View source for Astronomy:Regular moon

{{short description|Satellites which formed around their parent planet}}
[[File:Galilean_moons_around_Jupiter.gif|thumb|upright=1.5|Orbits of [[Astronomy:Jupiter|Jupiter]]'s Galilean moons, demonstrating the organized, low-eccentricity orbits typical of regular satellites]]
In astronomy, a '''regular moon''' or a '''regular satellite''' is a [[Astronomy:Natural satellite|natural satellite]] following a relatively close, stable, and circular orbit which is generally aligned to its [[Astronomy:Primary|primary]]'s equator. They formed within discs of debris and gas that once surrounded their primary, usually the aftermath of a [[Astronomy:Giant-impact hypothesis|large collision]] or leftover material accumulated from the protoplanetary disc. Regular moons then began to accumulate material within the circumplanetary disc in a process similar to [[Physics:Accretion (astrophysics)#Accretion of planets|planetary accretion]], as opposed to [[Astronomy:Irregular moon|irregular moon]]s, which formed independently before being captured into orbit around the primary. 

Regular moons are extremely diverse. The largest regular moons support varied and complex geology, and two regular moons—[[Astronomy:Ganymede (moon)|Ganymede]] and [[Astronomy:Titan (moon)|Titan]]—are larger than the planet [[Astronomy:Mercury (planet)|Mercury]], while Titan hosts a dense, hazy atmosphere and seas of liquid [[Chemistry:Hydrocarbon|hydrocarbon]]s. As a result, the rounded regular moons are often considered planetary objects in their own right by planetary scientists.<ref name="satelliteplanet"/> In contrast, the smallest regular moons lack active geology, and frequently are heavily cratered and irregular in shape, often resembling [[Astronomy:Asteroid|asteroid]]s and other minor bodies in appearance.

Six of the eight planets of the [[Astronomy:Solar System|Solar System]] host at least [[Astronomy:List of natural satellites|57 regular satellites]] combined, with the giant planets hosting the most extensive and complex regular satellite systems. At least four of the nine likeliest [[Astronomy:Dwarf planet|dwarf planet]]s also host regular moon systems: [[Astronomy:Moons of Pluto|Pluto]], [[Astronomy:Dysnomia (moon)|Eris]], [[Astronomy:Moons of Haumea|Haumea]], and [[Astronomy:Vanth (moon)|Orcus]]. 

== Origin and orbital characteristics ==

=== Formation ===
{{further|Astronomy:Circumplanetary disk|Astronomy:Giant-impact hypothesis}}
Regular moons are thought to have several different formation mechanisms. The regular moons of the giant planets are generally believed to have formed from accreting material within circumplanetary discs, growing progressively from smaller moonlets in a manner similar to the formation of [[Astronomy:Planet|planet]]s. Multiple generations of regular satellite systems may have formed around the giant planets, before interactions with the circumplanetary disc and with each other resulted in inward spiralling into the parent planet. As gas inflow into the parent planet begins to end, the effects of gas-induced migration decrease, allowing for a final generation of moons to survive.<ref name="Canup2008"/>

In contrast, Earth's [[Astronomy:Moon|Moon]] and Pluto's five satellites are thought to have originated from giant impacts between two protoplanets early in the Solar System's history, ejecting a dense disc of debris whence satellites can accrete.<ref name="Young2106"/><ref name="Stern2006"/> The giant impact model has also been applied to explain the origin of other dwarf planet satellite systems.<ref name="Arakawa2019"/>

Regular moons may also originate from secondary disruption events, being fragments of other regular moons following collisions or due to tidal disruption. The regular moons of [[Astronomy:Neptune|Neptune]] are likely examples of this, as the capture of Neptune's largest moon [[Astronomy:Triton (moon)|Triton]] would have severely disrupted the existing primordial moon system. Once Triton was tidally dampened into a lower-eccentricity orbit, the debris resulting from the disruption of the primordial moons re-accreted into the current regular moons of Neptune.<ref name="Naeye2006"/><ref name="BanfieldMurray"/><ref name="Goldreich1989"/>

==== Martian moons ====
Despite extensive exploration of [[Astronomy:Mars|Mars]], the origin of [[Astronomy:Moons of Mars|Mars's two moons]] remains the subject of ongoing debate. [[Astronomy:Phobos (moon)|Phobos]] and [[Astronomy:Deimos (moon)|Deimos]] were proposed to be captured asteroids originating from the neighboring [[Astronomy:Asteroid belt|asteroid belt]], and thus would not be classified as regular satellites. This model was further supported by their broad similarities to C- and D-type asteroids.<ref name="physorg2011"/>

However, the capture model is inconsistent with the small, low-eccentricity, low-inclination orbits of the two moons, which are more typical of regular satellites. The rubble-pile nature of Phobos and the basaltic composition of the Martian moons has further pointed against a captured origin.<ref name="madeira2023"/><ref name="EMMbasalt"/> As a result, various models for the ''in situ'' formation of Phobos and Deimos have been proposed to better explain their origins and current configuration, including a giant-impact scenario similar to the one which formed the Moon and a 'recycling' model for Phobos.<ref name="madeira2023"/>

=== Orbital characteristics ===
Regular moons are characterized by prograde orbits, usually with little [[Astronomy:Orbital inclination|orbital inclination]] or [[Astronomy:Orbital eccentricity|eccentricity]] relative to their parent body. These traits are largely constrained by their origins and tidal interactions with the parent body. In the case of the giant planet satellite systems, much like protoplanetary discs, infalling material surrounding a forming planet flattens out into a disc aligned with the planet's equator due to conservation of angular momentum.<ref name="pringle1981"/> As a consequence, any moons formed from the circumplanetary disc will orbit roughly coplanar with the planet's equator; even if future perturbations increase a moon's inclination, tidal effects work to eventually decrease it back to a coplanar state. Likewise, [[Tidal circularization|tidal circularization]] acts to decrease the eccentricity of the regular moons by dissipating energy towards a circular orbit, which is a minimum-energy state. Several regular moons do depart from these orbital traits, such as [[Astronomy:Hyperion (moon)|Hyperion's]] unusually eccentric orbit and [[Astronomy:Miranda (moon)|Miranda's]] unusually inclined orbit, but in these cases, orbital eccentricity and inclination are often increased by resonant interactions with neighboring moons.<ref name="Mignard1984"/><ref name="moons1994"/>

Orbital resonances are a common feature in regular moon systems and are a crucial aspect in their evolution and structure. Such resonances can excite the eccentricity and inclination of participating moons, leading to appreciable [[Astronomy:Tidal heating|tidal heating]] which can sustain geological activity. A particularly apparent example of this is the 1:2:4 mean-motion resonance (MMR) chain Io, Europa, and Ganymede participate in, contributing to Io's volcanism and Europa's liquid subsurface ocean.<ref name="Showman1997a"/> Orbital resonances and near-resonances can also act as a stabilizing and shepherding mechanism, allowing for moons to be closely packed whilst still remaining stable, as is thought to be the case with Pluto's small outer moons.<ref name="Kenyon2019"/> A small handful of regular moons have been discovered to participate in various [[Astronomy:Co-orbital configuration|co-orbital configuration]]s, such as the four trojan moons of Tethys and Dione within the Saturnian system.<ref name="Murray2005"/>

==== Shepherd moons ====
{{main|Astronomy:Shepherd moon}}
Regular moons which orbit near or within a [[Astronomy:Ring system|ring system]] can gravitationally interact with nearby material, either confining material into narrow ringlets or clearing out gaps within a ring in a process known as '[[Astronomy:Shepherd moon|shepherding]]'. Shepherd moons may also act as a direct source of ring material ejected from impacts. The material may then be corralled by the moon in its orbital path, as is the case with the [[Astronomy:Rings of Saturn#Janus/Epimetheus Ring|Janus-Epimetheus ring]] around Saturn.<ref name="NASAJPL2006"/>

== Physical characteristics ==

=== Geology ===
[[File:Enceladus_geysers_June_2009.jpg|thumb|upright=1.5|Active plumes on the south pole of Saturn's moon Enceladus, fed by a global subsurface ocean of liquid water]]
Of the nineteen regular moons large enough to be gravitationally rounded, several of them show geological activity, and many more exhibit signs of past activity. Several regular moons, such as [[Astronomy:Europa (moon)|Europa]], [[Astronomy:Titan (moon)|Titan]], and Enceladus are known to host global subsurface oceans of liquid water, maintained by tidal heating from their respective parent planets.<ref name="Hay2023"/><ref name="Iess2012"/><ref name=Thomas2015/> These subsurface oceans can drive a variety of geological processes, including widespread [[Astronomy:Cryovolcano|cryovolcanism]], resurfacing, and tectonics, acting as reservoirs of 'cryomagma' which can be erupted onto a moon's surface.<ref name="NASAcryovolcano"/><ref name="Figueredo2003"/>

[[Astronomy:Io (moon)|Io]] is unusual as, in contrast to most other regular moons of the giant planets, Io is rocky in composition with extremely little water. Io's high levels of volcanism instead erupt large basaltic flows which continuously resurfaces the moon, whilst also ejecting large volumes of sulfur and sulfur dioxide into its tenuous atmosphere. Analogous to the subsurface oceans of liquid water on icy moons such as Europa, Io may have a subsurface ocean of silicate magma beneath its crust, fuelling Io's volcanic activity.<ref name="Keszthelyi2001"/><ref name="geissler2007"/> 

One regular moon, Titan, hosts a dense nitrogen-dominated atmosphere as well as stable hydrocarbon lakes on its surface. The complex interactions between Titan's thick atmosphere, its surface, and its 'methane cycle' have led to the creation of many unusual features, including canyons eroded by rivers and large equatorial dune fields.<ref name="titanrivererosion"/><ref name="gunn2022"/>

=== Rotation ===
The majority of regular moons are [[Astronomy:Tidal locking|tidally locked]] to their parent planet, though several exceptions are known. One such exception is Saturn's [[Astronomy:Hyperion (moon)|Hyperion]], which exhibits [[Astronomy:Chaotic rotation|chaotic rotation]] due to Titan's gravitational influence on its irregular shape; Hyperion's chaotic rotation may be further facilitated by its 3:4 orbital resonance with Titan.<ref name="Mignard1984"/> The four small circumbinary moons of Pluto, which are similarly elongated, also rotate chaotically under the influence of [[Astronomy:Charon (moon)|Charon]] and generally have very high axial tilts.<ref name="NASA2015"/> Hi'iaka, the larger outer moon of Haumea, was revealed to have a very rapid rotational period of approximately 9.8 hours via lightcurve data, 120 times faster than its orbital period. Results for Namaka were less clear, potentially pointing towards a slower rotational period or a pole-on configuration, with a significant axial tilt relative to its orbital plane.<ref name="Hastings2016"/>

Uniquely, Charon is large enough to have also tidally locked Pluto, creating a mutual tidally locked state where Charon is only visible from one hemisphere of Pluto and vice versa. Similarly, [[Astronomy:Eris (dwarf planet)|Eris]] has been observed to be tidally locked to its satellite [[Astronomy:Dysnomia (moon)|Dysnomia]], which may indicate an unusually high density for the moon.<ref name="Szakats2022"/>

=== Parent-satellite interactions ===
[[File:Jupiter.Aurora.HST.mod.svg|thumb|upright=1.5|Bright auroral spots within Jupiter's northern aurorae, contributed by the Galilean moons]]
Due to their close nature and long, shared histories, regular moons can have a significant influence on their primary. A familiar example of this are the [[Astronomy:Tide|ocean tides]] raised by the Moon on the Earth. Just as Earth raises tidal bulges on the Moon which results in tidal locking, the Moon raises tidal bulges on the Earth which manifest most noticeably as the rising and falling of the local sea level roughly diurnally (though local coastal topography can result in semidiurnal or complex patterns).<ref name="noaatides"/>

Io's volcanic activity results in extreme interactions with Jupiter, constructing the [[Astronomy:Io (moon)#Interaction with Jupiter's magnetosphere|Io plasma torus]] in a roughly toroidal region surrounding Io's orbit as well as a neutral cloud of sulfur, oxygen, sodium, and potassium atoms which immediately surround the moon.<ref name="IobookChap11"/> Escaping ions from the [[Astronomy:Gas torus|plasma torus]] are responsible for Jupiter's unusually extensive magnetosphere, generating an internal pressure which inflates it from within.<ref name="Krimigis2002"/> Jupiter's intense magnetic field also couples an intense flux tube with Io's atmosphere and its associated neutral cloud to Jupiter's polar upper atmosphere, generating an intense region of auroral glow.<ref name="IobookChap11"/> Similar, albeit much weaker flux tubes were also discovered to be associated with the other Galilean moons.

== Exploration ==
Due to their ability to support large internal volumes of liquid water, regular moons are of particular interest to scientists as targets in the search for extraterrestrial life. Subsurface oceans are believed to be capable of hosting complex organic chemistry, an expectation which was supported after the potential indirect detection of various salts in Europa's ocean and the detection of organic compounds and hydrogen cyanide in Enceladus's plumes.<ref name="hao2022"/><ref name="peter2023"/><ref name="NASA2008"/><ref name="Trumbo2019"/> As a result, dedicated missions to investigate the nature and potential habitability of several regular moons' internal oceans have been proposed and launched.<ref name="NASA2020"/><ref name="Weiss2010"/>

=== Active missions ===
* ''[[Astronomy:Jupiter Icy Moons Explorer|Jupiter Icy Moons Explorer]]'' (''Juice'') is a mission developed and launched by the European Space Agency (ESA) which plans to study Europa, Ganymede, and Callisto and investigate their respective subsurface oceans. ''Juice'' is currently en route to Jupiter.<ref name="ESAL1"/>

=== Missions in development ===
* ''[[Astronomy:Europa Clipper|Europa Clipper]]'' is a mission currently under development by [[Organization:NASA|NASA]], intending to conduct 44 flybys of Europa to better investigate Europa's interior and plume activity. The spacecraft intends to launch on October 2024.<ref name="NASAclipper"/>
* ''[[Astronomy:Martian Moons eXploration|Martian Moons eXploration]]'' (''MMX'') is a sample-return mission being developed by [[Organization:JAXA|JAXA]]. The probe intends to launch in 2026, arriving at Mars by 2027 and collecting data about Phobos before collecting a surface sample from the moon and returning to Earth by 2031. A major goal of ''MMX'' is to better constrain the origins and history of Mars's moons.<ref name="JAXAMMX"/>
* ''[[Engineering:Dragonfly (spacecraft)|Dragonfly]]'' is a mission under development by NASA to send a robotic rotorcraft to the surface of Titan with the goal of researching Titan's complex atmospheric and ground chemistry.<ref name="LPSC 2017"/> ''Dragonfly'' currently plans to launch on July 2028.<ref name="spacenewsdragonfly"/>

== See also ==
* [[Astronomy:Irregular moon|Irregular moon]]
* [[Astronomy:Inner moon|Inner moon]]
* Orbital resonance

== References ==
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}} <!-- end of reflist -->

{{Solar System moons (compact)}}

[[Category:Moons]]

{{Sourceattribution|Regular moon}}