Planetary-mass moon

A planetary-mass moon is a planetary-mass object that is also a natural satellite. They are large and ellipsoidal (sometimes spherical) in shape. Two moons in the Solar System are larger than the planet Mercury (though less massive): Ganymede and Titan, and seven are larger and more massive than the dwarf planets Pluto and Eris.

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Planetary-mass moons larger than Pluto, the largest Solar dwarf planet.

The concept of satellite planets – the idea that planetary-mass objects, including planetary-mass moons, are planets – is used by some planetary scientists, such as Alan Stern, who are more concerned with whether a celestial body has planetary geology (that is, whether it is a planetary body) than its solar or non-solar orbit (planetary dynamics).[1] This conceptualization of planets as three classes of objects (classical planets, dwarf planets and satellite planets) has not been accepted by the International Astronomical Union (the IAU). In addition, the IAU definition of 'hydrostatic equilibrium' is quite restrictive – that the object's mass is sufficient for gravity to overcome rigid-body forces to become plastic. In contrast, planetary-mass moons may be in hydrostatic equilibrium due to tidal or radiogenic heating, in some cases forming a subsurface ocean.

Early history

The distinction between a satellite and a classical planet was not recognized until after the heliocentric model of the Solar System was established. When in 1610 Galileo discovered the first satellites of another planet (the four Galilean moons of Jupiter), he referred to them as "four planets flying around the star of Jupiter at unequal intervals and periods with wonderful swiftness."[2] Similarly, Christiaan Huygens, upon discovering Saturn's largest moon Titan in 1655, employed the terms "planeta" (planet), "Stella" (star), "luna" (moon), and the more modern "satellite" (attendant) to describe it.[3] Giovanni Cassini, in announcing his discovery of Saturn's moons Iapetus and Rhea in 1671 and 1672, described them as Nouvelles Planetes autour de Saturne ("New planets around Saturn").[4] However, when the Journal de Scavans reported Cassini's discovery of two new Saturnian moons (Tethys and Dione) in 1686, it referred to them strictly as "satellites", though sometimes to Saturn as the "primary planet".[5] When William Herschel announced his discovery of two objects in orbit around Uranus (Titania and Oberon) in 1787, he referred to them as "satellites" and "secondary planets".[6] All subsequent reports of natural satellite discoveries used the term "satellite" exclusively,[7] though the 1868 book Smith's Illustrated Astronomy referred to satellites as "secondary planets".[8]

Modern concept
Comparative masses of the seven largest moons. Values are ×1021 kg. The moons smaller than Triton would be barely visible at this scale.
The masses of the mid-sized moons, compared to Triton. Values are ×1021 kg. Dysnomia is given a value at the center of the known range (0.3–0.5). Unmeasured Vanth and Ilmarë are excluded. Enceladus, Miranda, and Mimas are nearly invisible at this scale.

In the modern era, Alan Stern considers satellite planets to be one of three categories of planets, along with dwarf planets and classical planets.[9] The term planemo ("planetary-mass object") covers all three populations.[10] Stern's and the IAU's definition of 'planet' depends on hydrostatic equilibrium – on the body's mass being sufficient to render it plastic, so that it relaxes into an ellipsoid under its own gravity. The IAU definition specifies that the mass is great enough to overcome 'rigid-body forces', and it does not address objects that may be in hydrostatic equilibrium due to a subsurface ocean or (in the case of Io) due to magma caused by tidal heating. Many of the larger icy moons could have subsurface oceans.[11]

The seven largest moons are more massive than the dwarf planet Pluto, which is known to be in hydrostatic equilibrium. (They are also known to be more massive than Eris, a dwarf planet even more massive than Pluto.) These seven are Earth's Moon, the four Galilean moons of Jupiter (Io, Europa, Ganymede and Callisto), and the largest moons of Saturn (Titan) and of Neptune (Triton). Ganymede and Titan are additionally larger than the planet Mercury, and Callisto is almost as large. All of these moons are ellipsoidal. That said, the two moons larger than Mercury have less than half its mass, and it is mass, along with composition and internal temperature, that determine whether a body is plastic enough to be in hydrostatic equilibrium. Io, Europa, Ganymede, Titan, and Triton are generally believed to be in hydrostatic equilibrium, but Earth's Moon is known not to be in hydrostatic equilibrium, and the situation for Callisto is unclear.

Another dozen moons are ellipsoidal as well, indicating that they achieved equilibrium at some point in their histories. However, it has been shown that some of these moons are no longer in equilibrium, due to them becoming increasingly rigid as they cooled over time. Dysnomia's shape is not known, but it appears to be dense enough that it must have collapsed to form a solid body.

Neptune's second-largest moon Proteus has occasionally been included by authors discussing or advocating geophysical conceptions of the 'planet'.[12][13] It is larger than Mimas but is quite far from being round.

Current equilibrium moons

Determining whether a moon is currently in hydrostatic equilibrium requires close observation, and is easier to disprove than to prove.

Earth's entirely rocky moon solidified out of equilibrium billions of years ago,[14] but most of the other six moons larger than Pluto, four of which are predominantly icy, are assumed to still be in equilibrium. (Ice has less tensile strength than rock, and is deformed at lower pressures and temperatures than rock.) The evidence is perhaps strongest for Ganymede, which has a magnetic field that indicates the fluid movement of electrically conducting material in its interior, though whether that fluid is a metallic core or a subsurface ocean is unknown.[15] One of the mid-sized moons of Saturn (Rhea) may also be in equilibrium,[16][11] as may a couple of the moons of Uranus (Titania and Oberon).[11] However, the other ellipsoidal moons of Saturn (Mimas, Enceladus, Tethys, Dione and Iapetus) are no longer in equilibrium.[16] In addition to not being in equilibrium, Mimas and Tethys have very low densities and it has been suggested that they may have non-negligible internal porosity,[17][18] in which case they would not be satellite planets. The situation for Uranus's three smaller ellipsoidal moons (Umbriel, Ariel and Miranda) is unclear, as is that of Pluto's moon Charon.[14]

The TNO moons Eris I Dysnomia, Orcus I Vanth, and possibly Varda I Ilmarë are at least the size of Mimas, the smallest ellipsoidal moon of Saturn. However, trans-Neptunian objects appear to become solid bodies at a larger size (around 900–1000 km diameter) than the moons of Saturn and Uranus (around 400 km diameter). Both Dysnomia and Vanth are dark bodies smaller than 900–1000 km, and Dysnomia is known to be low-density, suggesting that it cannot be solid. Consequently, these bodies have been excluded.[19]

List
Yes – believed to be in equilibrium
No – confirmed not to be in equilibrium
Maybe – uncertain evidence
List of ellipsoidal moons[20]
Moon Image Radius Mass Density Surface gravity Year of
discovery		 Hydrostatic
equilibrium?
Name Designation (km) (R☾) (1021 kg) (M☾) (g/cm3) (g)
GanymedeJupiter III
2634.1±0.3 156.4% 148.2 201.8% 1.942±0.005 0.146 1610 Yes
TitanSaturn VI
2574.7±0.1 148.2% 134.5 183.2% 1.882±0.001 0.138 1655 Maybe[21]
CallistoJupiter IV
2410.3±1.5 138.8% 107.6 146.6% 1.834±0.003 0.126 1610 Maybe[22]
IoJupiter I
1821.6±0.5 104.9% 89.3 121.7% 3.528±0.006 0.183 1610 Yes
MoonEarth I
1737.05 100% 73.4 100% 3.344±0.005 0.165 Prehistoric No[23]
EuropaJupiter II
1560.8±0.5 89.9% 48.0 65.4% 3.013±0.005 0.134 1610 Yes
TritonNeptune I
1353.4±0.9 79.9% 21.4 29.1% 2.059±0.005 0.080 1846 Yes
TitaniaUranus III
788.9±1.8 45.4% 3.40±0.06 4.6% 1.66±0.04 0.040 1787 Maybe[11]
RheaSaturn V
764.3±1.0 44.0% 2.31 3.1% 1.233±0.005 0.027 1672 Maybe[16]
OberonUranus IV
761.4±2.6 43.8% 3.08±0.09 4.2% 1.56±0.06 0.036 1787 Maybe[11]
IapetusSaturn VIII
735.6±1.5 42.3% 1.81 2.5% 1.083±0.007 0.022 1671 No[16]
CharonPluto I
603.6±1.4 34.7% 1.53 2.1% 1.664±0.012 0.029 1978 Maybe[14]
UmbrielUranus II
584.7±2.8 33.7% 1.28±0.03 1.7% 1.46±0.09 0.023 1851
ArielUranus I
578.9±0.6 33.3% 1.25±0.02 1.7% 1.59±0.09 0.028 1851
DioneSaturn IV
561.4±0.4 32.3% 1.10 1.5% 1.476±0.004 0.024 1684 No[16]
TethysSaturn III
533.0±0.7 30.7% 0.617 0.84% 0.973±0.004 0.015 1684 No[16]


EnceladusSaturn II
252.1±0.2 14.5% 0.108 0.15% 1.608±0.003 0.011 1789 No[16]
MirandaUranus V
235.8±0.7 13.6% 0.064±0.003 0.09% 1.21±0.11 0.008 1948
MimasSaturn I
198.2±0.4 11.4% 0.038 0.05% 1.150±0.004 0.006 1789 No[16]

(Saturn VII is Hyperion, which is not gravitationally rounded; it is smaller than Mimas.)

Atmospheres

Titan has a denser atmosphere (1.4 bar) than Earth; it is the only known moon with a significant atmosphere. Triton (14 μbar), Io (1.9 nbar), and Callisto (26 pbar) have very thin atmospheres, but still enough to have collisions between atmospheric molecules. Other planetary-mass moons only have exospheres at most.[24] Exospheres have been detected around Earth's Moon, Europa, Ganymede,[24] Enceladus,[25] Dione,[26] and Rhea.[27] An exosphere around Titania is a possibility, though it has not been confirmed.[28]

See also

Further reading

References
  1. "Should Large Moons Be Called 'Satellite Planets'?". News.discovery.com. 2010-05-14. Archived from the original on 2014-10-25.
  2. Galileo Galilei (1989). Siderius Nuncius. Albert van Helden. University of Chicago Press. p. 26.
  3. Christiani Hugenii (Christiaan Huygens) (1659). Systema Saturnium: Sive de Causis Miradorum Saturni Phaenomenon, et comite ejus Planeta Novo. Adriani Vlacq. pp. 1–50.
  4. Giovanni Cassini (1673). Decouverte de deux Nouvelles Planetes autour de Saturne. Sabastien Mabre-Craniusy. pp. 6–14.
  5. Cassini, G. D. (1686–1692). "An Extract of the Journal Des Scavans. On April 22 st. N. 1686. Giving an Account of Two New Satellites of Saturn, Discovered Lately by Mr. Cassini at the Royal Observatory in Paris". Philosophical Transactions of the Royal Society of London. 16 (179–191): 79–85. Bibcode:1686RSPT...16...79C. doi:10.1098/rstl.1686.0013. JSTOR 101844.
  6. William Herschel (1787). An Account of the Discovery of Two Satellites Around the Georgian Planet. Read at the Royal Society. J. Nichols. pp. 1–4.
  7. See primary citations in Timeline of discovery of Solar System planets and their moons
  8. Smith, Asa (1868). Smith's Illustrated Astronomy. Nichols & Hall. p. 23. secondary planet Herschel.
  9. "Should Large Moons Be Called 'Satellite Planets'?". News.discovery.com. May 14, 2010. Archived from the original on July 20, 2011. Retrieved November 4, 2011.
  10. Basri, Gibor; Brown, Michael E. (2006). "Planetesimals to Brown Dwarfs: What is a Planet?" (PDF). Annual Review of Earth and Planetary Sciences. 34: 193–216. arXiv:astro-ph/0608417. Bibcode:2006AREPS..34..193B. doi:10.1146/annurev.earth.34.031405.125058. S2CID 119338327. Archived from the original (PDF) on July 31, 2013.
  11. Hussmann, Hauke; Sohl, Frank; Spohn, Tilman (November 2006). "Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-Neptunian objects". Icarus. 185 (1): 258–273. Bibcode:2006Icar..185..258H. doi:10.1016/j.icarus.2006.06.005.
  12. Emily Lakdawalla et al., What Is A Planet? Archived 2022-01-22 at the Wayback Machine The Planetary Society, 21 April 2020
  13. Williams, Matt. "A geophysical planet definition". Phys.org. Retrieved 2022-05-25.
  14. Nimmo, Francis; et al. (2017). "Mean radius and shape of Pluto and Charon from New Horizons images". Icarus. 287: 12–29. arXiv:1603.00821. Bibcode:2017Icar..287...12N. doi:10.1016/j.icarus.2016.06.027. S2CID 44935431.
  15. Planetary Science Decadal Survey Community White Paper, Ganymede science questions and future exploration Archived 2022-01-21 at the Wayback Machine
  16. P.C. Thomas (2010) 'Sizes, shapes, and derived properties of the Saturnian satellites after the Cassini nominal mission' Archived 2018-12-23 at the Wayback Machine, Icarus 208: 395–401
  17. Leliwa-Kopystyński, J.; Kossacki, K. J. (2000). "Evolution of porosity in small icy bodies". Planetary and Space Science. 48 (7–8): 727–745. doi:10.1016/S0032-0633(00)00038-6.
  18. Schenk, Paul; Buratti, Bonnie; Clark, Roger; Byrne, Paul; McKinnon, William; Matsuyama, Isamu; Nimmo, Francis; Scipioni, Francesca (2022). "Red Streaks on Tethys: Evidence for Recent Activity". copernicus.org. Europlanet Science Congress 2022. Retrieved 20 November 2022.
  19. Grundy, W.M.; Noll, K.S.; Buie, M.W.; Benecchi, S.D.; Ragozzine, D.; Roe, H.G. (2019). "The Mutual Orbit, Mass, and Density of Transneptunian Binary Gǃkúnǁʼhòmdímà ((229762) 2007 UK126)". Icarus. 334: 30–38. doi:10.1016/j.icarus.2018.12.037. S2CID 126574999. Archived (PDF) from the original on 7 April 2019.
  20. Most figures are from the NASA/JPL list of Planetary Satellite Physical Parameters Archived 2019-01-04 at the Wayback Machine, apart from the masses of the Uranian moons, which are from Jacobson (2014).
  21. Durante, Daniele; Hemingway, D. J.; Racioppa, P.; Iess, L.; Stevenson, D. J. (2019). "Titan's gravity field and interior structure after Cassini" (PDF). Icarus. 326: 123–132. doi:10.1016/j.icarus.2019.03.003. hdl:11573/1281269. S2CID 127984873. Retrieved 3 April 2022.
  22. Castillo-Rogez, J. C.; et al. (2011). "How differentiated is Callisto" (PDF). 42nd Lunar and Planetary Science Conference: 2580. Retrieved 2 January 2020.
  23. Garrick-Bethell, I.; Wisdom, J; Zuber, MT (4 August 2006). "Evidence for a Past High-Eccentricity Lunar Orbit". Science. 313 (5787): 652–655. Bibcode:2006Sci...313..652G. doi:10.1126/science.1128237. PMID 16888135. S2CID 317360.
  24. A Moon with Atmosphere Archived 2022-02-08 at the Wayback Machine, Emily Lakdwalla, The Planetary Society (8 April 2015)
  25. Dougherty, M. K.; Khurana, K. K.; et al. (2006). "Identification of a Dynamic Atmosphere at Enceladus with the Cassini Magnetometer". Science. 311 (5766): 1406–9. Bibcode:2006Sci...311.1406D. doi:10.1126/science.1120985. PMID 16527966. S2CID 42050327.
  26. Ghosh, Pallab (2 March 2012). "Oxygen envelops Saturn's icy moon". BBC News. Retrieved 2012-03-02.
  27. Teolis, B. D.; Jones, G. H.; Miles, P. F.; Tokar, R. L.; Magee, B. A.; Waite, J. H.; Roussos, E.; Young, D. T.; Crary, F. J.; Coates, A. J.; Johnson, R. E.; Tseng, W. - L.; Baragiola, R. A. (2010). "Cassini Finds an Oxygen-Carbon Dioxide Atmosphere at Saturn's Icy Moon Rhea". Science. 330 (6012): 1813–1815. Bibcode:2010Sci...330.1813T. doi:10.1126/science.1198366. PMID 21109635. S2CID 206530211.
  28. Widemann, T.; Sicardy, B.; Dusser, R.; Martinez, C.; Beisker, W.; Bredner, E.; Dunham, D.; Maley, P.; Lellouch, E.; Arlot, J. -E.; Berthier, J.; Colas, F.; Hubbard, W. B.; Hill, R.; Lecacheux, J.; Lecampion, J. -F.; Pau, S.; Rapaport, M.; Roques, F.; Thuillot, W.; Hills, C. R.; Elliott, A. J.; Miles, R.; Platt, T.; Cremaschini, C.; Dubreuil, P.; Cavadore, C.; Demeautis, C.; Henriquet, P.; et al. (February 2009). "Titania's radius and an upper limit on its atmosphere from the September 8, 2001 stellar occultation" (PDF). Icarus. 199 (2): 458–476. Bibcode:2009Icar..199..458W. doi:10.1016/j.icarus.2008.09.011.
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