Rhea (moon)

Template:Short description Template:Distinguish {{#invoke:infobox|infoboxTemplate | class = vcard | titleclass = fn org | title = Rhea | image = {{#invoke:InfoboxImage|InfoboxImage|image=PIA07763 Rhea full globe5.jpg|upright={{#if:||1.1}}|alt=}} | caption = Mosaic of Rhea, assembled from Cassini imagery taken on 26 November 2005 | headerstyle = {{#if:|background-color:|background-color:#E0CCFF}} | labelstyle = max-width:{{#if:||11em}}; | autoheaders = y

| header1 = Discovery

| label2 = Discovered by | data2 = G. D. Cassini<ref name=space.com.Rhea/> | label3 = Discovery site | data3 = | label4 = Discovery date | data4 = December 23, 1672<ref name=space.com.Rhea>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> | label5 = Template:Longitem | data5 =

| header10 = {{#if:|Designations|Designations}}

| header20 = Orbital characteristics{{#ifeq:|yes| (barycentric)}} <ref name="NSES" />

| data21 = | data22 = {{#if: |Epoch {{{epoch}}}}} | data23 = {{#if: | Uncertainty parameter {{{uncertainty}}}}} | label24 = Observation arc | data24 = | label25 = Earliest precovery date | data25 = | label26 = {{#switch:{{{apsis}}} |apsis|gee|barion|center|centre|(apsis)=Apo{{{apsis}}} |Ap{{#if:|{{{apsis}}}|helion}}}} | data26 = | label27 = Peri{{#if:|{{{apsis}}}|helion}} | data27 = | label28 = Peri{{#if:|{{{apsis}}}|apsis}} | data28 = | label29 = {{#switch:{{{apsis}}} |helion|astron=Ap{{{apsis}}} |Apo{{#if:|{{{apsis}}}|apsis}}}} | data29 = | label30 = Periastron | data30 = | label31 = Apoastron | data31 = | label32 = Template:Longitem | data32 = Template:Val<ref name="NSSDC"/> | label33 = Template:Longitem | data33 = | label34 = Eccentricity | data34 = Template:Val<ref name="NSSDC"/> | label35 = Template:Longitem | data35 = Template:Val | label36 = Template:Longitem | data36 = | label37 = Template:Longitem | data37 = 8.48 km/sTemplate:Efn | label38 = Template:Longitem | data38 = | label39 = Template:Longitem | data39 = | label40 = Inclination | data40 = 0.35°<ref name="NSSDC"/> | label41 = Template:Longitem | data41 = | label42 = Template:Longitem | data42 = | label43 = Template:Longitem | data43 = | label44 = Template:Longitem | data44 = | label45 = Template:Longitem | data45 = | label46 = Template:Nowrap | data46 = | label47 = Satellite of | data47 = Saturn | label48 = Group | data48 = | label49 = {{#switch: |yes|true=Satellites |Known satellites}} | data49 = | label50 = Star | data50 = | label51 = Earth MOID | data51 = | label52 = Mercury MOID | data52 = | label53 = Venus MOID | data53 = | label54 = Mars MOID | data54 = | label55 = Jupiter MOID | data55 = | label56 = Saturn MOID | data56 = | label57 = Uranus MOID | data57 = | label58 = Neptune MOID | data58 = | label59 = T_Jupiter | data59 =

| header60 = Proper orbital elements

| label61 = Template:Longitem | data61 = {{#if: |{{{p_semimajor}}} AU}} | label62 = Template:Longitem | data62 = | label63 = Template:Longitem | data63 = | label64 = Template:Longitem | data64 = {{#if: |{{{p_mean_motion}}} deg Template:\yr}} | label65 = Template:Longitem | data65 = {{#if:|{{#expr:360/1 round 5}} yr
({{#expr:365.25*360/1 round 3}} d) }} | label66 = Template:Longitem | data66 = {{#if:|{{{perihelion_rate}}} arcsec Template:\yr }} | label67 = Template:Longitem | data67 = {{#if:|{{{node_rate}}} arcsec Template:\yr}}

| header70 = Template:Anchor{{#if:| Physical characteristics|Physical characteristics}}

| label71 = Dimensions | data71 = 1532.4 × 1525.6 × 1524.4 km <ref name="Roatsch et al. 2009" /> | label72 = Template:Longitem | data72 = | label73 = Template:Longitem | data73 = Template:Val<ref name="Jacobson2022">Template:Cite Q</ref> | label74 = Template:Longitem | data74 = | label75 = Template:Longitem | data75 = | label76 = Flattening | data76 = | label77 = Circumference | data77 = | label78 = Template:Longitem | data78 = Template:Val Template:Efn | label79 = Volume | data79 = | label80 = Mass | data80 = Template:Val<ref name="Jacobson2022"/> (~3.9Template:E Earths) | label81 = Template:Longitem | data81 = Template:Val<ref name="Jacobson2022"/> | label82 = Template:Longitem | data82 = Template:Gr m/s² Template:Efn | label83 = Template:Longitem | data83 = Template:Val<ref name="Anderson2007">Template:Cite journal</ref> (disputed/unclear<ref name="Anderson2008" />) | label84 = Template:Longitem | data84 = Template:V2 km/s | label85 = Template:Longitem | data85 = Template:Val
(synchronous) | label86 = Template:Longitem | data86 = | label87 = Template:Longitem | data87 = | label88 = Template:Longitem | data88 = zero | label89 = Template:Longitem | data89 = | label90 = Template:Longitem | data90 = | label91 = Template:Longitem | data91 = | label92 = Template:Longitem | data92 = | label93 = {{#if: |Template:Longitem |Albedo}} | data93 = Template:Val (geometric) <ref name="Verbiscer et al. 2007" /> | label94 = Temperature | data94 =

| data100 = {{#if:Kelvin|

Surface temp.	min	max
Kelvin	53 K	99 K
{{{temp_name2}}}
{{{temp_name3}}}
{{{temp_name4}}}

}}

| header110 = Atmosphere

| below = {{#if:||Template:Reflist }}

}}{{#invoke:Check for unknown parameters|check|unknown=Template:Main other|preview=Page using Template:Infobox planet with unknown parameter "_VALUE_"|ignoreblank=y| abs_magnitude | adjective | adjectives | albedo | allsatellites | alt_names | angular_dist | angular_size | aphelion | apoapsis | apsis | apoastron | arg_peri | asc_node | atmosphere | atmosphere_composition | atmosphere_ref | avg_speed | axial_tilt | background | barycentric | bgcolour | caption | circumference | declination | density | dimensions | discovered | discoverer | discovery_method | discovery_ref | discovery_site | earliest_precovery_date | eccentricity | epoch | equatorial_radius | escape_velocity | exosolar planets | extrasolarplanet | family | flattening | group | image | image_alt | image_scale | inclination | jupiter_moid | label_width | long_periastron | magnitude | mars_moid | mass | max_temp_1 | max_temp_2 | max_temp_3 | max_temp_4 | mean_anomaly | mean_diameter | mean_motion | mean_orbit_radius | mean_radius | mean_temp_1 | mean_temp_2 | mean_temp_3 | mean_temp_4 | mercury_moid | min_temp_1 | min_temp_2 | min_temp_3 | min_temp_4 | minorplanet | moid | moment_of_inertia_factor | mp_category | mp_name | mpc_name | name | named_after | neptune_moid | node_rate | note | observation_arc | orbit_diagram | orbit_ref | p_eccentricity | p_inclination | p_mean_motion | p_orbit_ref | p_semimajor | periapsis | periastron | perihelion | perihelion_rate | period | physical_ref | polar_radius | pole_ecliptic_lat | pole_ecliptic_lon | pronounce | pronounced | right_asc_north_pole | rot_velocity | rotation | satellite_of | satellites | saturn_moid | scale_height | semi-amplitude | semimajor | sidereal_day | single_temperature | spectral_type | star | surface_area | surface_grav | surface_pressure | surface_absorbed_dose_rate | surface_equivalent_dose_rate | symbol | synodic_period | temp_name1 | temp_name2 | temp_name3 | temp_name4 | time_periastron | tisserand | uncertainty | uranus_moid | venus_moid | volume }}

Rhea (Template:IPAc-en) is the second-largest moon of Saturn and the ninth-largest moon in the Solar System, with a surface area that is comparable to the area of Australia. It is the smallest body in the Solar System for which precise measurements have confirmed a shape consistent with hydrostatic equilibrium. Rhea has a nearly circular orbit around Saturn, but it is also tidally locked, like Saturn's other major moons; that is, it rotates with the same period it revolves (orbits), so one hemisphere always faces towards the planet.

The moon itself has a fairly low density, composed of roughly three-quarters ice and only one-quarter rock. The surface of Rhea is heavily cratered, with distinct leading and trailing hemispheres. Like the moon Dione, it has high-albedo ice cliffs that appear as bright wispy streaks visible from space. The surface temperature varies between −174 °C and −220 °C.

Rhea was discovered in 1672 by Giovanni Domenico Cassini. Since then, it has been visited by both Voyager probes and was the subject of close targeted flybys by the Cassini orbiter in 2005, 2007, 2010, 2011, and once more in 2013.

DiscoveryEdit

Rhea was discovered by Giovanni Domenico Cassini on 23 December 1672, with a Template:Convert telescope made by Giuseppe Campani.<ref name=space.com.Rhea/><ref>Template:Cite Q</ref> Cassini named the four moons he discovered (Tethys, Dione, Rhea, and Iapetus) Sidera Lodoicea (the stars of Louis) to honor King Louis XIV.<ref name="space.com.Rhea" /> Rhea was the second moon of Saturn that Cassini discovered, and the third moon discovered around Saturn overall.<ref name=space.com.Rhea/>

NameEdit

Rhea is named after the Titan Rhea of Greek mythology, the mother of the first generation of Olympian gods and wife of Cronus, the Greek counterpart of the god Saturn. It is also designated Saturn V (being the fifth major moon going outward from the planet, after Mimas, Enceladus, Tethys, and Dione).<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name=":0">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Astronomers fell into the habit of referring to them and Titan as Saturn I through Saturn V.<ref name=space.com.Rhea/> Once Mimas and Enceladus were discovered, in 1789, the numbering scheme was extended to Saturn VII, and then to Saturn VIII with the discovery of Hyperion in 1848.<ref name=":0" />

Rhea was not named until 1847, when John Herschel (son of William Herschel, discoverer of the planet Uranus and two other moons of Saturn, Mimas and Enceladus) suggested in Results of Astronomical Observations made at the Cape of Good Hope that the names of the Titans, sisters and brothers of Kronos (Saturn, in Roman mythology), be used.<ref name="Lassell1848" /><ref name=space.com.Rhea/>

Planetary moons other than Earth's were never given symbols in the astronomical literature. Denis Moskowitz, a software engineer who designed most of the dwarf planet symbols, proposed a Greek rho (the initial of Rhea) combined with the crook of the Saturn symbol as the symbol of Rhea (File:Rhea symbol (fixed width).svg). This symbol is not widely used.<ref name=moons>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

OrbitEdit

The orbit of Rhea has very low eccentricity (0.001), meaning it is nearly circular. It has a low inclination of less than a degree, inclined by only 0.35° from Saturn's equatorial plane.<ref name="NSSDC"/>

Rhea is tidally locked and rotates synchronously; that is, it rotates at the same speed it revolves (orbits), so one hemisphere is always facing towards Saturn. This is called the near pole. Equally, one hemisphere always faces forward, relative to the direction of movement; this is called the leading hemisphere; the other side is the trailing hemisphere, which faces backwards relative to the moon's motion.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Physical characteristicsEdit

Size, mass, and internal structureEdit

File:Rhea, Earth & Moon size comparison.jpg

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. Rhea is an icy body with a density of about 1.236 g/cm³. This low density indicates that it is made of ~25% rock (density ~3.25 g/cm³) and ~75% water ice (density ~0.93 g/cm³). 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 Template:Convert beneath the surface.<ref>Template:Cite Q</ref><ref>Template:Cite book</ref><ref>Template:Cite book</ref> Although Rhea is the ninth-largest moon in the Solar System, it is only the tenth-most massive. Indeed, 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 JPLSSD.</ref> The surface area of the moon can be estimated at Template:Convert, about the size of Australia (7,688,287 km²).<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>Template:Efn

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 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.Template:Refn<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.Template:Refn 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 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 internal liquid-water ocean through heating by radioactive decay; such an ocean would have to be at about 176 K, the eutectic temperature for the water–ammonia system.<ref> Template:Cite journal</ref> More recent indications are that Rhea has a homogeneous interior and hence that this ocean does not exist.<ref name=Anderson2008/>

Surface featuresEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} Template:Multiple image Rhea's features resemble those of Dione, with distinct and dissmillar leading and trailing hemispheres, suggesting similar composition and histories. The temperature on Rhea is 99 K (−174 °C) in direct sunlight and between 73 K (−200 °C) and 53 K (−220 °C) in the shade.

File:PIA18310-SaturnMoon-Rhea-20150210.jpg

Surface features on Rhea well defined due to the lighting

Rhea has a rather typical heavily cratered surface,<ref name="MooreSchenk_2004" /> with the exceptions of a few large Dione-type chasmata or fractures (formerly known as wispy terrain) on the trailing hemisphere (the side facing away from the direction of motion along Rhea's orbit)<ref name="Wagner2008" /> and a very faint "line" of material at Rhea's equator that may have been deposited by material deorbiting from its rings.<ref name="Schenk2009" /> Rhea has two very large impact basins on its hemisphere facing away from Saturn, which are about 400 and 500 km across.<ref name="Wagner2008" /> The more northerly and less degraded of the two, called Tirawa, is roughly comparable in size to the basin Odysseus on Tethys.<ref name="MooreSchenk_2004" /> There is a 48 km-diameter impact crater at 112°W that is prominent because of an extended system of bright rays, which extend up to Template:Convert away from the crater, across most of one hemisphere.<ref name="Wagner2008" /><ref>Template:Cite journal</ref> This crater, called Inktomi, is nicknamed "The Splat", and may be one of the youngest craters on the inner moons of Saturn. This was hypothesized in a 2007 paper published by Lunar and Planetary Science.<ref name="Wagner2008" /> Rhea's impact craters are more crisply defined than the flatter craters that are pervasive on Ganymede and Callisto; it is theorized that this is due to a much lower surface gravity (0.26 m/s², compared to Ganymede's 1.428 m/s² and Callisto's 1.235 m/s²) and a stiffer crust of ice. Similarly, ejecta blankets – asymmetrical blankets of ejected particles surrounding impact craters – are not present on Rhea, potentially another result of the moon's low surface gravity.<ref>Template:Cite Q</ref>

File:Rhea at approximately 2,348 miles (3,778 kilometers) away.jpg

Closeup showing two craters on Rhea's surface taken in 2013 by Cassini spacecraft

Its surface can be divided into two geologically different areas based on crater density; the first area contains craters which are larger than 40 km in diameter, whereas the second area, in parts of the polar and equatorial regions, has only craters under that size. This suggests that a major resurfacing event occurred some time during its formation. The leading hemisphere is heavily cratered and uniformly bright. As on Callisto, the craters lack the high relief features seen on the Moon and Mercury. It has been theorized that these cratered plains are up to four billion years old on average.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> On the trailing hemisphere there is a network of bright swaths on a dark background, and fewer craters.<ref>Template:Cite journal</ref> It is believed, based on data from the Cassini probe, that these are tectonic features: depressions (graben) and troughs, with ice-covered cliff sides causing the lines' whiteness (more technically their albedo).<ref>Template:Cite journal</ref> The extensive dark areas are thought to be deposited tholins, which are a mix of complex organic compounds generated on the ice by pyrolysis and radiolysis of simple compounds containing carbon, nitrogen and hydrogen.<ref name='Cruikshank 2005'>A spectroscopic study of the surfaces of Saturn's large satellites: H2O ice, tholins, and minor constituents (PDF). Dale P. Cruikshank, Tobias Owen, Cristina Dalle Ore, Thomas R. Geballe, Ted L. Roush, Catherine de Bergh, Scott A. Sandford, Francois Poulet, Gretchen K. Benedix, Joshua P. Emery. Icarus, 175, pages: 268–283, 2 March 2005.</ref> The trailing side of Rhea's surface is irradiated by Saturn's magnetosphere, which may cause chemical-level changes on the surface, including radiolysis (see Template:Section link). Particles from Saturn's E-ring are also flung onto the moon's leading hemisphere, coating it.<ref name=":1" />

Rhea has some evidence of endogenic activity – that is, activity originating from within the moon, such as heating and cryovolcanic activity: there are fault systems and craters with uplifted bases (so-called "relaxed" craters), although the latter is apparently only present in large craters more than Template:Convert across.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref name=":1">Template:Cite journal</ref>

FormationEdit

The moons of Saturn are thought to have formed through co-accretion, a similar process to that believed to have formed the planets in the Solar System. As the young giant planets formed, they were surrounded by discs of material that gradually coalesced into moons. However, a model proposed by Erik Asphaug and Andreas Reufer for the formation of Titan may also shine a new light on the origin of Rhea and Iapetus. In this model, Titan was formed in a series of giant impacts between pre-existing moons, and Rhea and Iapetus are thought to have formed from part of the debris of these collisions.<ref>{{#invoke:citation/CS1|citation |CitationClass=web

}}</ref>

Rhea ice cliffs.jpg

Image of the wispy hemisphere, showing ice cliffs – Powehiwehi (upper center); chasmata stretch from upper left to right center – Onokoro Catenae (lower right)
PIA08148 (Rhea-Splat).jpg

View of Rhea's leading hemisphere with crater Inktomi and its prominent ray system just below center; impact basin Tirawa is at upper left

AtmosphereEdit

On November 27, 2010, NASA announced the discovery of an extremely tenuous atmosphere—an exosphere. It consists of oxygen and carbon dioxide in proportion of roughly 5 to 2. The surface density of the exosphere is from 10⁵ to 10⁶ molecules in a cubic centimeter, depending on local temperature. The main source of oxygen is radiolysis of water ice at the surface via irradiation from the magnetosphere of Saturn. The source of the carbon dioxide is less clear, but it may be related to oxidation of the organics present in ice or to outgassing of the moon's interior.<ref name=":1" /><ref name="atmosphere" /><ref name="Teolis2010" />

Possible ring systemEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}}

File:Rings of Rhea (23988178290).jpg

Rings of Rhea

On March 6, 2008, NASA announced that Rhea may have a weak ring system. This would mark the first discovery of rings around a moon. The rings' existence was inferred by observed changes in the flow of electrons trapped by Saturn's magnetic field as Cassini passed by Rhea.<ref name="rings" /><ref name="Jones2008" /><ref name="LakdawallaE" /> Dust and debris could extend out to Rhea's Hill sphere, but were thought to be denser nearer the moon, with three narrow rings of higher density. The case for a ring was strengthened by the subsequent finding of the presence of a set of small ultraviolet-bright spots distributed along Rhea's equator (interpreted as the impact points of deorbiting ring material).<ref name="Lakdawalla2" /> However, when Cassini made targeted observations of the putative ring plane from several angles, there was no evidence of ring material found, suggesting that another explanation for the earlier observations is needed.<ref name="Tiscareno2010" /><ref name="norings" />

ExplorationEdit

The first images of Rhea were obtained by Voyager 1 & 2 spacecraft in 1980–1981.

There were five close targeted fly-bys by the Cassini orbiter, which was one part of the dual orbiter and lander Cassini–Huygens mission. Launched in 1997, Cassini–Huygens was targeted at the Saturn system; in total it took more than 450 thousand images.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Cassini passed Rhea at a distance of 500 km on November 26, 2005; at a distance of 5,750 km on August 30, 2007; at a distance of 100 km on March 2, 2010; at 69 km flyby on January 11, 2011;<ref name="CookJ2011" /> and a last flyby at 992 km on March 9, 2013.<ref name="LastFlybyRhea" />