Rhea (moon)
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Cassini view of Rhea showing two adjacent impact basins
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Discovery
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| Discovered by | G. D. Cassini | ||||||
| Discovery date | December 23, 1672 | ||||||
| Semi-major axis | 527,108 km | ||||||
| Eccentricity | 0.0012583 | ||||||
| Orbital period | 4.518212 d | ||||||
| Inclination | 0.345° (to Saturn's equator) | ||||||
| Satellite of | Saturn | ||||||
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Physical characteristics
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| Dimensions | 1535.2 × 1525 × 1526.4 km[2] | ||||||
| Mean radius | 764.30 ± 1.10 km[3] | ||||||
| Surface area | 7,337,000 km² | ||||||
| Mass | 2.306518±0.000353×1021 kg[3] (~3.9×10-4 Earths) | ||||||
| Mean density | 1.2333 ± 0.0053 g/cm³[3] | ||||||
| Equatorial surface gravity | 0.264 m/s2 | ||||||
| Escape velocity | 0.635 km/s | ||||||
| Rotation period | 4.518212 d (synchronous) |
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| Axial tilt | zero | ||||||
| Albedo | 0.949 ± 0.003 (geometric)[4] | ||||||
| Surface temp. Kelvin |
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| Apparent magnitude | 10 [5] | ||||||
| Adjectives | Rhean | ||||||
Rhea (pronounced /ˈriːə/ REE-ə, or as Greek Ῥέᾱ) is the second-largest moon of Saturn and the ninth largest moon in the Solar System. It was discovered in 1672 by Giovanni Domenico Cassini.
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[edit] Name
Rhea is named after the Titan Rhea of Greek mythology. It is also designated Saturn V.
Cassini named the four moons he discovered (Tethys, Dione, Rhea and Iapetus) Sidera Lodoicea (the stars of Louis) to honor 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, wherein he suggested the names of the Titans, sisters and brothers of Cronos (in Roman = Saturn), be used.[6]
[edit] Physical characteristics
Rhea is an icy body with a density of about 1,233 kg/m3. This low density indicates that it is made of ~25% 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.[7] However measurements taken during a close flyby by the Cassini orbiter (see below) determined the axial moment of inertia coefficient as 0.3911 ± 0.0045 kg m2 i.e. only slightly less than 0.4 kg m2.[8][9] Such a value indicates that Rhea has 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.[7] The triaxial shape of Rhea is also consistent with a homogeneous body in hydrostatic equilibrium.
Rhea 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.
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.
[edit] Possible ring system
On March 6, 2008, NASA announced that Rhea may have a tenuous ring system. This would mark the first discovery of rings about a moon. The rings' existence is inferred by observed changes in the flow of electrons trapped by Saturn's magnetic field as Cassini passed by Rhea.[10][11][12] Dust and debris appear to extend out to Rhea's Hill sphere, but are denser nearer the moon, and contain three narrow rings of yet higher density.
[edit] Exploration
Rhea has been imaged several times from moderate distances by the Cassini orbiter. There was one close targeted fly-by during the primary mission, at a distance of 500 km on November 26, 2005. An additional close flyby at a distance of 5,750 km was performed on August 30, 2007. One targeted encounter is planned for Cassini's two-year extended mission, a 100 km pass on March 2, 2010.
[edit] Gallery
 Cassini color image of Rhea, showing the wispy trailing hemisphere |
 Higher-resolution image of the wispy hemisphere, showing ice cliffs |
 An artist impression of Rhea's rings |
 Composite image map of Rhea's surface |
[edit] See also
[edit] References
- ^ http://cfa-www.harvard.edu/iau/NatSats/NaturalSatellites.html Cfa-www.harvard.edu
- ^ Thomas, P. C.;Veverka, J.; Helfenstein, P.; Porco, C.; Burns, J. A.; Denk, T.; Turtle, E.; Jacobson, R. A.; and the ISS Science team; Shapes of the Saturnian Icy Satellites, Lunar and Planetary Science XXXVII (2006)
- ^ 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.
- ^ Verbiscer, A.; et al.; Enceladus: Cosmic Graffiti Artist Caught in the Act, Science, Vol. 315 (2007), p. 815 (supporting online material, table S1)
- ^ Classic Satellites of the Solar System. Observatorio ARVAL. Retrieved on 2007-09-28.
- ^ As reported by William Lassell, Monthly Notices of the Royal Astronomical Society, Vol. 8, No. 3, pp. 42–43 (1848 January 14)
- ^ 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.
- ^ http://www.nasa.gov/mission_pages/cassini/media/rhea20080306.html NASA - Saturn's Moon Rhea Also May Have Rings
- ^ Jones, G. H.; et al. (2008-03-07). "The Dust Halo of Saturn's Largest Icy Moon, Rhea". Science 319 (5868): 1380-1384. AAAS. doi:.
- ^ Lakdawalla, E. (2008-03-06). A Ringed Moon of Saturn? Cassini Discovers Possible Rings at Rhea. The Planetary Society web site. Planetary Society. Retrieved on 2008-03-09.
[edit] External links
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