Habitability of natural satellites

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Europa, a potentially habitable moon of Jupiter Europa - Perijove 45 (53255790801).png
Europa, a potentially habitable moon of Jupiter

The habitability of natural satellites is the potential of moons to provide habitats for life, though it is not an indicator that they harbor it. 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. There are, nevertheless, significant environmental variables specific to moons.

Contents

It is projected that parameters for surface habitats will be comparable to those of planets like Earth, namely stellar properties, orbit, planetary mass, atmosphere and geology. Of the natural satellites in the Solar System's habitable zone – the Moon, two Martian satellites (though some estimates put those outside it) [1] and numerous minor-planet moons – all lack the conditions for surface water. Unlike the Earth, all planetary mass moons of the Solar System are tidally locked and it is not yet known to what extent this and tidal forces influence habitability.

Research suggests that deep biospheres like that of Earth are possible. [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. While the lunar surface is hostile to life as we know it, a deep lunar biosphere (or that of similar bodies) cannot yet be ruled out; [6] [7] deep exploration would be required for confirmation.

Exomoons are not yet confirmed to exist and their detection may be limited to transit-timing variation, which is not currently sufficiently sensitive. [8] It is possible that some of their attributes could be found through study of their transits. [9] Despite this, some scientists estimate that there are as many habitable exomoons as habitable exoplanets. [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. [12]

Tidal forces are likely to play as significant a role providing heat as stellar radiation. [13] [14]

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 the planetary habitable zone. [15]

Liquid water

Liquid water is thought by most astrobiologists to be an essential prerequisite for extraterrestrial life. There is growing evidence of subsurface 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 been confirmed to date.

Orbital stability

For a stable orbit the ratio between the moon's orbital period Ps around its primary 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. [16] [17] 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. [18]

Atmosphere

An atmosphere is considered by astrobiologists to be important in developing prebiotic chemistry, 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. [19]

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 about 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. [20] One way to decrease loss from sputtering is for the moon to have a strong magnetic field of its own that can deflect stellar wind and radiation belts. 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. [18] Alternatively, the moon's atmosphere may be constantly replenished by gases from subsurface sources, as thought by some scientists to be the case with Titan. [21]

Tidal effects

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. Scientists at the NASA Ames Research Center modelled the temperature on tide-locked exoplanets in the habitability zone of red dwarf stars. They found that an atmosphere with a carbon dioxide (CO
2) pressure of only 1–1.5 standard atmospheres (15–22 psi) not only allows habitable temperatures, but allows liquid water on the dark side of the satellite. The temperature range of a moon that is tidally locked to a gas giant could be less extreme than with a planet locked to a star. Even though no studies have been done on the subject, modest amounts of CO
2 are speculated to make the temperature habitable. [18]

Tidal effects could also allow a moon to sustain plate tectonics, which would cause volcanic activity to regulate the moon's temperature [22] [23] and create a geodynamo effect which would give the satellite a strong magnetic field. [24]

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. [25] 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.

An artist rendering of an exomoon with an Earth-like atmosphere with liquid water filling its craters and water clouds. It orbits a Jupiter-like gas giant exoplanet in the habitable zone, mostly white due to water vapor clouds (Class II, in Sudarsky's exoplanet classification) Upsilon Andromedae Habitable Moon Surface.jpg
An artist rendering of an exomoon with an Earth-like atmosphere with liquid water filling its craters and water clouds. It orbits a Jupiter-like gas giant exoplanet in the habitable zone, mostly white due to water vapor clouds (Class II, in Sudarsky's exoplanet classification)

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. [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. [27] [28]

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. [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.

Habitable edge

In 2012, scientists introduced a concept to define the habitable orbits of moons. [30] The concept is similar to the circumstellar habitable zone for planets orbiting a star, but for moons orbiting a planet. This inner border, which they call the circumplanetary habitable edge, delimits the region in which a moon can be habitable around its planet. Moons closer to their planet than the habitable edge are uninhabitable.

Magnetosphere

The magnetic environment of exomoons, which is critically triggered by the intrinsic magnetic field of the host planet, has been identified as another factor of exomoon habitability. [31] Most notably, it was found that moons at distances between about 5 and 20 planetary radii from a giant planet could be habitable from an illumination and tidal heating point of view, [31] but still the planetary magnetosphere would critically influence their habitability. [31]

Tidal-locking

Earth-sized exoplanets in the habitable zone around red dwarfs are often tidally locked to the host star. This has the effect that one hemisphere always faces the star, while the other remains in darkness. Like an exoplanet, an exomoon can potentially become tidally locked to its primary. 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.

Scientists consider tidal heating as a threat for the habitability of exomoons. [32]

In the Solar System

The following is a list of natural satellites and environments in the Solar System with a possibility of hosting habitable environments:

NameSystemArticleNotes
Europa Jupiter Colonization of Europa Thought to have a subsurface ocean maintained by geologic activity, tidal heating, and irradiation. [33] [34] The moon may have more water and oxygen than Earth and an oxygen exosphere. [35]
Enceladus Saturn Enceladus – potential habitability Thought to have a subsurface liquid water ocean due to tidal heating [36] or geothermal activity. [37] Free molecular hydrogen (H2) has been detected, providing another potential energy source for life. [38]
Titan Saturn Colonization of Titan Its atmosphere is considered similar to that of the early Earth, although somewhat thicker. The surface is characterized by hydrocarbon lakes, cryovolcanos, and methane rain and snow. Like Earth, Titan is shielded from the solar wind by a magnetosphere, in this case its parent planet for most of its orbit, but the interaction with the moon's atmosphere remains sufficient to facilitate the creation of complex organic molecules. It has a remote possibility of an exotic methane-based biochemistry. [39]
Callisto Jupiter Callisto – potential habitability Thought to have a subsurface ocean heated by tidal forces. [40] [41]
Ganymede Jupiter Ganymede – Subsurface oceans Thought to have a magnetic field, with ice and subterranean oceans stacked up in several layers, with salty water as a second layer on top of the rocky iron core. [42] [43]
Io JupiterDue 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 outgassing generates a trace atmosphere. [44]
Triton NeptuneIts high orbital inclination with respect to Neptune's equator drives significant tidal heating, [45] which suggests a layer of liquid water or a subsurface ocean. [46]
Dione SaturnSimulations made in 2016 suggest an internal water ocean under 100 kilometres of crust possibly suitable for microbial life. [47]
Charon PlutoPossible internal ocean of water and ammonia, based on suspected cryovolcanic activity. [48]

Extrasolar

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

A small list of exomoon candidates has been assembled by various exoastronomy teams, but none of them have been confirmed. Given the general planet-to-satellite(s) mass ratio of 10,000, Large Saturn or Jupiter sized gas planets in the habitable zone are believed to be the best candidates to harbour Earth-like moons with more than 120 such planets by 2018. [12] 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 will depend on stellar and planetary illumination on moons as well as the effect of eclipses on their orbit-averaged surface illumination. [49] Beyond that, tidal heating might play a role for a moon's habitability. In 2012, scientists introduced a concept to define the habitable orbits of moons; [49] they define an inner border of an habitable moon around a certain planet and call it the circumplanetary "habitable edge". Moons closer to their planet than the habitable edge are uninhabitable. When effects of eclipses as well as constraints from a satellite's orbital stability are used to model the runaway greenhouse limit of hypothetical moons, it is estimated that — depending on a moon's orbital eccentricity — there is a minimum mass of roughly 0.20 solar masses for stars to host habitable moons within the stellar habitable zone. [17] The magnetic environment of exomoons, which is critically triggered by the intrinsic magnetic field of the host planet, has been identified as another factor of exomoon habitability. [31] Most notably, it was found that moons at distances between about 5 and 20 planetary radii from a giant planet could be habitable from an illumination and tidal heating point of view, [31] but still the planetary magnetosphere would critically influence their habitability. [31]

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

See also

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An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not then recognized as such. The first confirmation of the detection occurred in 1992. A different planet, first detected in 1988, was confirmed in 2003. As of 7 November 2024, there are 5,787 confirmed exoplanets in 4,320 planetary systems, with 969 systems having more than one planet. The James Webb Space Telescope (JWST) is expected to discover more exoplanets, and to give more insight into their traits, such as their composition, environmental conditions, and potential for life.

<span class="mw-page-title-main">Rare Earth hypothesis</span> Hypothesis that complex extraterrestrial life is improbable and extremely rare

In planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the origin of life and the evolution of biological complexity, such as sexually reproducing, multicellular organisms on Earth, and subsequently human intelligence, required an improbable combination of astrophysical and geological events and circumstances. According to the hypothesis, complex extraterrestrial life is an improbable phenomenon and likely to be rare throughout the universe as a whole. The term "Rare Earth" originates from Rare Earth: Why Complex Life Is Uncommon in the Universe (2000), a book by Peter Ward, a geologist and paleontologist, and Donald E. Brownlee, an astronomer and astrobiologist, both faculty members at the University of Washington.

<span class="mw-page-title-main">Habitable zone</span> Orbits where planets may have liquid surface water

In astronomy and astrobiology, the habitable zone (HZ), or more precisely the circumstellar habitable zone (CHZ), is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure. The bounds of the HZ are based on Earth's position in the Solar System and the amount of radiant energy it receives from the Sun. Due to the importance of liquid water to Earth's biosphere, the nature of the HZ and the objects within it may be instrumental in determining the scope and distribution of planets capable of supporting Earth-like extraterrestrial life and intelligence.

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<span class="mw-page-title-main">Gliese 876 b</span> Extrasolar planet orbiting Gliese 876

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<span class="mw-page-title-main">Kepler-90h</span> Exoplanet in the constellation Draco

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<span class="mw-page-title-main">Satellite system (astronomy)</span> Set of gravitationally bound objects in orbit

A satellite system is a set of gravitationally bound objects in orbit around a planetary mass object or minor planet, or its barycenter. Generally speaking, it is a set of natural satellites (moons), although such systems may also consist of bodies such as circumplanetary disks, ring systems, moonlets, minor-planet moons and artificial satellites any of which may themselves have satellite systems of their own. Some bodies also possess quasi-satellites that have orbits gravitationally influenced by their primary, but are generally not considered to be part of a satellite system. Satellite systems can have complex interactions including magnetic, tidal, atmospheric and orbital interactions such as orbital resonances and libration. Individually major satellite objects are designated in Roman numerals. Satellite systems are referred to either by the possessive adjectives of their primary, or less commonly by the name of their primary. Where only one satellite is known, or it is a binary with a common centre of gravity, it may be referred to using the hyphenated names of the primary and major satellite.

<span class="mw-page-title-main">Superhabitable world</span> Hypothetical type of planet or moon that may be better-suited for life than Earth

A superhabitable world is a hypothetical type of planet or moon that is better suited than Earth for the emergence and evolution of life. The concept was introduced in a 2014 paper by René Heller and John Armstrong, in which they criticized the language used in the search for habitable exoplanets and proposed clarifications. The authors argued that knowing whether a world is located within the star's habitable zone is insufficient to determine its habitability, that the principle of mediocrity cannot adequately explain why Earth should represent the archetypal habitable world, and that the prevailing model of characterization was geocentric or anthropocentric in nature. Instead, they proposed a biocentric approach that prioritized astrophysical characteristics affecting the abundance and variety of life on a world's surface.

<span class="mw-page-title-main">TRAPPIST-1d</span> Small Venus-like exoplanet orbiting TRAPPIST-1

TRAPPIST-1d is a small exoplanet, which orbits on the inner edge of the habitable zone of the ultracool dwarf star TRAPPIST-1, located 40.7 light-years away from Earth in the constellation of Aquarius. The exoplanet was found by using the transit method. The first signs of the planet were announced in 2016, but it was not until the following years that more information concerning the probable nature of the planet was obtained. TRAPPIST-1d is the second-least massive planet of the system and is likely to have a compact hydrogen-poor atmosphere similar to Venus, Earth, or Mars. It receives just 4.3% more sunlight than Earth, placing it on the inner edge of the habitable zone. It has about <5% of its mass as a volatile layer, which could consist of atmosphere, oceans, and/or ice layers. A 2018 study by the University of Washington concluded that TRAPPIST-1d might be a Venus-like exoplanet with an uninhabitable atmosphere. The planet is an eyeball planet candidate.

HIP 57274 d is an exoplanet orbiting the K-type main sequence star HIP 57274 about 84.5 light-years (26 parsecs, or nearly 8.022×1016 km) from Earth in the constellation Cetus. It orbits within the outer part of its star's habitable zone, at a distance of 1.01 AU. The exoplanet was found by using the radial velocity method, from radial-velocity measurements via observation of Doppler shifts in the spectrum of the planet's parent star.

<span class="mw-page-title-main">Proxima Centauri b</span> Terrestrial planet orbiting Proxima Centauri

Proxima Centauri b, also referred to as Alpha Centauri Cb, is an exoplanet orbiting within the habitable zone of the red dwarf star Proxima Centauri, which is the closest star to the Sun and part of the larger triple star system Alpha Centauri. It is about 4.2 light-years from Earth in the constellation Centaurus, making it and Proxima d, along with the currently disputed Proxima c, the closest known exoplanets to the Solar System.

<span class="mw-page-title-main">Habitable zone for complex life</span>

A Habitable Zone for Complex Life (HZCL) is a range of distances from a star suitable for complex aerobic life. Different types of limitations preventing complex life give rise to different zones. Conventional habitable zones are based on compatibility with water. Most zones start at a distance from the host star and then end at a distance farther from the star. A planet would need to orbit inside the boundaries of this zone. With multiple zonal constraints, the zones would need to overlap for the planet to support complex life. The requirements for bacterial life produce much larger zones than those for complex life, which requires a very narrow zone.

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