The habitability of icy exomoons

We present surface illumination maps, tidal heating models and melting depth models for several icy Solar System moons and use these to discuss the potential subsurface habitability of exomoons. Small icy moons like Saturn’s Enceladus may maintain oceans under their ice shells, heated by endogenic processes. Under the right circumstances, these environments might sustain extraterrestrial life. We investigate the influence of multiple orbital and physical characteristics of moons on the subsurface habitability of these bodies and model how the ice melting depth changes. Assuming a conduction only model, we derive an analytic expression for the melting depth dependent on seventeen physical and orbital parameters of a hypothetical moon. We find that small to mid-sized icy satellites (Enceladus up to Uranus’ Titania) locked in an orbital resonance and in relatively close orbits to their host planet are best suited to sustaining a subsurface habitable environment, and may do so largely irrespective of their host’s distance from the parent star: endogenic heating is the primary key to habitable success, either by tidal heating or radiogenic processes. We also find that the circumplanetary habitable edge as formulated by Heller and Barnes (2013) might be better described as a manifold criterion, since the melting depth depends on seventeen (more or less) free parameters. We conclude that habitable exomoons, given the right physical characteristics, may be found at any orbit beyond the planetary habitable zone, rendering the habitable zone for moons (in principle) arbitrarily large. As such, habitable exomoons may outnumber habitable exoplanets and may thus take precedence over exoplanets as potential sites of extraterrestrial life.