Natural satellite habitability
Natural satellite habitability is the measure of a natural satellite's potential to sustain life. It is an emerging study which is considered important to astrobiology for several reasons, foremost being that natural satellites are predicted to greatly outnumber planets and that it is hypothesized that habitability factors are likely to be similar to those of planets. There, however, key environmental differences which have a bearing on moons as potential sites for extraterrestrial life.
The strongest candidates of natural satellite habitability are currently icy satellites such as those of Jupiter and Saturn - Europa and Enceladus respectively. Though life there if existed, would likely be confined to sub-surface habitats, in contrast to the primarily surface biosphere of Earth. These candidates exist outside the circumstellar habitable zone which has historically defined the limits of life within the Solar System as the zone in which water can exist as liquid at the surface. In the Solar System's habitable zone only three natural satellites may be found - Earth's Moon, and Mars' moons Phobos and Deimos - none of which sustain an atmosphere or water in liquid form. Tidal forces are likely to play as significant role as stellar radiation in the potential habitability of natural satellites.
Extrasolar moons are not yet confirmed to exist. Detecting them is difficult if not impossible, though current methods are limited to transit timing. It is possible that some of their attributes could be determined by similar methods as those of transiting planets. Despite this some scientists estimate that there are as many habitable exomoons as habitable exoplanets. The identification of candidate moons such as KOI-433.02m, which has an Earth Similarity Index equal (ESI) equal to that of the highest scoring exoplanet KOI-1686.01, and the categorising of a similar number of habitable exomoon candidates as habitable exoplanets, would appear to indicate that the distribution of planetary mass exomoons within the habitable zone is at least the same if not greater than that of planets.
The conditions of habitability for natural satellites are similar to those of planetary habitability however there are several factors which differentiate natural satellite habitability and additionally extend their habitability outside of the planetary habitable zone.
Liquid water is suggested by many astrobiologists as a prerequisite for extraterrestrial life. There is growing evidence of sub-surface 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 received final confirmation to date.
For a stable orbit the ratio between the moon's orbital period Ps around its primary and that of the primary around its 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.
An atmosphere is considered by astrobiologists to be important in developing primal biochemistry, 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 around 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 strong magnetic field which can deflect stellar wind and radiation belts. NASA's Galileo's measurements hints large moons can have magnetic fields; it found Ganymede has its own magnetosphere, even though its mass is only 2.5% of Earth's. An exception is if the moon's atmosphere is constantly replenished by gases from sub-surface sources (as believed by some scientists to be the case with Titan).
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. Monoj Joshi and Robert Haberle (NASA/Ames Research Center) and their colleagues modelled the temperature on tide-locked exoplanets in the habitability zone of red dwarfs. They found that an atmosphere with a carbon-dioxide pressure of only 1 to 1.5 atmospheres not only allows habitable temperatures but allows liquid water on the dark side. The temperature range of a moon that is tidally locked to a gas giant could be less extreme than with a planet that locked to a sun. Even though no studies have been done on the subject, modest amounts of CO2 would make the temperature habitable.
Furthermore, 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.
Axial tilt and climate
Provided gravitational interaction of a moon with other satellites can be neglected, moons tend to be tidally locked with their planets. In addition to the rotational locking mentioned above, there will also be a process termed 'tilt erosion', which has originally been coined for the tidal erosion of planetary obliquity against a planet's orbit around its host star. The final spin state of a moon then consists of a rotational period equal to its orbital period around the planet and a rotational axis that is perpendicular to the orbital plane.
If the moon's mass is not too low compared to the planet, it may in turn stabilize the planet's axial tilt, i.e. its obliquity against the orbit around the star. On Earth, the Moon has played an important role in stabilizing the axial tilt of the Earth, thereby reducing the impact of gravitational perturbations from the other planets and ensuring only moderate climate variations throughout the planet. 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.
Being tidally locked to a giant planet or sub-brown-dwarf would allow for more moderate climates on a moon than there would be if the moon were a similar-sized planet orbiting in locked rotation in the habitable zone of the star. 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.
Complex conditions thought to be required for abiogenesis are not known to exist anywhere within the solar system. However several candidates beyond Sol's habitable zone have been identified that have some of the ingredients thought necessary for life to exist. The alternate theory of panspermia suggests life may have been introduced to such environments.
There is also the theoretical possibility of extraterrestrial biochemistries exotic beyond current human speculation.
Deliberate or accidental future forward-contamination by organisms originating from Earth is a distinct possibility in these potentially habitable environments. Such cases would make it difficult to determine where the origin of life was.
In the Solar System
The following is a list of natural satellites and environments in the Solar System with a possibility of harboring extraterrestrial life.
|Europa||Jupiter||Life on Europa||A subsurface ocean is maintained by geologic activity, tidal heating, and irradiation. The moon may have more water and oxygen than Earth and a thin oxygen atmosphere.|
|Enceladus||Saturn||Water could be maintained liquid under the surface due to geothermal activity.|
|Titan||Saturn||Life on Titan||Its atmosphere is considered similar to the early Earth although it is somewhat thicker. The surface is characterized by hydrocarbon lakes, cryovolcanos, and eventually by rain and snow. It has a remote possibility of an exotic methane-based biochemistry.|
|Callisto||Jupiter||Thought to have a sub-surface ocean heated by tidal forces.|
|Io||Jupiter||Due to its proximity to Jupiter, it is subject to intense tidal heating which makes it the most volcanically active object in the Solar System. The outgasing generates a trace atmosphere.|
|Triton||Neptune||Triton orbits retrograde around Neptune and cannot haved formed in-situ. It is believed to be a captured member from a former binary. Its high orbital inclination with respect to Neptune's equator drives significant tidal heating, responsible for the dashing surface arrangements observed by the Voyager 2 space probe. This heating possibly maintains a layer of liquid water or a subterranean ocean.|
|Charon||Pluto||Possible internal ocean of water and ammonia evidenced by possible cryovolcanic activity.|
No extrasolar natural satellites have yet been detected. Large planets in the Solar System like Jupiter and Saturn are known to have large moons with some of the conditions for life. Therefore some scientists speculate that large extrasolar planets (and double planets) may have similarly large moons that are potentially habitable. A moon with sufficient mass may support an atmosphere like Titan and may also sustain liquid water on the surface.
Massive exoplanets known to be located within a habitable zone (such as Gliese 876 b, 55 Cancri f, Upsilon Andromedae d, 47 Ursae Majoris b, HD 28185 b and HD 37124 c) are of particular interest as they may potentially possess natural satellites with liquid water on the surface.
Habitability of extrasolar moons has been considered in at least two studies published in peer-reviewed journals. René Heller & Rory Barnes considered stellar and planetary illumination on moons as well as the effect of eclipses on their orbit-averaged surface illumination. They also considered tidal heating as a threat for their habitability. In Sect. 4 in their paper, they introduce a new concept to define the habitable orbits of moons. Referring to the concept of the circumstellar habitable zone for planets, they define an inner border for a moon to be habitable around a certain planet and call it the circumplanetary "habitable edge". Moons closer to their planet than the habitable edge are uninhabitable. In a second study, René Heller  then included the effect of eclipses into this concept as well as constraints from a satellite's orbital stability. He found that, depending on a moon's orbital eccentricity, there is a minimum mass for stars to host habitable moons and located it around 0.2 solar masses.
The concept of habitable exo-moons has been popularized by the moons of Star Wars such as Return of the Jedi's forest moon of Endor, and Yavin 4 from A New Hope, several habitable planets and habitable moons in Stargate SG1, Pandora from the 2009 film Avatar, a broad range of terraformed habitable planets and moons (such as Whitefall, Persephone) in the Firefly TV series and related movie Serenity.
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