Habitability of natural satellites

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.

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.

[2] The strongest candidates therefore are currently icy satellites[3] such as those of Jupiter and Saturn—Europa[4] and Enceladus[5] respectively, in which subsurface liquid water is thought to exist.

[10][11] 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.

There is growing evidence of subsurface liquid water on several moons in the Solar System orbiting the gas giants Jupiter, Saturn, Uranus, and Neptune.

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.

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.

Scientists at the NASA Ames Research Center modelled the temperature on tide-locked exoplanets in the habitability zone of red dwarf stars.

[26] On Mars, however, a planet without significant tidal effects from its relatively low-mass moons Phobos and Deimos, axial tilt can undergo extreme changes from 13° to 40° on timescales of 5 to 10 million years.

[29] This is especially true of red dwarf systems, where comparatively high gravitational forces and low luminosities leave the habitable zone in an area where tidal locking would occur.

If tidally locked, one rotation about the axis may take a long time relative to a planet (for example, ignoring the slight axial tilt of Earth's Moon and topographical shadowing, any given point on it has two weeks – in Earth time – of sunshine and two weeks of night in its lunar day) but these long periods of light and darkness are not as challenging for habitability as the eternal days and eternal nights on a planet tidally locked to its star.

However, since the exomoon's primary is an exoplanet, it would continue to rotate relative to its star after becoming tidally locked, and thus would still experience a day-night cycle indefinitely.

[31] Natural satellites that host life are common in (science-fictional) written works, films, television shows, video games, and other popular media.