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

TL;DR

This study models tidally heated, hydrogen-rich exomoons around free-floating planets, showing they can sustain habitable conditions for billions of years without stellar energy, due to effective heat trapping and stable atmospheres.

Contribution

It introduces a self-consistent model of hydrogen-dominated atmospheres on tidally heated exomoons, addressing previous challenges with atmospheric collapse and expanding potential habitability scenarios.

Findings

Hydrogen atmospheres can maintain liquid water for up to 4.3 billion years.

Collision-induced absorption effectively traps heat in H2-rich atmospheres.

Stable, habitable conditions are possible without stellar radiation.

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.