hycean-like but detectable class of worlds whose habitability hinges on atmospheric CIA and tidal forcing, and highlights future work needed to include clouds, moist adiabats, and additional CIA opacities for a more complete assessment.

Abstract

Exomoons around free-floating planets (FFPs) can survive their host planet's ejection. Such ejections can increase their orbital eccentricity, providing significant tidal heating in the absence of any stellar energy source. Previous studies suggested that liquid water could exist on such moons under thick CO$_2$-dominated atmospheres, but these models faced challenges with CO$_2$ condensation and atmospheric collapse, particularly in the high-pressure regimes that favoured long-term habitability. To address this, we employ a self-consistent model, including radiative transfer and equilibrium chemistry with condensation, to simulate a more stable hydrogen-dominated atmosphere for a range of initial chemical compositions, including C, O, and N. We find that such atmospheres can effectively trap heat via collision-induced absorption of H$_2$, maintaining surface temperatures suitable for liquid water for time-scales of up to 4.3 Gyr, depending on the surface pressure, while not prone to condensation-induced collapse. Wet-dry cycling caused by the strong tides together with the alkalinity of dissolved NH$_3$ could create favourable conditions for RNA polymerisation and thus support the emergence of life.

Figures (5)

• Figure 1: Example run of the coupled HELIOS-GGchem code with P$_\mathrm{surf}$ = 10 bar, T$_\mathrm{int}$ = 100 K, $X_\mathrm{C+O}$ = 10$^{-2}$, C/O = 0.59 and $X_\mathrm{N}$ = 10$^{-4}$. Left: Evolution of the pressure-temperature profile T(P) with the radiative-convective boundary (RCB) of the last step. Right: Final VMR profiles of all molecules with a maximum VMR > 10$^{-12}$.
• Figure 2: Surface temperatures T$_\mathrm{surf}$ ($X_\mathrm{C+O}$, C/O) for varying P$_\mathrm{surf}$ (from left to right: 1, 10, and 100 bar). For comparability, the internal temperatures T$_\mathrm{int}$ (185, 104, and 56 K, respectively) were chosen such that the minimum T$_\mathrm{surf}$ for each pressure is at the freezing point of water, 273 K. For this, T$_\mathrm{surf}$(T$_\mathrm{int}$) was linearly interpolated.
• Figure 3: The surface temperature T$_\mathrm{surf}$ increasing with the tidally provided internal temperature T$_\mathrm{int}$ for Earth- vs. Io-like gravity for $X_\mathrm{C+O}$ = 10$^{-2}$ and C/O=0.59. In a lower-gravity environment, more atmospheric mass is required to maintain a constant surface pressure P$_\mathrm{surf}$, resulting in an optically thicker and hence hotter atmosphere.
• Figure 4: The surface temperature T$_\mathrm{surf}$ increasing with the tidally provided internal temperature T$_\mathrm{int}$ for varying abundances of N for $X_\mathrm{C+O}$ = 10$^{-2}$ and C/O=0.59. The produced NH$_3$ only increases temperatures if it can reach beyond the optically thick convective part of the atmosphere before condensing, thereby shifting the radiative-convective boundary (cf. RCB in Fig. \ref{['fig:iteration']}).
• Figure 5: Updated time spent in habitable zone for $X_\mathrm{C+O}$ = 10$^{-2}$ and C/O=0.59 using the semi-major axis and eccentricity distribution and their evolution from Roccetti_2023 for Earth-mass moons. Of the total sample of 6945 moons [20, 31, 43] % reach the HZ at any point during their evolution and stay there for a maximum time of [95, 699, 4341] Myr for the surface pressures of [1, 10, 100] bar.