Tidally heated moons: from icy worlds to temperate habitats

摘要

Moons of giant planets may represent an alternative to the classical picture of habitable worlds. Within our own solar system Europa has long served as an intriguing candidate for a subsurface liquid water ocean. Sustained by tidal heating, such an ocean can exist well beyond the range at which stellar heating could raise surface temperatures to similar levels. For exoplanets, with their extraordinarily diverse orbital architectures, the same situation may arise, along with a host of other possibilities - including those where a combination of tidal and stellar heating results in water rich moons experiencing temperate surface conditions. The next generation of space-based planet finders and ground based large telescopes should begin to probe the population of moons around exoplanets - thereby opening up a new avenue in the search for life. We discuss some of these possibilities by investigating the dynamical constraints on moon systems of giant planets and by studying the characteristics of a set of 74 known extrasolar giant planets located beyond 0.6 AU from their parent stars - where moons should be long-lived with respect to removal by stellar tides. By estimating the stellar insolation that moons would experience for these exoplanet systems, and the implications for sublimation loss of volatiles, we find that between 15 and 27% of all known exoplanets may be capable of harboring small, icy, moons. In addition, by applying a simplified energy balance model, we find that some 22-28% of all known exoplanets could potentially harbor moons which, if large, could experience temperate surface conditions due to a combination of tidal and stellar heating. Large moons (0.1M?), at orbital radii commensurate with those of the Galilean satellites, could maintain temperate, or habitable, surface conditions during episodes of tidal heat dissipation ranging from that seen on Europa to 10-100 times greater. We discuss the implications of these findings in the context of habitability.