Saturn’s Moon Rhea

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The Cronian complement (i.e. Saturn and a complement of rings and moons) is monumental to spy and intriguing to study. Besides a immeasurable and pleasing ring system, it also has a second-most satellites of any world in a Solar System. In fact, Saturn has an estimated 150 moons and moonlets– and usually 53 of them have been strictly named – that creates it second usually to Jupiter.

Saturn’s moon Rhea, as imaged by a Cassini-Huygens spacecraft. Credit: NASA/JPL-Caltech

Saturn’s moon Rhea, as imaged by a Cassini-Huygens spacecraft. Credit: NASA/JPL-Caltech

For a many part, these moons are small, icy bodies that are believed to residence interior oceans. And in all cases, quite Rhea, their engaging appearances and compositions make them a primary aim for systematic research. In further to being means to tell us most about a Cronian complement and a formation, moons like Rhea can also tell us most about a story of a Solar System.

Discovery and Naming:
Rhea was detected by Italian astronomer Giovanni Domenico Cassini on Dec 23rd, 1672. Together with a moons of Iapetus, Tethys and Dione, that he detected between 1671 and 1672, he named them allSidera Lodoicea (“the stars of Louis”) in respect of his patron, King Louis XIV of France. However, these names were not widely famous outward of France.

In 1847, John Herschel (the son of famed astronomer William Herschel, who detected Uranus, Enceladus and Mimas) suggested a name Rhea – that initial seemed in his dissertation Results of Astronomical Observations done during a Cape of Good Hope. Like all a other Cronian satellites, Rhea was named after a Titan from Greek mythology, a “mother of a gods” and one a sisters of Cronos (Saturn, in Roman mythology).

The moons of Saturn, from left to right: Mimas, Enceladus, Tethys, Dione, Rhea; Titan (background), Iapetus (top), and Hyperion (bottom). Credit: NASA/JPL/Space Science Institute

The moons of Saturn, from left to right: Mimas, Enceladus, Tethys, Dione, Rhea; Titan (background), Iapetus (top), and Hyperion (bottom). Credit: NASA/JPL/Space Science Institute

Size, Mass and Orbit:
With a meant radius of 763.8±1.0 km and a mass of 2.3065 ×1021 kg, Rhea is allied in stretch to 0.1199 Earths (and 0.44 Moons), and about 0.00039 times as vast (or 0.03139 Moons). It orbits Saturn during an normal stretch (semi-major axis) of 527,108 km, that places it outward a orbits of  Dione and Tethys, and has a scarcely round circuit with a unequivocally teenager oddity (0.001).

With an orbital quickness of about 30,541 km/h, Rhea takes approximately 4.518 days to finish a singular circuit of a primogenitor planet. Like many of Saturn’s moons, its rotational duration is synchronous with a orbit, definition that a same face is always forked towards it.

Composition and Surface Features:
With a meant firmness of about 1.236 g/cm³, Rhea is estimated to be stoical of 75% H2O ice (with a firmness of roughly 0.93 g/cm³) and 25% of silicate stone (with a firmness of around 3.25 g/cm³). This low firmness means that nonetheless Rhea is a ninth-largest moon in a Solar System, it is also a tenth-most massive.

In terms of a interior, Rhea was creatively suspected of being differentiated between a hilly core and an icy mantle. However, some-more new measurements would seem to prove that Rhea is possibly usually partly differentiated, or has a allied interior – expected consisting of both silicate stone and ice together (similar to Jupiter’s moon Callisto).

Views of Saturn’s moon Rhea, with false-color picture display betterment information during a right. Credit: NASA/JPL/Space Science Institute

Views of Saturn’s moon Rhea, with false-color picture display betterment information during a right. Credit: NASA/JPL/Space Science Institute

Models of Rhea’s interior also advise that it competence have an inner liquid-water ocean, identical to Enceladus and Tethys. This liquid-water ocean, should it exist, would expected be located during a core-mantle boundary, and would be postulated by a heating caused by from spoil of hot elements in a core.

Rhea’s aspect facilities resemble those of Dione, with separate appearances existent between their heading and trailing hemispheres – that suggests that a dual moons have identical compositions and histories. Images taken of a aspect have led astronomers to order it into dual regions – a heavily cratered and splendid terrain, where craters are incomparable than 40 km (25 miles) in diameter; and a frigid and equatorial regions where craters are noticeably smaller.

Another disproportion between Rhea’s heading and trailing hemisphere is their coloration. The heading hemisphere is heavily cratered and regularly splendid while a trailing hemisphere has networks of splendid swaths on a dim credentials and few manifest craters. It had been suspicion that these splendid areas (aka. wispy terrain) competence be element ejected from ice volcanoes early in Rhea’s story when a interior was still liquid.

However, observations of Dione, that has an even darker trailing hemisphere and identical though some-more distinguished splendid streaks, has expel this into doubt. It is now believed that a wispy turf are tectonically-formed ice cliffs (chasmata) that resulted from endless fracturing of a moon’s surface. Rhea also has a unequivocally gloomy “line” of element during a equator that was suspicion to be deposited by element deorbiting from a rings (see below).

Hemispheric tone differences on Saturn’s moon Rhea are apparent in this false-color perspective of a anti-Cronian side, from NASA’s Cassini spacecraft. Image Credit: NASA/JPL/SSI

Hemispheric tone differences on Saturn’s moon Rhea are apparent in this false-color perspective of a anti-Cronian side, from NASA’s Cassini spacecraft. Image Credit: NASA/JPL/SSI

Rhea has dual quite vast impact basins, both of that are situated on Rhea’s anti-Cronian side (aka. a side confronting divided from Saturn). These are famous as Tirawa and Mamaldi basins, that magnitude roughly 360 and 500 km (223.69 and 310.68 mi) across. The some-more primeval and reduction degraded dish of Tirawa overlaps Mamaldi – that lies to a southwest – and is roughly allied to a Odysseus void on Tethys (which gives it a “Death-Star” appearance).

Atmosphere:
Rhea has a gossamer atmosphere (exosphere) that consists of oxygen and CO dioxide, that exists in a 5:2 ratio. The aspect firmness of a exosphere is from 105 to 106 molecules per cubic centimeter, depending on internal temperature. Surface temperatures on Rhea normal 99 K (-174 °C/-281.2 °F) in approach sunlight, and between 73 K (-200 °C/-328 °F) and 53 K (-220 °C/-364 °F) when object is absent.

The oxygen in a atmosphere is combined by a communication of aspect H2O ice and ions granted from Saturn’s magnetosphere (aka. radiolysis). These ions means a H2O ice to mangle down into oxygen gas (O²) and component hydrogen (H), a former of that is defended while a latter escapes into space. The source of a CO dioxide is reduction clear, and could be possibly a outcome of organics in a aspect ice being oxidized, or from outgassing from a moon’s interior.

Saturn’s second-largest moon Rhea, graphic by a Cassini examine on Mar 29, 2012. Credit: NASA/JPL

Saturn’s second-largest moon Rhea, graphic by a Cassini examine on Mar 29, 2012. Credit: NASA/JPL

Rhea competence also have a gossamer ring system, that was unspoken formed on celebrated changes in a upsurge of electrons trapped by Saturn’s captivating field. The existence of a ring complement was temporarily bolstered by a detected participation of a set of tiny ultraviolet-bright spots distributed along Rhea’s equator (which were interpreted as a impact points of deorbiting ring material).

However, some-more new observations done by a Cassini probe have expel doubt on this. After holding images of a world from mixed angles, no justification of ring element was found, suggesting that there contingency be another means for a celebrated nucleus upsurge and UV splendid spots on Rhea’s equator. If such a ring complement were to exist, it would be a initial instance where a ring complement was found orbiting a moon.

Exploration:
The initial images of Rhea were performed by a Voyager 1 and 2 booster while they complicated a Cronian system, in 1980 and 1981, respectively. No successive missions were done until a attainment of a Cassini orbiter in 2005. After it’s attainment in a Cronian system, a orbiter done 5 tighten targeted fly-bys and took many images of Saturn from prolonged to assuage distances. 

The Cronian complement is unequivocally a fascinating place, and we’ve unequivocally usually begun to blemish a aspect in new years. In time, some-more orbiters and maybe landers will be roving to a system, seeking to learn some-more about Saturn’s moons and what exists underneath their icy surfaces. One can usually wish that any such goal includes a closer demeanour during Rhea, and a other “Death Star Moon”, Dione.

Source: Universe Today, created by Matt Williams