Planetary-mass moon
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A planetary-mass moon is a
The concept of satellite planets – the idea that planetary-mass objects, including moons, are
Early history
The distinction between a satellite and a classical planet was not recognized until after the
Modern concept
Graphs are unavailable due to technical issues. There is more info on Phabricator and on MediaWiki.org. |
Graphs are unavailable due to technical issues. There is more info on Phabricator and on MediaWiki.org. |
In the modern era, Alan Stern considers satellite planets to be one of three categories of planets, along with
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.
Neptune's second-largest moon Proteus (Neptune VIII) 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 (
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
Satellites of planets | ||
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Satellite of Earth | Satellites of Jupiter | Satellites of Uranus |
Satellites of Saturn | Satellites of Neptune | |
Satellites of generally agreed dwarf planets | ||
Satellites of Pluto |
Moon | Image | Radius | Mass | Density | Surface gravity | Year of discovery |
Hydrostatic equilibrium? | |||
---|---|---|---|---|---|---|---|---|---|---|
Name | Designation | (km) | ( R☾ )
|
( 1021 kg)
|
( M☾ )
|
(g/cm3) | (g) | |||
Ganymede | Jupiter III | 2634.1±0.3 | 156.4% | 148.2 | 201.8% | 1.942±0.005 | 0.146 | 1610 | ||
Titan | Saturn VI | 2574.7±0.1 | 148.2% | 134.5 | 183.2% | 1.882±0.001 | 0.138 | 1655 | [21] | |
Callisto | Jupiter IV | 2410.3±1.5 | 138.8% | 107.6 | 146.6% | 1.834±0.003 | 0.126 | 1610 | [22] | |
Io | Jupiter I | 1821.6±0.5 | 104.9% | 89.3 | 121.7% | 3.528±0.006 | 0.183 | 1610 | ||
Moon (Luna) | Earth I | 1737.05 | 100% | 73.4 | 100% | 3.344±0.005 | 0.165 | Prehistoric | [23] | |
Europa | Jupiter II | 1560.8±0.5 | 89.9% | 48.0 | 65.4% | 3.013±0.005 | 0.134 | 1610 | ||
Triton | Neptune I | 1353.4±0.9 | 79.9% | 21.4 | 29.1% | 2.059±0.005 | 0.080 | 1846 | ||
Titania | Uranus III | 788.9±1.8 | 45.4% | 3.40±0.06 | 4.6% | 1.66±0.04 | 0.040 | 1787 | [11] | |
Rhea | Saturn V | 764.3±1.0 | 44.0% | 2.31 | 3.1% | 1.233±0.005 | 0.027 | 1672 | [16] | |
Oberon | Uranus IV | 761.4±2.6 | 43.8% | 3.08±0.09 | 4.2% | 1.56±0.06 | 0.036 | 1787 | [11] | |
Iapetus | Saturn VIII | 735.6±1.5 | 42.3% | 1.81 | 2.5% | 1.083±0.007 | 0.022 | 1671 | [16] | |
Charon | Pluto I | 603.6±1.4 | 34.7% | 1.53 | 2.1% | 1.664±0.012 | 0.029 | 1978 | [14] | |
Umbriel | Uranus II | 584.7±2.8 | 33.7% | 1.28±0.03 | 1.7% | 1.46±0.09 | 0.023 | 1851 | ||
Ariel | Uranus I | 578.9±0.6 | 33.3% | 1.25±0.02 | 1.7% | 1.59±0.09 | 0.028 | 1851 | ||
Dione | Saturn IV | 561.4±0.4 | 32.3% | 1.10 | 1.5% | 1.476±0.004 | 0.024 | 1684 | [16] | |
Tethys | Saturn III | 533.0±0.7 | 30.7% | 0.617 | 0.84% | 0.973±0.004 | 0.015 | 1684 | [16]
| |
Enceladus | Saturn II | 252.1±0.2 | 14.5% | 0.108 | 0.15% | 1.608±0.003 | 0.011 | 1789 | [16] | |
Miranda | Uranus V | 235.8±0.7 | 13.6% | 0.064±0.003 | 0.09% | 1.21±0.11 | 0.008 | 1948 | ||
Mimas |
Saturn I | 198.2±0.4 | 11.4% | 0.038 | 0.05% | 1.150±0.004 | 0.006 | 1789 | [16] |
Methone, Pallene, and, with less certainty, Aegaeon are in hydrostatic equilibrium.[24] However, as they are not planetary-mass objects, these are not included as planetary-mass moons.
Atmospheres
Titan has a denser atmosphere than Earth, with a surface pressure of 1.4 bar, while Triton has a relatively thinner atmosphere of 14 μbar; Titan and Triton are the only known moons to have atmospheres significant enough to drive weather and climate processes.[25] 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.[26] Exospheres have been detected around Earth's Moon, Europa, Ganymede,[26] Enceladus,[27] Dione,[28] and Rhea.[29] An exosphere around Titania is a possibility, though it has not been confirmed.[30]
See also
References
- ^ "Should Large Moons Be Called 'Satellite Planets'?". News.discovery.com. 2010-05-14. Archived from the original on 2014-10-25.
- ^ Galileo Galilei (1989). Siderius Nuncius. Albert van Helden. University of Chicago Press. p. 26.
- ^ Christiani Hugenii (Christiaan Huygens) (1659). Systema Saturnium: Sive de Causis Miradorum Saturni Phaenomenon, et comite ejus Planeta Novo. Adriani Vlacq. pp. 1–50.
- ^ Giovanni Cassini (1673). Decouverte de deux Nouvelles Planetes autour de Saturne. Sabastien Mabre-Craniusy. pp. 6–14.
- JSTOR 101844.
- ^ 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.
- ^ See primary citations in Timeline of discovery of Solar System planets and their moons
- ^ Smith, Asa (1868). Smith's Illustrated Astronomy. Nichols & Hall. p. 23.
secondary planet Herschel.
- ^ "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.
- S2CID 119338327. Archived from the original(PDF) on July 31, 2013.
- ^ a b c d e Hussmann, Hauke; Sohl, Frank; Spohn, Tilman (November 2006). "Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-Neptunian objects". .
- ^ Emily Lakdawalla et al., What Is A Planet? Archived 2022-01-22 at the Wayback Machine The Planetary Society, 21 April 2020
- ^ Williams, Matt. "A geophysical planet definition". Phys.org. Retrieved 2022-05-25.
- ^ a b c
Nimmo, Francis; et al. (2017). "Mean radius and shape of Pluto and Charon from New Horizons images". Icarus. 287: 12–29. S2CID 44935431.
- ^ Planetary Science Decadal Survey Community White Paper, Ganymede science questions and future exploration Archived 2022-01-21 at the Wayback Machine
- ^ a b c d e f g h 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
- .
- ^ 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.
- (PDF) from the original on 7 April 2019.
- ^ 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).
- S2CID 127984873. Retrieved 3 April 2022.
- ^ Castillo-Rogez, J. C.; et al. (2011). "How differentiated is Callisto" (PDF). 42nd Lunar and Planetary Science Conference: 2580. Retrieved 2 January 2020.
- S2CID 317360.
- Bibcode:2013LPI....44.1598T.
- S2CID 4250378.
- ^ a b A Moon with Atmosphere Archived 2022-02-08 at the Wayback Machine, Emily Lakdwalla, The Planetary Society (8 April 2015)
- S2CID 42050327.
- ^ Ghosh, Pallab (2 March 2012). "Oxygen envelops Saturn's icy moon". BBC News. Retrieved 2012-03-02.
- ^
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. S2CID 206530211.
- ^ 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. .
Further reading
- Moons Are Planets: Scientific Usefulness Versus Cultural Teleology in the Taxonomy of Planetary Science, Philip T. Metzger, William M. Grundy, Mark Sykes, S. Alan Stern, James F. Bell III, Charlene E. Detelich, Kirby D. Runyon, Michael Summers, 22 Oct 2021