Concept
The habitability of natural satellites describes the study of a moon's potential to provide habitats for life, though is not an indicator that it harbors it. Natural satellites are expected to outnumber planets by a large margin and the study is therefore important to astrobiology and the search for extraterrestrial life. There are, nevertheless, significant environmental variables specific to moons. It is projected that parameters for surface habitats will be comparable to those of planets like Earth - stellar properties, orbit, planetary mass, atmosphere and geology. Of the natural satellites in the Solar System's habitable zone —the Moon, two Martian satellites (though some estimates put those outside it) and numerous Minor-planet moons — all lack the conditions for surface water. Unlike the Earth, all planetary mass moons of the Solar System are tidally locked and it is not yet known to what extent this and tidal forces influence habitability. Research suggests that deep biospheres like that of Earth are possible. The strongest candidates therefore are currently icy satellites such as those of Jupiter and Saturn—Europa and Enceladus respectively, in which subsurface liquid water is thought to exist. While the Lunar surface is hostile to life as we know it, a deep Lunar biosphere (or that of similar bodies) cannot yet be ruled out deep exploration would be required for confirmation. Exomoons are not yet confirmed to exist and their detection may be limited to transit-timing variation which is not currently sufficiently sensitive. It is possible that some of their attributes could be found through study of their transits. Despite this, some scientists estimate that there are as many habitable exomoons as habitable exoplanets. Given the general planet-to-satellite(s) mass ratio of 10,000, gas giants in the habitable zone are thought to be the best candidates to harbour Earth-like moons. Tidal forces are likely to play as significant a role providing heat as stellar radiation.
