Editing Rhea (moon) (section)

=== Size, mass, and internal structure ===
[[File:Rhea, Earth & Moon size comparison.jpg|left|thumb|250x250px|Size comparison of [[Earth]] (right), the [[Moon]] (left top), and Rhea (left down)]]
Rhea is the second largest moon of Saturn, but with a mean diameter of 1,528 kilometers (949 miles) it is less than a third the radius of Saturn's largest moon, [[Titan (moon)|Titan]]. Rhea is an icy body with a [[density]] of about 1.236 g/cm<sup>3</sup>. This low density indicates that it is made of ~25% rock (density ~3.25 g/cm<sup>3</sup>) and ~75% water ice (density ~0.93 g/cm<sup>3</sup>). A layer of [[Ice II]] (a high-pressure and extra-low temperature form of ice) is believed, based on the moon's temperature profile, to start around {{Convert|350 to 450|km|mi}} beneath the surface.<ref>{{Cite Q|Q126417371}}</ref><ref>{{Cite book |url=https://books.google.com/books?id=Rg6dBAAAQBAJ&dq=%22surface+gravity%22+of+%22rhea%22&pg=RA4-PA628 |title=Treatise on Geophysics |date=2015-04-17 |publisher=Elsevier |isbn=978-0-444-53803-1 |language=en}}</ref><ref>{{Cite book |last=Hobbs |first=Peter Victor |url=https://books.google.com/books?id=7Is6AwAAQBAJ&pg=PA61 |title=Ice Physics |date=2010-05-06 |publisher=OUP Oxford |isbn=978-0-19-958771-1 |language=en}}</ref> Although Rhea is the ninth-largest moon in the Solar System, it is only the tenth-most massive. Indeed, [[Oberon (moon)|Oberon]], the second-largest moon of Uranus, has almost the same size, but is significantly denser than Rhea (1.63 vs 1.24) and thus more massive, although Rhea is slightly larger by volume.<ref group="lower-alpha">The moons more massive than Rhea are: the [[Moon]], the four [[Galilean moons]], Titan, Triton, Titania, and Oberon. Oberon, Uranus's second-largest moon, has a radius that is ~0.4% smaller than Rhea's, but a density that is ~26% greater. See [http://ssd.jpl.nasa.gov/?sat_phys_par JPLSSD.]</ref> The surface area of the moon can be estimated at {{Convert|7330000|km2|mi2|sigfig=3|round=10}}, about the size of Australia (7,688,287 km<sup>2</sup>).<ref>{{Cite web |last=Australia |first=Geoscience |date=2014-06-27 |title=Area of Australia - States and Territories |url=https://www.ga.gov.au/scientific-topics/national-location-information/dimensions/area-of-australia-states-and-territories |access-date=2024-06-07 |website=Geoscience Australia |language=en}}</ref>{{Efn|The [[surface area]] can be estimated, given the radius, with the formula {{math|4''&pi;r''<sup>2</sup>}}|name=surface_area}}

Before the ''[[Cassini–Huygens]]'' mission, it was assumed that Rhea had a rocky core.<ref name="Anderson2003" />  However, measurements taken during a close flyby by the [[Cassini–Huygens|''Cassini'']] orbiter in 2005 cast this into doubt. In a paper published in 2007 it was claimed that the axial dimensionless [[moment of inertia]] coefficient was 0.4.{{refn | group = lower-alpha | More precisely, 0.3911.<ref name="Anderson_2007" /> }}<ref name="Anderson_2007" />  Such a value indicated that Rhea had an almost homogeneous interior (with some compression of ice in the center) while the existence of a rocky core would imply a moment of inertia of about 0.34.<ref name="Anderson2003" />  In the same year, another paper claimed the moment of inertia was about 0.37.{{refn | group = lower-alpha | More precisely, 0.3721.<ref name="Iess2007" /> }} Rhea being either partially or fully differentiated would be consistent with the observations of the ''Cassini'' probe.<ref name="Iess2007" /> A year later, yet another paper claimed that the moon may not be in [[hydrostatic equilibrium]], meaning that the moment of inertia cannot be determined from the gravity data alone.<ref name="MacKenzie2008" />  In 2008, an author of the first paper tried to reconcile these three disparate results. He concluded that there is a systematic error in the ''Cassini'' radio Doppler data used in the analysis, but, after restricting the analysis to a subset of data obtained closest to the moon, he arrived at his old result that Rhea was in hydrostatic equilibrium and had a moment of inertia of about 0.4, again implying a homogeneous interior.<ref name="Anderson2008" />

The [[Triaxial ellipsoid|triaxial]] shape of Rhea is consistent with a homogeneous body in [[hydrostatic equilibrium]] rotating at Rhea's angular velocity.<ref name="Thomas2007" /> Modelling in 2006 suggested that Rhea could be barely capable of sustaining an [[subsurface ocean|internal liquid-water ocean]] through heating by [[radioactive decay]]; such an ocean would have to be at about 176&nbsp;K, the [[eutectic temperature]] for the water–ammonia system.<ref>
{{cite journal| doi = 10.1016/j.icarus.2006.06.005| last1 = Hussmann| first1 = Hauke| last2 = Sohl| first2 = Frank| last3 = Spohn| first3 = Tilman| date = November 2006| title = Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-neptunian objects| journal = [[Icarus (journal)|Icarus]]| volume = 185| issue = 1| pages = 258–273| url = https://www.researchgate.net/publication/225019299| bibcode = 2006Icar..185..258H| ref = {{sfnRef|Hussmann Sohl et al.|2006}}}}</ref> More recent indications are that Rhea has a homogeneous interior and hence that this ocean does not exist.<ref name=Anderson2008/>