Rhea (moon)

Cassini named the four moons he discovered (Tethys, Dione, Rhea and Iapetus) Sidera Lodoicea ("the stars of Louis") to honour king Louis XIV. Astronomers fell into the habit of referring to them and Titan as Saturn I through Saturn V. Once Mimas and Enceladus were discovered, in 1789, the numbering scheme was extended to Saturn VII.

The names of all seven satellites of Saturn then known come from John Herschel (son of William Herschel, discoverer of Mimas and Enceladus) in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope ([3]), wherein he suggested the names of the Titans, sisters and brothers of Cronos (the Greek Saturn), be used.

[edit] Physical characteristics

Cassini color image of Rhea, showing the wispy trailing hemisphere

Rhea is an icy body with a density of about 1,233 kg/m³. This low density indicates that it is made of ~ 25% of rocks (density 3,250 kg/m³) and ~75% water ice (density 1000 kg/m³). Earlier it was assumed that Rhea had a rocky core in the center.^[3] However measurements during a close "Cassini" flyby (see below) enabled to determine the moment of inertia which is 0.3911 ± 0.0045 i.e. only slightly less then 0.4.^[4]^[5] Such a value indicates that Rhea has almost homogenious interior (with some compression of ice in the center) while the existence of a rocky core would imply the moment of inertia about 0.34.^[3] The triaxial shape of Rhea is also consistent with a homogeneous body in hydrostatic equilibrium.

Rhean features resemble those of Dione, with dissimilar 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.

Rhea is heavily cratered and has bright wispy markings on its surface. 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 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. On the trailing hemisphere there is a network of bright swaths on a dark background and few visible craters. It has been thought that these bright swaths may be material ejected from ice volcanoes early in Rhea's history when it was still liquid inside. However, recent observations of Dione, which has an even darker trailing hemisphere and similar but more prominent bright streaks, show that the streaks are in fact ice cliffs, and it is plausible to assume that the bright streaks on the Rhean surface are also ice cliffs.

Higher-resolution image of the wispy hemisphere, showing ice cliffs

The January 17, 2006 distant flyby by the Cassini spacecraft yielded images of the wispy hemisphere at better resolution and a lower sun angle than previous observations. While scientific analysis is still pending, raw images from the flyby seem to show that Rhea's streaks in fact are ice cliffs similar to those of Dione.

The Cassini orbiter performed a targeted flyby of Rhea on November 25, 2005 with an altitude of 500 kilometers.

Rhea, from Cassini

[edit] See also

List of geological features on Rhea

[edit] References

^ ^a ^b ^c Jacobson, R. A.; Antreasian, P. G.; Bordi, J. J.; Criddle, K. E.; et.al. (December 2006). "The gravity field of the saturnian system from satellite observations and spacecraft tracking data". The Astronomical Journal 132: 2520-2526.
^ A. Verbiscer et al. (2007). "Enceladus: Cosmic Graffiti Artist Caught in the Act". Science 315: 815.
^ ^a ^b Anderson, J. D.; Rappaport, N. J.; Giampieri, G.; et.al. (2003). "Gravity field and interior structure of Rhea". Physics of the Earth and Planetary Interiors 136: 201-213.
^ Schubert, G.; Anderson, J.D.; Palguta, J.; Travis, B.J. (December 2006). "Internal Structure of Rhea and Enceladus". American Geophysical Union, Fall Meeting 2006, abstract #P31D-06.
^ Anderson, J. D.; Schubert, J. (2007). "Saturn's satellite Rhea is a homogeneous mix of rock and ice". GEOPHYSICAL RESEARCH LETTERS 34: L02202.

[edit] External links

The Planetary Society: Rhea

... | Polydeuces, Dione and Helene | Rhea | Titan | ...

v • d • e Moons of Saturn
Generally listed in increasing distance from Saturn
Ring shepherds	Pan · Daphnis · Atlas · Prometheus · S/2004 S 6? · S/2004 S 4? · S/2004 S 3? · Pandora
Co-orbitals	Epimetheus · Janus
Inner large and Trojan	Mimas · Methone · Pallene · Enceladus · Tethys (trojans Telesto, Calypso) · Dione (trojans Helene, Polydeuces)
Outer large	Rhea · Titan · Hyperion · Iapetus
Inuit group	Kiviuq · Ijiraq · Paaliaq · Siarnaq
Norse group	Phoebe · Skathi · Skoll · S/2004 S 13 · S/2006 S 4 · Hyrokkin · Mundilfari · S/2006 S 6 · S/2006 S 1 · S/2004 S 17 · Narvi · Bergelmir · Aegir · Suttungr · S/2004 S 12 · Bestla · Farbauti · Hati · S/2004 S 7 · Thrymr · S/2006 S 3 · Surtur · Kari · Fenrir · Ymir · Loge · Fornjot
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