The habitability of natural satellites is the potential of moons to provide habitats for life, though it is not an indicator that they harbor it. Natural satellites are expected to outnumber planets by a large margin and the study of their habitability 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 terrestrial planets like Earth and Mars, namely 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.
Presumed conditions The conditions of habitability for natural satellites are similar to those of planetary habitability. However, there are several factors which differentiate a natural satellite's habitability and additionally extend their habitability outside the planetary habitable zone.
Liquid water
Liquid water is thought by most astrobiologists to be an essential prerequisite for extraterrestrial life. There is growing evidence of subsurface liquid water on several moons in the Solar System orbiting the gas giants Jupiter, Saturn, Uranus, and Neptune. However, none of these subsurface bodies of water has been confirmed to date.
Orbital stability For a stable orbit the ratio between the moon's orbital period Ps around its primary star Pp must be < 1⁄9, e.g. if a planet takes 90 days to orbit its star, the maximum stable orbit for a moon of that planet is less than 10 days. Simulations suggest that a moon with an orbital period less than about 45 to 60 days will remain safely bound to a massive giant planet or brown dwarf that orbits 1 AU from a Sun-like star.
Atmosphere An atmosphere is considered by astrobiologists to be important in developing prebiotic chemistry, sustaining life and for surface water to exist. Most natural satellites in the Solar System lack significant atmospheres, the sole exception being Saturn's moon Titan. Sputtering, a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles, presents a significant problem for natural satellites. All the gas giants in the Solar System, and likely those orbiting other stars, have magnetospheres with radiation belts potent enough to completely erode an atmosphere of an Earth-like moon in just a few hundred million years. Strong stellar winds can also strip gas atoms from the top of an atmosphere causing them to be lost to space. To support an Earth-like atmosphere for about 4.6 billion years (Earth's current age), a moon with a Mars-like density is estimated to need at least 7% of Earth's mass. One way to decrease loss from sputtering is for the moon to have a strong magnetic field of its own that can deflect stellar wind and radiation belts. NASA's Galileo's measurements suggest that large moons can have magnetic fields; it found Ganymede has its own magnetosphere, even though its mass is only 2.5% of Earth's. Alternatively, the moon's atmosphere may be constantly replenished by gases from subsurface sources, as thought by some scientists to be the case with Titan.
Tidal effects While the effects of tidal acceleration are relatively modest on planets, it can be a significant source of energy for natural satellites and an alternative energy source for sustaining life. Moons orbiting gas giants or brown dwarfs are likely to be tidally locked to their primary: that is, their days are as long as their orbits. While tidal locking may adversely affect planets within habitable zones by interfering with the distribution of stellar radiation, it may work in favour of satellite habitability by allowing tidal heating. Scientists at the NASA Ames Research Center modelled the temperature on tide-locked exoplanets in the habitability zone of red dwarf stars. They found that an atmosphere with a carbon dioxide (CO2) pressure of only 1–1.5 standard atmospheres (15–22 psi) not only allows habitable temperatures, but allows liquid water on the dark side of the satellite. The temperature range of a moon that is tidally locked to a gas giant could be less extreme than with a planet locked to a star. Even though no studies have been done on the subject, modest amounts of CO2 are speculated to make the temperature habitable. Tidal effects could also allow a moon to sustain plate tectonics, which would cause volcanic activity to regulate the moon's temperature and create a geodynamo effect which would give the satellite a strong magnetic field.
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