Natural satellite habitability
Encyclopedia
Natural satellite habitability is the measure of a natural satellite
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

's potential to sustain life
Life
Life is a characteristic that distinguishes objects that have signaling and self-sustaining processes from those that do not, either because such functions have ceased , or else because they lack such functions and are classified as inanimate...

. The study of natural satellite habitability is important to astrobiology
Astrobiology
Astrobiology is the study of the origin, evolution, distribution, and future of life in the universe. This interdisciplinary field encompasses the search for habitable environments in our Solar System and habitable planets outside our Solar System, the search for evidence of prebiotic chemistry,...

 for several reasons. While theoretical conditions under which life
Life
Life is a characteristic that distinguishes objects that have signaling and self-sustaining processes from those that do not, either because such functions have ceased , or else because they lack such functions and are classified as inanimate...

 might be sustained on natural satellite
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

s (moons) are similar to those of planets
Planetary habitability
Planetary habitability is the measure of a planet's or a natural satellite's potential to sustain life. Life may develop directly on a planet or satellite or be transferred to it from another body, a theoretical process known as panspermia...

 there are key environmental differences which can make moons of particular interest in the search for extraterrestrial life
Extraterrestrial life
Extraterrestrial life is defined as life that does not originate from Earth...

.

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
Habitable zone
In astronomy and astrobiology, a habitable zone is an umbrella term for regions that are considered favourable to life. The concept is inferred from the empirical study of conditions favourable for Life on Earth...

 the only such objects are The Moon (Luna), Phobos
Phobos (moon)
Phobos is the larger and closer of the two natural satellites of Mars. Both moons were discovered in 1877. With a mean radius of , Phobos is 7.24 times as massive as Deimos...

 and Deimos
Deimos (moon)
Deimos is the smaller and outer of Mars's two moons . It is named after Deimos, a figure representing dread in Greek Mythology. Its systematic designation is '.-Discovery:Deimos was discovered by Asaph Hall, Sr...

 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
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

 and Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

.

No extrasolar moon
Extrasolar moon
An extrasolar moon, or exomoon, is a natural satellite that orbits an extrasolar planet or other extrasolar body. Although no extrasolar moons have yet been observed, it can be inferred from the empirical study of natural satellites in the Solar System that they are likely to be common elements of...

s 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.

Natural satellites are considered potential candidates for space colonization
Space colonization
Space colonization is the concept of permanent human habitation outside of Earth. Although hypothetical at the present time, there are many proposals and speculations about the first space colony...

 by human
Human
Humans are the only living species in the Homo genus...

s as humans can inhabit moons through artificial environment, having already briefly inhabited our moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

 (Luna). However artificial environments are not considered in the definition of habitability. The most Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

-like moon in our solar system is Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....

, which is extremely hostile to human life. Terraforming
Terraforming
Terraforming of a planet, moon, or other body is the hypothetical process of deliberately modifying its atmosphere, temperature, surface topography or ecology to be similar to those of Earth, in order to make it habitable by terrestrial organisms.The term is sometimes used more generally as a...

 of moons may be possible but outside the limits of current technology.

Possible origins

Complex conditions thought to be required for abiogenesis
Abiogenesis
Abiogenesis or biopoesis is the study of how biological life arises from inorganic matter through natural processes, and the method by which life on Earth arose...

 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
Forward-contamination
Forward-contamination is the contamination of other worlds with Terrestrial microbes. The risk of forward-contamination is twofold: that human beings may accidentally seed a previously sterile world, thus creating "extraterrestrials" that are really of terrestrial origin ; or that an...

 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.

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
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...

 and Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

; 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
Orbital period
The orbital period is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.There are several kinds of...

 around its primary and that of the primary around its sun cannot be too small. 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
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

 from a Sun-like star.

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
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

's mass to retain most of its atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

 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
Sputtering
Sputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles. It is commonly used for thin-film deposition, etching and analytical techniques .-Physics of 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
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

, 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
Sputtering
Sputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles. It is commonly used for thin-film deposition, etching and analytical techniques .-Physics of sputtering:...

 is for the moon to have strong magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

. NASA's Galileo's measurements hints large moons can have strong magnetic field. It detected Earth-like magnetic field around Ganymede
Ganymede (moon)
Ganymede is a satellite of Jupiter and the largest moon in the Solar System. It is the seventh moon and third Galilean satellite outward from Jupiter. Completing an orbit in roughly seven days, Ganymede participates in a 1:2:4 orbital resonance with the moons Europa and Io, respectively...

 even though its mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

 is only 2.5% of Earth's

Tidal effects

While the effects of tidal acceleration
Tidal acceleration
Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite , and the primary planet that it orbits . The "acceleration" is usually negative, as it causes a gradual slowing and recession of a satellite in a prograde orbit away from the primary, and a corresponding...

 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 giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...

s or brown dwarf
Brown dwarf
Brown dwarfs are sub-stellar objects which are too low in mass to sustain hydrogen-1 fusion reactions in their cores, which is characteristic of stars on the main sequence. Brown dwarfs have fully convective surfaces and interiors, with no chemical differentiation by depth...

s 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
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

/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.

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.
Name System Article Notes
Europa
Europa (moon)
Europa Slightly smaller than Earth's Moon, Europa is primarily made of silicate rock and probably has an iron core. It has a tenuous atmosphere composed primarily of oxygen. Its surface is composed of ice and is one of the smoothest in the Solar System. This surface is striated by cracks and...

 
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
Enceladus
Enceladus (moon)
Enceladus is the sixth-largest of the moons of Saturn. It was discovered in 1789 by William Herschel. Until the two Voyager spacecraft passed near it in the early 1980s very little was known about this small moon besides the identification of water ice on its surface...

 
Saturn Has water and geothermal activity
Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....

 
Saturn Life on Titan
Life on Titan
Whether or not there is life on Titan, the largest moon of Saturn, is at present an open question and a topic of scientific discussion and research....

 
Considered similar to an early Earth with a thicker atmosphere, hydrocarbon lakes, cryovolcano
Cryovolcano
A cryovolcano is a volcano that erupts volatiles such as water, ammonia or methane, instead of molten rock. Collectively referred to as cryomagma or ice-volcanic melt, these substances are usually liquids and form plumes, but can also be in vapour form...

s, with a remote possibility of an exotic methane-based biochemistry
Callisto
Callisto (moon)
Callisto named after the Greek mythological figure of Callisto) is a moon of the planet Jupiter. It was discovered in 1610 by Galileo Galilei. It is the third-largest moon in the Solar System and the second largest in the Jovian system, after Ganymede. Callisto has about 99% the diameter of the...

 
Jupiter Thought to have a sub-surface ocean heated by radiation.
Io
Io (moon)
Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus....

 
Jupiter Volcanically active, generating heat energy with a trace atmosphere
Triton
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...

 
Neptune Possible layer of liquid water or subterranean ocean and geologically active.
Charon
Charon (moon)
Charon is the largest satellite of the dwarf planet Pluto. It was discovered in 1978 at the United States Naval Observatory Flagstaff Station. Following the 2005 discovery of two other natural satellites of Pluto , Charon may also be referred to as Pluto I...

 
Pluto Possible internal ocean of water and ammonia evidenced by possible cryovolcanic activity.

Extrasolar

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 planet
Double planet
In astronomy, double planet and binary planet are informal terms used to describe a binary system of two astronomical objects that each satisfy the definition of planet and that are near enough to each other to have a significant gravitational effect on each other compared with the effect of the...

s) may have similarly large moons that are potentially habitable.

Large exoplanets known to be located within a habitable zone (such as Gliese 876 b
Gliese 876 b
Gliese 876 b is an extrasolar planet orbiting the red dwarf star Gliese 876. It completes one orbit in approximately 61 days. Discovered in June 1998, Gliese 876 b was the first planet to be discovered orbiting a red dwarf star.- Discovery :...

, 55 Cancri f
55 Cancri f
55 Cancri f is an extrasolar planet approximately 41 light-years away from Earth in the constellation of Cancer...

, Upsilon Andromedae d
Upsilon Andromedae d
Upsilon Andromedae d is an extrasolar planet orbiting the Sun-like star Upsilon Andromedae A. Its discovery in April 1999 by Geoffrey Marcy and R. Paul Butler made Upsilon Andromedae the first star to be known to host a multiple-planet planetary system...

, 47 Ursae Majoris b
47 Ursae Majoris b
47 Ursae Majoris b is an extrasolar planet approximately 46 light-years from Earth in the constellation of Ursa Major. The planet was discovered located in a long-period orbit around the star 47 Ursae Majoris in January 1996 and as of 2011 is the innermost of three known planets in its planetary...

 HD 28185 b
HD 28185 b
HD 28185 b is an extrasolar planet approximately 138 light-years away from Earth in the constellation of Eridanus. The planet was discovered orbiting the Sun-like star HD 28185 in April 2001 as a part of the CORALIE survey for southern extrasolar planets, and its existence was independently...

, 16 Cygni Bb
16 Cygni Bb
16 Cygni Bb or 16 Cyg Bb is an extrasolar planet approximately 70 light-years away in the constellation of Cygnus. The planet was discovered orbiting the sun-like star 16 Cygni B, one of two solar-mass components of the triple star system 16 Cygni...

, HD 37124 c
HD 37124 c
HD 37124 c is an extrasolar planet approximately 108 light-years away in the constellation of Taurus. The planet was discovered in 2002 orbiting the star HD 37124. The planet is most likely to be a gas giant ....

, 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
Star Wars
Star Wars is an American epic space opera film series created by George Lucas. The first film in the series was originally released on May 25, 1977, under the title Star Wars, by 20th Century Fox, and became a worldwide pop culture phenomenon, followed by two sequels, released at three-year...

 and Pandora from Fictional universe of Avatar
Fictional universe of Avatar
In the 2009 science fiction film Avatar, director James Cameron conceived a fictional universe in which humans seek to mine unobtanium on the fictional exoplanetary moon, Pandora. The Earth-like moon is inhabited by a sapient indigenous humanoid species called the Na'vi, and varied fauna and flora...

(2009 film).
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