Astronomy:Regular moon

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Short description: Satellites which formed around their parent planet
Orbits of 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 natural satellite following a relatively close, stable, and circular orbit which is generally aligned to its primary's equator. They formed within discs of debris and gas that once surrounded their primary, usually the aftermath of a 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 planetary accretion, as opposed to irregular moons, 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—Ganymede and Titan—are larger than the planet Mercury, while Titan hosts a dense, hazy atmosphere and seas of liquid hydrocarbons. As a result, the rounded regular moons are often considered planetary objects in their own right by planetary scientists.[1] In contrast, the smallest regular moons lack active geology, and frequently are heavily cratered and irregular in shape, often resembling asteroids and other minor bodies in appearance.

Six of the eight planets of the Solar System host at least 57 regular satellites combined, with the giant planets hosting the most extensive and complex regular satellite systems. At least four of the nine likeliest dwarf planets also host regular moon systems: Pluto, Eris, Haumea, and Orcus.

Origin and orbital characteristics

Formation

Further information: Astronomy:Circumplanetary disk and 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 planets. 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.[2]

In contrast, Earth's 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.[3][4] The giant impact model has also been applied to explain the origin of other dwarf planet satellite systems.[5]

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 Neptune are likely examples of this, as the capture of Neptune's largest 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.[6][7][8]

Martian moons

Despite extensive exploration of Mars, the origin of Mars's two moons remains the subject of ongoing debate. Phobos and Deimos were proposed to be captured asteroids originating from the neighboring 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.[9]

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.[10][11] 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.[10]

Orbital characteristics

Regular moons are characterized by prograde orbits, usually with little orbital inclination or 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.[12] 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 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 Hyperion's unusually eccentric orbit and Miranda's unusually inclined orbit, but in these cases, orbital eccentricity and inclination are often increased by resonant interactions with neighboring moons.[13][14]

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 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.[15] 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.[16] A small handful of regular moons have been discovered to participate in various co-orbital configurations, such as the four trojan moons of Tethys and Dione within the Saturnian system.[17]

Shepherd moons

Main page: Astronomy:Shepherd moon

Regular moons which orbit near or within a 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 '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 Janus-Epimetheus ring around Saturn.[18]

Physical characteristics

Geology

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 Europa, Titan, and Enceladus are known to host global subsurface oceans of liquid water, maintained by tidal heating from their respective parent planets.[19][20][21] These subsurface oceans can drive a variety of geological processes, including widespread cryovolcanism, resurfacing, and tectonics, acting as reservoirs of 'cryomagma' which can be erupted onto a moon's surface.[22][23]

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.[24][25]

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.[26][27]

Rotation

The majority of regular moons are tidally locked to their parent planet, though several exceptions are known. One such exception is Saturn's Hyperion, which exhibits 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.[13] The four small circumbinary moons of Pluto, which are similarly elongated, also rotate chaotically under the influence of Charon and generally have very high axial tilts.[28] 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.[29]

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, Eris has been observed to be tidally locked to its satellite Dysnomia, which may indicate an unusually high density for the moon.[30]

Parent-satellite interactions

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 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).[31]

Io's volcanic activity results in extreme interactions with Jupiter, constructing the 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.[32] Escaping ions from the plasma torus are responsible for Jupiter's unusually extensive magnetosphere, generating an internal pressure which inflates it from within.[33] 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.[32] 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.[34][35][36][37] As a result, dedicated missions to investigate the nature and potential habitability of several regular moons' internal oceans have been proposed and launched.[38][39]

Active missions

  • 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.[40]

Missions in development

  • Europa Clipper is a mission currently under development by 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.[41]
  • Martian Moons eXploration (MMX) is a sample-return mission being developed by 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.[42]
  • 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.[43] Dragonfly currently plans to launch on July 2028.[44]

See also

References

  1. ↑ Villard, Ray (2010-05-14). "Should Large Moons Be Called 'Satellite Planets'?". Discovery News. http://news.discovery.com/space/should-large-moons-be-called-satellite-planets.html. 
  2. ↑ Canup, Robin M.; Ward, William R. (2008). Origin of Europa and the Galilean Satellites. University of Arizona Press. p. 59. ISBN 978-0-8165-2844-8. Bibcode: 2009euro.book...59C. 
  3. ↑ Young, Edward D.; Kohl, Issaku E.; Warren, Paul H.; Rubie, David C.; Jacobson, Seth A.; Morbidelli, Alessandro (2016-01-29). "Oxygen isotopic evidence for vigorous mixing during the Moon-forming giant impact" (in en). Science (Washington DC: American Association for the Advancement of Science) 351 (6272): 493–496. doi:10.1126/science.aad0525. ISSN 0036-8075. PMID 26823426. Bibcode: 2016Sci...351..493Y. 
  4. ↑ Stern, SA; Weaver, HA; Steffl, AJ; Mutchler, MJ et al. (2006). "A giant impact origin for Pluto's small natural satellites and satellite multiplicity in the Kuiper belt". Nature 439 (7079): 946–49. doi:10.1038/nature04548. PMID 16495992. Bibcode: 2006Natur.439..946S. 
  5. ↑ Arakawa, Sota et al. (2019). "Early formation of moons around large trans-Neptunian objects via giant impacts". Nature 3 (9): 802–807. doi:10.1038/s41550-019-0797-9. Bibcode: 2019NatAs...3..802A. 
  6. ↑ Naeye, R. (September 2006). "Triton Kidnap Caper". Sky & Telescope 112 (3): 18. Bibcode: 2006S&T...112c..18N. 
  7. ↑ Banfield, Don; Murray, Norm (October 1992). "A dynamical history of the inner Neptunian satellites". Icarus 99 (2): 390–401. doi:10.1016/0019-1035(92)90155-Z. Bibcode: 1992Icar...99..390B. 
  8. ↑ Goldreich, P.; Murray, N.; Longaretti, P. Y.; Banfield, D. (1989). "Neptune's story". Science 245 (4917): 500–504. doi:10.1126/science.245.4917.500. PMID 17750259. Bibcode: 1989Sci...245..500G. 
  9. ↑ "New Views of Martian Moons". http://www.physorg.com/news115483748.html. 
  10. ↑ 10.0 10.1 Madeira, Gustavo; Charnoz, Sébastian; Zhang, Yun; Hyodo, Ryuki; Michel, Patrick; Genda, Hidenori; Giuliatti Winter, Silvia (April 2023). "Exploring the Recycling Model of Phobos Formation: Rubble-pile Satellites". The Astronomical Journal 165 (4): 161. doi:10.3847/1538-3881/acbf53. Bibcode: 2023AJ....165..161M. 
  11. ↑ "EMM unveils new Deimos observations at EGU23, extends mission". Sharjah24. 24 April 2023. https://www.sharjah24.ae/en/Articles/2023/04/24/EMM-unveils-new-Deimos-observations-at-EGU23-extends-mission. 
  12. ↑ Pringle, J.E. (1981). "Accretion discs in astrophysics". Annual Review of Astronomy and Astrophysics 19: 137–162. doi:10.1146/annurev.aa.19.090181.001033. Bibcode: 1981ARA&A..19..137P. 
  13. ↑ 13.0 13.1 Wisdom, J.; Peale, S. J.; Mignard, F. (1984). "The chaotic rotation of Hyperion". Icarus 58 (2): 137–152. doi:10.1016/0019-1035(84)90032-0. Bibcode: 1984Icar...58..137W. 
  14. ↑ Michele Moons and Jacques Henrard (June 1994). "Surfaces of Section in the Miranda-Umbriel 3:1 Inclination Problem". Celestial Mechanics and Dynamical Astronomy 59 (2): 129–148. doi:10.1007/bf00692129. Bibcode: 1994CeMDA..59..129M. 
  15. ↑ Showman, Adam P.; Malhotra, Renu (1997). "Tidal Evolution into the Laplace Resonance and the Resurfacing of Ganymede". Icarus 127 (1): 93–111. doi:10.1006/icar.1996.5669. Bibcode: 1997Icar..127...93S. http://www.lpl.arizona.edu/~showman/publications/showman-malhotra-1997.pdf. Retrieved January 22, 2008. 
  16. ↑ Kenyon, Scott J.; Bromley, Benjamin C. (28 January 2019). "A Pluto-Charon Sonata: The Dynamical Architecture of the Circumbinary Satellite System". The Astrophysical Journal 157 (2): 79. doi:10.3847/1538-3881/aafa72. Bibcode: 2019AJ....157...79K. 
  17. ↑ Murray, C. D.; Cooper, N. J.; Evans, M. W.; Beurle, K. (December 2005). "S/2004 S 5: A new co-orbital companion for Dione". Icarus 179 (1): 222–234. doi:10.1016/j.icarus.2005.06.009. Bibcode: 2005Icar..179..222M. 
  18. ↑ "NASA Finds Saturn's Moons May Be Creating New Rings". Jet Propulsion Lab. 2006-10-11. https://saturn.jpl.nasa.gov/news/2372/nasa-finds-saturns-moons-may-be-creating-new-rings/. 
  19. ↑ Hay, H. C. F. C. et al. (2023). "Turbulent Drag at the Ice-Ocean Interface of Europa in Simulations of Rotating Convection: Implications for Nonsynchronous Rotation of the Ice Shell". JGR Planets 128 (6376): 21. doi:10.1029/2022JE007648. PMID 34737306. Bibcode: 2023JGRE..12807648H. 
  20. ↑ Iess, L.; Jacobson, R. A.; Ducci, M.; Stevenson, D. J.; Lunine, Jonathan I.; Armstrong, J. W.; Asmar, S. W.; Racioppa, P. et al. (2012). "The Tides of Titan". Science 337 (6093): 457–9. doi:10.1126/science.1219631. PMID 22745254. Bibcode: 2012Sci...337..457I. 
  21. ↑ Thomas, P. C. et al. (2016). "Enceladus's measured physical libration requires a global subsurface ocean". Icarus 264: 37–47. doi:10.1016/j.icarus.2015.08.037. Bibcode: 2016Icar..264...37T. 
  22. ↑ "Cassini Spots Potential Ice Volcano on Saturn Moon". NASA. http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20101214/. 
  23. ↑ Figueredo, Patricio H.; Greeley, Ronald (February 2004). "Resurfacing history of Europa from pole-to-pole geological mapping". Icarus 167 (2): 287–312. doi:10.1016/j.icarus.2003.09.016. Bibcode: 2004Icar..167..287F. 
  24. ↑ Keszthelyi, L. (2001). "Imaging of volcanic activity on Jupiter's moon Io by Galileo during the Galileo Europa Mission and the Galileo Millennium Mission". J. Geophys. Res. 106 (E12): 33025–33052. doi:10.1029/2000JE001383. Bibcode: 2001JGR...10633025K. 
  25. ↑ Geissler, P. E.; Goldstein, D. B. (2007). "Plumes and their deposits". in Lopes, R. M. C.; Spencer, J. R.. Io after Galileo. Springer-Praxis. pp. 163–192. ISBN 978-3-540-34681-4. 
  26. ↑ Chu, Jennifer (July 2012). "River networks on Titan point to a puzzling geologic history". MIT Research. https://web.mit.edu/newsoffice/2012/river-networks-on-titan-0720.html. 
  27. ↑ Gunn, Andrew; Jerolmack, Douglas J. (19 May 2022). "Conditions for aeolian transport in the Solar System". Nature Astronomy 6 (8): 923–929. doi:10.1038/s41550-022-01669-0. Bibcode: 2022NatAs...6..923G. https://www.nature.com/articles/s41550-022-01669-0. Retrieved 15 December 2023. 
  28. ↑ Northon, Karen (3 June 2015). "NASA's Hubble Finds Pluto's Moons Tumbling in Absolute Chaos". https://www.nasa.gov/press-release/nasa-s-hubble-finds-pluto-s-moons-tumbling-in-absolute-chaos. 
  29. ↑ Hastings, Danielle M.; Ragozzine, Darin; Fabrycky, Daniel C.; Burkhart, Luke D.; Fuentes, Cesar; Margot, Jean-Luc; Brown, Michael E.; Holman, Matthew (December 2016). "The Short Rotation Period of Hiʻiaka, Haumea's Largest Satellite". The Astronomical Journal 152 (6): 12. doi:10.3847/0004-6256/152/6/195. 195. OCLC 6889796157. Bibcode: 2016AJ....152..195H. 
  30. ↑ Szakáts, R.Expression error: Unrecognized word "etal". (2023). "Tidally locked rotation of the dwarf planet (136199) Eris discovered from long-term ground based and space photometry". Astronomy & Astrophysics L3: 669. doi:10.1051/0004-6361/202245234. Bibcode: 2023A&A...669L...3S. 
  31. ↑ "Types and causes of tidal cycles". U.S. National Oceanic and Atmospheric Administration (NOAA) National Ocean Service (Education section). http://oceanservice.noaa.gov/education/kits/tides/tides07_cycles.html. 
  32. ↑ 32.0 32.1 Schneider, N. M.; Bagenal, F. (2007). "Io's neutral clouds, plasma torus, and magnetospheric interactions". in Lopes, R. M. C.; Spencer, J. R.. Io after Galileo. Springer-Praxis. pp. 265–286. ISBN 978-3-540-34681-4. 
  33. ↑ Krimigis, S. M. (2002). "A nebula of gases from Io surrounding Jupiter". Nature 415 (6875): 994–996. doi:10.1038/415994a. PMID 11875559. Bibcode: 2002Natur.415..994K. 
  34. ↑ Hao, Jihua; Glein, Christopher R.; Huang, Fang; Yee, Nathan; Catling, David C.; Postberg, Frank; Hillier, Jon K.; Hazen, Robert M. (27 September 2022). "Abundant phosphorus expected for possible life in Enceladus's ocean". Proceedings of the National Academy of Sciences 119 (39): e2201388119. doi:10.1073/pnas.2201388119. ISSN 0027-8424. PMID 36122219. Bibcode: 2022PNAS..11901388H. 
  35. ↑ Peter, Jonah S. (14 December 2023). "Detection of HCN and diverse redox chemistry in the plume of Enceladus". Nature Astronomy: 1–10. doi:10.1038/s41550-023-02160-0. Bibcode: 2023NatAs.tmp..259P. Archived from the original on 15 December 2023. https://archive.today/20231215144349/https://www.nature.com/articles/s41550-023-02160-0. Retrieved 15 December 2023. 
  36. ↑ "Cassini Tastes Organic Material at Saturn's Geyser Moon". NASA. March 26, 2008. http://www.nasa.gov/mission_pages/cassini/media/cassini-20080326.html. 
  37. ↑ Trumbo, Samantha K.; Brown, Michael E.; Hand, Kevin P. (12 June 2019). "Sodium chloride on the surface of Europa". Science Advances 5 (6): eaaw7123. doi:10.1126/sciadv.aaw7123. PMID 31206026. Bibcode: 2019SciA....5.7123T. 
  38. ↑ Pat Brennan (November 10, 2020). "Life in Our Solar System? Meet the Neighbors". NASA. https://exoplanets.nasa.gov/news/1665/life-in-our-solar-system-meet-the-neighbors/. Retrieved March 30, 2023. 
  39. ↑ Weiss, P.; Yung, K. L.; Kömle, N.; Ko, S. M.; Kaufmann, E.; Kargl, G. (2011). "Thermal drill sampling system onboard high-velocity impactors for exploring the subsurface of Europa". Advances in Space Research 48 (4): 743. doi:10.1016/j.asr.2010.01.015. Bibcode: 2011AdSpR..48..743W. 
  40. ↑ "ESA—Selection of the L1 mission". ESA. 17 April 2012. http://planetary.s3.amazonaws.com/assets/resources/ESA/ESA-SPC_20120417_selection-L1-mission.pdf. 
  41. ↑ "Europa Clipper". NASA (JPL). https://www.jpl.nasa.gov/missions/europa-clipper/.  This article incorporates text from this source, which is in the public domain.
  42. ↑ "MMX - Martian Moons eXploration". 26 December 2023. https://www.mmx.jaxa.jp/en/. 
  43. ↑ Dragonfly: Exploring Titan's Prebiotic Organic Chemistry and Habitability E. P. Turtle, J. W. Barnes, M. G. Trainer, R. D. Lorenz, S. M. MacKenzie, K. E. Hibbard, D. Adams, P. Bedini, J. W. Langelaan, K. Zacny, and the Dragonfly Team Lunar and Planetary Science Conference 2017
  44. ↑ Foust, Jeff (28 November 2023). "NASA postpones Dragonfly review, launch date". SpaceNews.com. https://spacenews.com/nasa-postpones-dragonfly-review-launch-date/. 



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