Natural satellite

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A natural satellite, or moon, is a celestial body that orbits another body, e.g. a planet, which is called its primary. There are 173 known natural satellites orbiting planets in the Solar System,[1][2] as well as at least eight orbiting IAU-listed dwarf planets.[3] As of January 2012, over 200 minor-planet moons have been discovered.[4] There are 76 known objects in the asteroid belt with satellites (five with two satellites each), four Jupiter trojans, 39 near-Earth objects (two with two satellites each), and 14 Mars-crossers.[4] There are also 84 known natural satellites of trans-Neptunian objects.[4] Some 150 additional small bodies have been observed within rings of Saturn, but only a few were tracked long enough to establish orbits. Planets around other stars are likely to have satellites as well, though numerous candidates have been detected to date, none have yet been confirmed.

Of the inner planets, Mercury and Venus have no natural satellites; Earth has one large natural satellite, known as the Moon; and Mars has two tiny natural satellites, Phobos and Deimos. The large gas giants have extensive systems of natural satellites, including half a dozen comparable in size to Earth's Moon: the four Galilean moons, Saturn's Titan, and Neptune's Triton. Saturn has an additional six mid-sized natural satellites massive enough to have achieved hydrostatic equilibrium, and Uranus has five. It has been suggested that some satellites may potentially harbour life, though there is currently no direct evidence of life.

The Earth–Moon system is unique in that the ratio of the mass of the Moon to the mass of the Earth is much greater than that of any other natural-satellite–planet ratio in the Solar System (although there are minor-planet systems with even greater ratios, notably the PlutoCharon system).

Among the dwarf planets, Ceres and Makemake have no known natural satellites. Pluto has the relatively large natural satellite Charon and four smaller natural satellites.[5] Haumea has two natural satellites, and Eris has one. The Pluto–Charon system is unusual in that the center of mass lies in open space between the two, a characteristic sometimes associated with a double-planet system.

Nineteen natural satellites are large enough to be round, and one, Saturn's moon, Titan, has a substantial atmosphere.

Origin and orbital characteristics[edit]

Two moons: Saturn's natural satellite Dione occults Enceladus

The natural satellites orbiting relatively close to the planet on prograde, uninclined circular orbits (regular satellites) are generally believed to have been formed out of the same collapsing region of the protoplanetary disk that created its primary. In contrast, irregular satellites (generally orbiting on distant, inclined, eccentric and/or retrograde orbits) are thought to be captured asteroids possibly further fragmented by collisions. Most of the major natural satellites of the Solar System have regular orbits, while most of the small natural satellites have irregular orbits.[6] The Earth's Moon[7] and possibly Charon[8] are exceptions among large bodies in that they are believed to have originated by the collision of two large proto-planetary objects (see the giant impact hypothesis). The material that would have been placed in orbit around the central body is predicted to have reaccreted to form one or more orbiting natural satellites. As opposed to planetary-sized bodies, asteroid moons are thought to commonly form by this process. Triton is another exception; although large and in a close, circular orbit, its motion is retrograde and it is thought to be a captured dwarf planet.

Tidal locking[edit]

Main article: Tidal locking

Most regular moons (natural satellites following relatively close and prograde orbits with small orbital inclination and eccentricity) in the Solar System are tidally locked to their respective primaries, meaning that the same side of the natural satellite always faces its planet. The only known exception is Saturn's natural satellite Hyperion, which rotates chaotically because of the gravitational influence of Titan.

In contrast, the outer natural satellites of the gas giants (irregular satellites) are too far away to have become locked. For example, Jupiter's natural satellite Himalia, Saturn's natural satellite Phoebe, and Neptune's natural satellite Nereid have rotation periods in the range of ten hours, while their orbital periods are hundreds of days.

Satellites of satellites[edit]

Artist impression of Rhea's proposed rings

No "moons of moons" (natural satellites that orbit a natural satellite of another body) are currently known as of 2014. In most cases, the tidal effects of the primary would make such a system unstable.

However, calculations performed after the recent detection[9] of a possible ring system around Saturn's natural satellite Rhea indicate that satellites orbiting Rhea would have stable orbits. Furthermore, the suspected rings are thought to be narrow,[10] a phenomenon normally associated with shepherd moons. However, targeted images taken by the Cassini spacecraft failed to detect any rings associated with Rhea.[11]

It has also been proposed that Saturn's satellite Iapetus possessed a subsatellite in the past; this is one of several hypotheses that have been put forward to account for its equatorial ridge.[12]

Trojan satellites[edit]

Two natural satellites are known to have small companions at their L4 and L5 Lagrangian points, sixty degrees ahead and behind the body in its orbit. These companions are called trojan moons, as their orbits are analogous to the Trojan asteroids of Jupiter. The trojan moons are Telesto and Calypso, which are the leading and following companions, respectively, of Tethys; and Helene and Polydeuces, the leading and following companions of Dione.

Asteroid satellites[edit]

Main article: Asteroid moon

The discovery of 243 Ida's natural satellite Dactyl in the early 1990s confirmed that some asteroids have natural satellites; indeed, 87 Sylvia has two. Some, such as 90 Antiope, are double asteroids with two comparably sized components.

Shape[edit]

The relative masses of the natural satellites of the Solar System. Mimas, Enceladus, and Miranda are too small to be visible at this scale. All the irregularly shaped natural satellites, even added together, would also be too small to be visible.

Neptune's moon Proteus is the largest irregularly shaped natural satellite. All other known natural satellites that are at least the size of Uranus's Miranda have lapsed into rounded ellipsoids under hydrostatic equilibrium, i.e. are "round/rounded satellites". The larger natural satellites, being tidally locked, tend toward ovoid (egg-like) shapes: squat at their poles and with longer equatorial axes in the direction of their primaries (their planets) than in the direction of their motion. Saturn's moon Mimas, for example, has a major axis 9% greater than its polar axis and 5% greater than its other equatorial axis. Methone, another of Saturn's moons, is only around 3 km in diameter and visibly egg-shaped. The effect is smaller on the largest natural satellites, where their own gravity is greater relative to the effects of tidal distortion, especially those that orbit less massive planets or, as in the case of the Moon, at greater distances.

Name Satellite of Difference in axes
km
% of mean
diameter
Mimas Saturn 33.4 (20.4 / 13.0) 8.4 (5.1 / 3.3)
Enceladus Saturn 16.6 3.3
Miranda Uranus 14.2 3.0
Tethys Saturn 25.8 2.4
Io Jupiter 29.4 0.8
Moon (Luna) Earth 4.3 0.1

Geological activity[edit]

Of the nineteen known natural satellites in the Solar System that are massive enough to have lapsed into hydrostatic equilibrium, several remain geologically active today. Io is the most volcanically active body in the Solar System, while Europa, Enceladus, Titan and Triton display evidence of ongoing tectonic activity and cryovolcanism. In the first three cases, the geological activity is powered by the tidal heating resulting from having eccentric orbits close to their gas giant primaries. (This mechanism would have also operated on Triton in the past, before its orbit was circularized.) Many other natural satellites, such as Earth's Moon, Ganymede, Tethys and Miranda, show evidence of past geological activity, resulting from energy sources such as the decay of their primordial radioisotopes, greater past orbital eccentricities (due in some cases to past orbital resonances), or the differentiation or freezing of their interiors. Enceladus and Triton both have active features resembling geysers, although in the case of Triton solar heating appears to provide the energy. Titan and Triton have significant atmospheres; Titan also has hydrocarbon lakes, and presumably methane rain. Four of the largest natural satellites, Europa, Ganymede, Callisto, and Titan, are thought to have subsurface oceans of liquid water, while smaller Enceladus may have localized subsurface liquid water.

Natural satellites of the Solar System[edit]

Discovery image of Styx, photographed by Hubble’s Wide Field Camera 3.[5]

The seven largest natural satellites in the Solar System (those bigger than 2,500 km across) are Jupiter's Galilean moons (Ganymede, Callisto, Io, and Europa), Saturn's moon Titan, Earth's moon, and Neptune's captured natural satellite Triton. Triton, the smallest of these, has more mass than all smaller natural satellites together. Similarly in the next size group of nine natural satellites, between 1,000 km and 1,600 km across, Titania, Oberon, Rhea, Iapetus, Charon, Ariel, Umbriel, Dione, and Tethys, the smallest, Tethys, has more mass than all smaller natural satellites together. As well as the natural satellites of the various planets, there are also over 80 known natural satellites of the dwarf planets, asteroids and other small Solar System bodies. Some studies estimate that up to 15% of all trans-Neptunian objects could have satellites.

The following is a comparative table classifying the natural satellites in the Solar System by diameter. The column on the right includes some notable planets, dwarf planets, asteroids, and trans-Neptunian objects for comparison. The natural satellites of the planets are named after mythological figures. These are predominately Greek, except for the Uranian natural satellites, which are named after Shakespearean characters. The nineteen bodies massive enough to have achieved hydrostatic equilibrium are in bold in the table below. Minor planets and satellites suspected but not proven to have achieved a hydrostatic equilibrium are italicized in the table below.

Mean
diameter
(km)
Satellites of planets Satellites of dwarf planets Satellites of
other
minor planets
Non-satellites
for comparison
Earth Mars Jupiter Saturn Uranus Neptune Pluto Haumea Eris
4,000–6,000 Ganymede
Callisto
Titan Mercury
3,000–4,000 Moon Io
Europa
2,000–3,000 Triton Eris
Pluto
1,000–2,000 Rhea
Iapetus
Dione
Tethys
Titania
Oberon
Umbriel
Ariel
Charon Makemake
Haumea
2007 OR10,
Quaoar
500–1,000 Enceladus Dysnomia 90377 Sedna, Ceres,
Salacia, Orcus,
2 Pallas4 Vesta
many more TNOs
250–500 Mimas
Hyperion
Miranda Proteus
Nereid
Hiʻiaka Orcus I Vanth 10 Hygiea
704 Interamnia
87 Sylvia
and many others
100–250 Amalthea
Himalia
Thebe
Phoebe
Janus
Epimetheus
Sycorax
Puck
Portia
Larissa
Galatea
Despina
Namaka S/2005 (82075) 1
Sila–Nunam I
Salacia I Actaea
Ceto I Phorcys
Patroclus I Menoetius
~21 more moons of TNOs
3 Juno
1992 QB1
5 Astraea
42355 Typhon
and many others
50–100 Elara
Pasiphae
Prometheus
Pandora
Caliban
Juliet
Belinda
Cressida
Rosalind
Desdemona
Bianca
Thalassa
Halimede
Neso
Naiad
Hydra
Nix
Quaoar I Weywot
90 Antiope I
Typhon I Echidna
Logos I Zoe
5 more moons of TNOs
90 Antiope
58534 Logos
253 Mathilde
and many others
25–50 Carme
Metis
Sinope
Lysithea
Ananke
Siarnaq
Helene
Albiorix
Atlas
Pan
Ophelia
Cordelia
Setebos
Prospero
Perdita
Stephano
Sao
Laomedeia
Psamathe
22 Kalliope I Linus 1036 Ganymed
243 Ida
and many others
10–25 Phobos
Deimos
Leda
Adrastea
Telesto
Paaliaq
Calypso
Ymir
Kiviuq
Tarvos
Ijiraq
Erriapus
Mab
Cupid
Francisco
Ferdinand
Margaret
Trinculo
S/2004 N 1 Kerberos
Styx
762 Pulcova I
Sylvia I Romulus
624 Hektor I
Eugenia I Petit-Prince
121 Hermione I
283 Emma I
1313 Berna I
107 Camilla I
433 Eros
1313 Berna
and many others
< 10 51 moons 36 moons Sylvia II Remus
Ida I Dactyl
and many others
many

Terminology[edit]

Montage of planets and natural satellites of the Solar System and the dwarf planet Pluto

The first known natural satellite was the Moon, but it was considered a "planet" until Copernicus' introduction of heliocentrism in 1543. Until the discovery of the Galilean satellites in 1610, however, there was no opportunity for referring to such objects as a class. Galileo chose to refer to his discoveries as Planetæ ("planets"), but later discoverers chose other terms to distinguish them from the objects they orbited.

Christiaan Huygens, the discoverer of Titan, was the first to use the term moon for such objects, calling Titan Luna Saturni or Luna Saturnia – "Saturn's moon" or "The Saturnian moon", because it stood in the same relation to Saturn as the Moon did to the Earth.[citation needed]

The first to use of the term satellite to describe orbiting bodies was the German astronomer Johannes Kepler in his pamphlet Narratio de Observatis a se quatuor Iouis satellitibus erronibus ("Narration About Four Satellites of Jupiter Observed") in 1610. He derived the term from the Latin word satelles, meaning "guard", "attendant", or "companion", because the satellites accompanied their primary planet in their journey through the heavens.

As additional natural satellites of Saturn were discovered the term "moon" was abandoned. Giovanni Domenico Cassini sometimes referred to his discoveries as planètes in French, but more often as satellites.[citation needed]

The term satellite thus became the normal one for referring to an object orbiting a planet, as it avoided the ambiguity of "moon". In 1957, however, the launching of the artificial object Sputnik created a need for new terminology. The terms man-made satellite or artificial moon were very quickly abandoned in favor of the simpler satellite, and as a consequence, the term has become linked primarily with artificial objects flown in space – including, sometimes, even those not in orbit around a planet.

Because of this shift in meaning, the term moon, which had continued to be used in a generic sense in works of popular science and in fiction, has regained respectability and is now used interchangeably with natural satellite, even in scientific articles. When it is necessary to avoid both the ambiguity of confusion with the Earth's natural satellite the Moon and the natural satellites of the other planets on the one hand, and artificial satellites on the other, the term natural satellite (using "natural" in a sense opposed to "artificial") is used. To further avoid ambiguity, the convention is to capitalize the word Moon when referring to the Earth's natural satellite, but not when referring to other natural satellites.

The definition of a moon[edit]

Comparison of Earth and the Moon

There is not an established lower limit on what is considered a "moon", as natural satellites will be referred to in this section. Every natural celestial body with an identified orbit around a planet of the Solar System, some as small as a kilometer across, has been identified as a moon, though objects a tenth that size within Saturn's rings, which have not been directly observed, have been called moonlets. Small asteroid moons (natural satellites of asteroids), such as Dactyl, have also been called moonlets.[13]

The upper limit is also vague. Two orbiting bodies are sometimes described as a double body rather than primary and satellite. Asteroids such as 90 Antiope are considered double asteroids, but they have not forced a clear definition of what constitutes a moon. Some authors consider the Pluto–Charon system to be a double (dwarf) planet. The most common[citation needed] dividing line on what is considered a moon rests upon whether the barycentre is below the surface of the larger body, though this is somewhat arbitrary, as it relies on distance as well as relative mass.

Visual summary[edit]

Solar System moons
Ganymede g1 true 2.jpg
Two Halves of Titan.png
Callisto.jpg
Io highest resolution true color.jpg
FullMoon2010.jpg
Europa-moon.jpg
Triton Voyager 2.jpg
Ganymede
(moon of Jupiter)
Titan
(moon of Saturn)
Callisto
(moon of Jupiter)
Io
(moon of Jupiter)
Moon
(moon of Earth)
Europa
(moon of Jupiter)
Triton
(moon of Neptune)
Titania (moon) color cropped.jpg
PIA07763 Rhea full globe5.jpg
Voyager 2 picture of Oberon.jpg
Iapetus as seen by the Cassini probe - 20071008.jpg
PIA00040 Umbrielx2.47.jpg
Ariel-NASA.jpg
Dione (Mond) (30823363).jpg
Titania
(moon of Uranus)
Rhea
(moon of Saturn)
Oberon
(moon of Uranus)
Iapetus
(moon of Saturn)
Umbriel
(moon of Uranus)
Ariel
(moon of Uranus)
Dione
(moon of Saturn)
Inset-sat tethys-large.jpg
Enceladus from Voyager.jpg
Miranda.jpg
Proteus Voyager 2 croped.jpg
Mimas PIA12569.jpg
Hyperion in natural colours.jpg
Phoebe cassini.jpg
Tethys
(moon of Saturn)
Enceladus
(moon of Saturn)
Miranda
(moon of Uranus)
Proteus
(moon of Neptune)
Mimas
(moon of Saturn)
Hyperion
(moon of Saturn)
Phoebe
(moon of Saturn)
PIA12714 Janus crop.jpg
Amalthea (moon).png
PIA09813 Epimetheus S. polar region.jpg
Thebe.jpg
Prometheus 12-26-09a.jpg
Flying By Pandora.jpg
Leading hemisphere of Helene - 20110618.jpg
Janus
(moon of Saturn)
Amalthea
(moon of Jupiter)
Epimetheus
(moon of Saturn)
Thebe
(moon of Jupiter)
Prometheus
(moon of Saturn)
Pandora
(moon of Saturn)
Helene
(moon of Saturn)
Atlas (NASA).jpg
Telesto cassini closeup.jpg
N00151485 Calypso crop.jpg
Phobos colour 2008.jpg
Deimos-MRO.jpg
Methone PIA14633.jpg
Atlas
(moon of Saturn)
Telesto
(moon of Saturn)
Calypso
(moon of Saturn)
Phobos
(moon of Mars)
Deimos
(moon of Mars)
Methone
(moon of Saturn)

See also[edit]

Moons of planets[edit]

Moons of dwarf planets and small Solar System bodies[edit]

References[edit]

  1. ^ Sheppard, Scott S. "The Giant Planet Satellite and Moon Page". Departament of Terrestrial Magnetism at Carniege Institution for science. Retrieved 2012-09-11. 
  2. ^ "How Many Solar System Bodies". NASA/JPL Solar System Dynamics. Retrieved 2012-01-26. 
  3. ^ "Planet and Satellite Names and Discoverers". International Astronomical Union (IAU) Working Group for Planetary System Nomenclature (WGPSN). Retrieved 2012-01-27. 
  4. ^ a b c Wm. Robert Johnston (2012-01-11). "Asteroids with Satellites". Johnston's Archive. Retrieved 2012-01-26. 
  5. ^ a b "Hubble Discovers New Pluto Moon". ESA/Hubble Press Release. Retrieved 13 July 2012. 
  6. ^ Encyclopedia of the Solar System, page 366, Academic Press, 2007, Lucy-Ann Adams McFadden, Paul Robert Weissman, Torrence V. Johnson
  7. ^ R. Canup and E. Asphaug, RM; Asphaug, E (2001). "Origin of the Moon in a giant impact near the end of the Earth's formation". Nature 412 (6848): 708–712. Bibcode:2001Natur.412..708C. doi:10.1038/35089010. PMID 11507633. 
  8. ^ S. Stern, H. Weaver, A. Steffl, M. Mutchler, W. Merline, M. Buie, E. Young, L. Young, and J. Spencer, SA; Weaver, HA; Steffl, AJ; Mutchler, MJ; Merline, WJ; Buie, MW; Young, EF; Young, LA; Spencer, JR (2006). "A giant impact origin for Pluto's small natural satellites and satellite multiplicity in the Kuiper belt". Nature 439 (7079): 946–949. Bibcode:2006Natur.439..946S. doi:10.1038/nature04548. PMID 16495992. 
  9. ^ "The Dust Halo of Saturn's Largest Icy Moon, Rhea – Jones et al. 319 (5868): 1380 – Science". Retrieved 2008-03-06. 
  10. ^ "Saturn satellite reveals first moon rings – 06 March 2008 – New Scientist". Retrieved 2008-03-06. 
  11. ^ "Cassini imaging search rules out rings around Rhea - Tiscareno - 2010 - Geophysical Research Letters - Wiley Online Library". Agu.org. Retrieved 2013-10-29. 
  12. ^ "How Iapetus, Saturn’s outermost moon, got its ridge". Retrieved 2010-12-18. 
  13. ^ F. Marchis et al. (2005). "Discovery of the triple asteroidal system 87 Sylvia". Nature 436 (7052): 822–4. Bibcode:2005Natur.436..822M. doi:10.1038/nature04018. PMID 16094362. 

External links[edit]

All moons[edit]

Jupiter's moons[edit]

Saturn's moons[edit]