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 the 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.

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

Main article: Extraterrestrial 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 around its primary and that of the primary around its sun cannot be too small.^{[citation needed]} 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. ^[3]

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.

A moon is estimated to need at least 7% of Earth's mass^[4] to retain most of its atmosphere for 4.6 billion years (Earth's current age) if it had a Mars-like density and an Earth-like atmospheric temperature structure, because some of the gas atoms at the top of an atmosphere will get kicked by random thermal collisions to faster than the moon's escape velocity and fly away.

A moon can also lose its atmosphere through sputtering, a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles. 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.

One way to decrease loss of atmosphere by 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^[3]

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. ^[3]

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.

Name	System	Article	Notes
Europa	Jupiter	Life on Europa	Has an ocean heated by volcanic activity, tidal energy and radiation. Also may have more water and oxygen than Earth, including an oxygen atmosphere[5]
Enceladus	Saturn		Has water and geothermal activity[6]
Titan	Saturn	Life on Titan	Considered similar to an early Earth with a thicker atmosphere, hydrocarbon lakes, cryovolcanos, with a remote possibility of an exotic methane-based biochemistry[7]
Callisto	Jupiter		Thought to have a sub-surface ocean heated by radiation.[8]
Io	Jupiter		Volcanically active, generating heat energy with a trace atmosphere[9]
Triton	Neptune		Possible layer of liquid water or subterranean ocean[10] and geologically active.
Charon	Pluto		Possible internal ocean of water and ammonia evidenced by possible cryovolcanic activity.[11]

Extrasolar

Further information: Extrasolar moon

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.

Large 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, 16 Cygni Bb, HD 37124 c, HD 29587 b and HIP 57050 b) 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 popularised by Return of the Jedi's Endor (forest moon) from Star Wars and Pandora from Fictional universe of Avatar (2009 film).

See Also

• Earth analog

References

1. ^ Shriber, Michael Detecting Life Friendly Moons Astrobiology Magazine. 10/26/09
2. ^ Scharf, Caleb Exomoons Ever Closer Scientific American. October 4, 2011
3. ^ ^{a b c} Andrew J. LePage. "Habitable Moons:What does it take for a moon — or any world — to support life?". SkyandTelescope.com. http://www.skyandtelescope.com/resources/seti/3304591.html?showAll=y&c=y. Retrieved 2011-07-11. 
4. ^ "In Search Of Habitable Moons". Pennsylvania State University. http://www.xs4all.nl/~carlkop/habit.html. Retrieved 2011-07-11. 
5. ^ "Moon of Jupiter could support life:Europa has a liquid ocean that lies beneath several miles of ice". msnbc.com. http://www.msnbc.msn.com/id/33226921/ns/technology_and_science-space/. Retrieved 2011-07-10. 
6. ^ "Liquid water on Saturn moon could support life:Cassini spacecraft sees signs of geysers on icy Enceladus". msnbc.com. http://www.msnbc.msn.com/id/11736311/ns/technology_and_science-space/. Retrieved 2011-07-10. 
7. ^ "Life On Titan? New Clues to What's Consuming Hydrogen, Acetylene On Saturn's Moon". Science Daily. 2010-06-07. http://www.sciencedaily.com/releases/2010/06/100606103125.htm. Retrieved 2011-07-10. 
8. ^ Phillips, T. (1998-10-23). "Callisto makes a big splash". Science@NASA. http://science.nasa.gov/newhome/headlines/ast22oct98_2.htm. 
9. ^ Charles Q. Choi (2010-06-07). "Chance For Life On Io". Science Daily. http://www.spacedaily.com/reports/Chance_For_Life_On_Io_999.html. Retrieved 2011-07-10. 
10. ^ Louis Neal Irwin; Dirk Schulze-Makuch (June 2001). "Assessing the Plausibility of Life on Other Worlds". Astrobiology 1 (2): 143–60. Bibcode 2001AsBio...1..143I. doi:10.1089/153110701753198918. PMID 12467118. 
11. ^ "Water on Pluto moon". The Sydney Morning Herald. 2007-07-19. http://www.smh.com.au/news/science/water-on-pluto-sparks-hopes-of-marine-life/2007/07/19/1184559914231.html.