Natural satellite habitability

Artist's impression of a hypothetical habitable moon of Upsilon Andromedae d

Natural satellite habitability is the measure of a natural satellite's potential to sustain life. The study of natural satellite habitability is important to astrobiology for several reasons. While theoretical conditions under which life might be sustained on natural satellites (moons) are similar to those of planets there are key environmental differences which can make moons of particular interest in the search for extraterrestrial life.

Scientists generally consider the probability of life on natural satellites within our own solar system to be remote, though the possibility has not been ruled out. Within our solar system's habitable zone the only such objects are The Moon (Luna), Phobos and Deimos and none have either an atmosphere or water in liquid form. Significantly however, some of the strongest known candidates for harbouring extraterrestrial life are located outside of the solar habitable zone, on satellites of Jupiter and Saturn.

No extrasolar moons are yet known to exist and there is no way of knowing how common they may be, what their attributes are or how many could be considered habitable. However some scientists estimate that there are as many habitable exomoons as habitable planets.^[1]

Natural satellites are considered potential candidates for space colonization by humans as humans can inhabit moons through artificial environment, having already briefly inhabited our moon (Luna). However artificial environments are not considered in the definition of habitability. The most Earth-like moon in our solar system is Titan, which is extremely hostile to human life. Terraforming of moons may be possible but outside the limits of current technology.

Possible origins

Complex conditions thought to be required for abiogenesis are not known to exist on natural satellites within our solar system. However several candidates outside the solar system's habitable zone have been identified that have some of the ingredients thought necessary for life to exist. The theory of panspermia suggests that life may have been introduced to such environments. There is also the theoretical possibility of exotic biochemistries.

Deliberate or accidental future forward-contamination by organisms originating from Earth is a distinct possibility in these potentially habitable environments. Such cases may make it difficult to determine where the origin of life was.

Presumed conditions

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.^[2]

Liquid water

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 our solar system orbiting the gas giant planets Jupiter, Saturn, Uranus and Neptune; however, none of these sub-surface moon water bodies has received final confirmation to date.

Orbital stability

For a stable orbit the ratio between the moon's orbital period P_s around its primary and that of the primary around its star P_p must be < 1/9.^[3] 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.^[4]

Atmosphere

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 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 that are in our solar system, and likely others, have magnetospheres with radiation belts potent enough to completely erode an atmosphere of an Earth-like moon in just a few hundred million years. Stellar winds can strip gas atoms from the top of an atmosphere and random thermal collisions faster than the moon's escape velocity can cause 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.^[5]

One way to decrease loss from sputtering is for the moon to have strong magnetic field. NASA's Galileo's measurements hints large moons can have strong magnetic field. It detected Earth-like magnetic field around Ganymede even though its mass is only 2.5% of Earth's^[4]

The 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).

Tidal effects

While the effects of tidal acceleration is relatively modest on planets, it can be a significant source of energy on natural satellites and an alternative energy source for sustaining life.

Moons orbiting gas giants or brown dwarfs are likely to be tidally locked to its primary: that is, its day is as long as its orbit. While tidal locking may adversely affects planets within habitable zones by interfering with the distribution of stellar radiation, it may work in favour of satellite habitability by allowing for more consistent exposure to this powerful energy source. 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 should be less extreme than with a planet that locked to a sun. Even though no studies have been done on the subject, just modest amounts of CO₂ would make the temperature habitable.^[4]

Furthermore, tidal effects could also allow the moon to sustain plate tectonics, which would cause volcanic activity to regulate the moon's temperature^[6][7] and create a geodynamo effect which would give the satellite a strong magnetic field.^[8]

Axial Tilt

Provided gravitational interaction of a moon with other satellites can be neglected, the moon tends to be tidally locked with the planet. 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.^[9] 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.

In case 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.^[10] 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.^[11][12]

Temperature

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.^[13] 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 could occur. While, depending on the orbital period around the planetary body, one rotation about the axis may be longer than a day like that on Earth (if the moon is tidally locked), the effect would be less severe than if the planet were locked to the star, resulting in an eternal day side and night side.

In the Solar System

Europa, a moon of Jupiter, with a possibility of having life

The following is a list of natural satellites and environments in the Solar System with a possibility of harboring extraterrestrial life.

Extrasolar

Artist's impression of a hypothetical moon around a Saturn-like exoplanet that could be habitable.

No extrasolar natural satellites have yet been detected. Large planets within our 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 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.

In fiction

The concept of habitable exomoons has been popularized by Star Wars, including moons such as Return of the Jedi's Endor (forest moon) and Yavin 4 from A New Hope, and Pandora from the fictional universe of Avatar (2009 film).

See also

• Earth analog

References