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A planetary-mass moon is a planetary-mass object that is a natural satellite of another such object. They are large and ellipsoidal (sometimes spherical) in shape. Moons may be in hydrostatic equilibrium due to tidal or radiogenic heating, in some cases forming a subsurface ocean. Two moons in the Solar System, Ganymede and Titan, are larger than the planet Mercury, and a third, Callisto, is just slightly smaller than it, although all three are less massive. Additionally, seven – Ganymede, Titan, Callisto, Io, Moon, Europa, and Triton – are larger and more massive than the dwarf planets and .
The concept of satellite planets – the idea that all planetary-mass objects, including moons, are – 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). This conceptualization of planets as three classes of objects (classical planets, and satellite planets) has not been accepted by the International Astronomical Union (the IAU).
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 , 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'.Emily Lakdawalla et al., What Is A Planet? The Planetary Society, 21 April 2020 It is larger than Mimas but is quite far from being round.
Earth's entirely rocky moon solidified out of equilibrium billions of years ago, 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.Planetary Science Decadal Survey Community White Paper, Ganymede science questions and future exploration
One of the mid-sized moons of Saturn (Rhea) may also be in equilibrium,P.C. Thomas (2010) 'Sizes, shapes, and derived properties of the Saturnian satellites after the Cassini nominal mission' , Icarus 208: 395–401
as may a couple of the moons of Uranus (Titania and Oberon).
However, the other ellipsoidal moons of Saturn (Mimas, Enceladus, Tethys, Dione and Iapetus) are no longer in equilibrium. 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, 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.
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.
Methone, Pallene, and, with less certainty, Aegaeon are in hydrostatic equilibrium. However, as they are not planetary-mass objects, these are not included as planetary-mass moons.
List
Moon of Earth Satellites of Jupiter Satellites of Uranus Satellites of Saturn Satellites of Neptune Satellites of Pluto +List of ellipsoidal moonsMost figures are from the NASA/JPL list of Planetary Satellite Physical Parameters , apart from the masses of the Uranian moons, which are from Jacobson (2014).
! colspan=2 Moon
! rowspan=2 class="unsortable" Image
! colspan=2 Radius
! colspan=2 Mass
! data-sort-type="number" Density
! data-sort-type="number" Surface gravity
! rowspan=2 data-sort-type="number" Year of
discovery
! rowspan=2Hydrostatic
equilibrium? 156.4% 201.8% 0.146 1610 148.2% 183.2% 0.138 1655
138.8% 146.6% 0.126 1610
104.9% 121.7% 0.183 1610 100% 100% 0.165 Prehistoric
89.9% 65.4% 0.134 1610 79.9% 29.1% 0.080 1846 45.4% 4.6% 0.040 1787
44.0% 3.1% 0.027 1672
43.8% 4.2% 0.036 1787
42.3% 2.5% 0.022 1671
34.7% 2.1% 0.029 1978
33.7% 1.7% 0.023 1851 33.3% 1.7% 0.028 1851 32.3% 1.5% 0.024 1684
30.7% 0.84% 0.015 1684
14.5% 0.15% 0.011 1789
13.6% 0.09% 0.008 1948 11.4% 0.05% 0.006 1789
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. Io (1.9 ) and Callisto (26 ) have very thin atmospheres, but still enough to have collisions between atmospheric molecules. Other planetary-mass moons only have at most. A Moon with Atmosphere , Emily Lakdwalla, The Planetary Society (8 April 2015) Exospheres have been detected around Earth's Moon, Europa, Ganymede, Enceladus, Dione, and Rhea. An exosphere around Titania is a possibility, though it has not been confirmed.
See also
Further reading
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