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Rhea is covered with ]s, including several large impact basins such as ]. It also has bright ] on its surface. Its surface can be divided into two geologically different areas based on ] 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. Rhea is covered with ]s, including several large impact basins such as ]. It also has bright ] on its surface. Its surface can be divided into two geologically different areas based on ] 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 ], the craters lack the high relief features seen on the ] and ]. On the trailing hemisphere there is a network of bright swaths on a dark background and few visible craters. It had been thought that these bright areas might be material ejected from ice ]es early in Rhea's history when its interior was still liquid. 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 resulting from extensive fracturing of the moon's surface. It is plausible to assume that the bright streaks on the Rhean surface are also tectonically formed ice cliffs. The leading hemisphere is heavily cratered and uniformly bright. As on ], the craters lack the high relief features seen on the ] and ]. On the trailing hemisphere there is a network of bright swaths on a dark background and few visible craters. It had been thought that these bright areas might be material ejected from ice ]es early in Rhea's history when its interior was still liquid. However, recent observations of Dione, which has an even darker trailing hemisphere and similar but more i love feta cdheese ven though it makes ur breathe smelly prominent bright streaks of lavender gold and lilly lure, show that the streaks are in fact ice cliffs resulting from extensive fracturing of the moon's surface. It is plausible to assume that the bright streaks on the Rhean surface are also tectonically formed ice cliffs.


The January 17, 2006 distant flyby by the ] 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 January 17, 2006 distant flyby by the ] 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.

Revision as of 17:32, 23 November 2009

Template:Distinguish2

Rhea
Cassini view of Rhea showing two adjacent impact basins
Discovery
Discovered byG. D. Cassini
Discovery dateDecember 23, 1672
Designations
Alternative namesSaturn V
AdjectivesRhean
Orbital characteristics
Semi-major axis527 108 km
Eccentricity0.001 258 3
Orbital period (sidereal)4.518 212 d
Inclination0.345° (to Saturn's equator)
Satellite ofSaturn
Physical characteristics
Dimensions1535.2 × 1525 × 1526.4 km
Mean radius764.30 ± 1.10 km
Surface area7 337 000 km²
Mass(2.306 518 ± 0.000 353)×10 kg (~3.9×10 Earths)
Mean density1.233 3 ± 0.005 3 g/cm³
Surface gravity0.264 m/s²
Escape velocity0.635 km/s
Synodic rotation period4.518 212 d
(synchronous)
Axial tiltzero
Albedo0.949 ± 0.003 (geometric)
Surface temp. min mean max
Kelvin 53 K   99 K
Apparent magnitude10
Giovanni Domenico Cassini, discoverer of Rhea

Rhea (Template:Pron-en, or as Greek Template:Polytonic) 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.

Name

Rhea is named after the Titan Rhea of Greek mythology, "mother of the gods". 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 the planet Uranus, and two other Saturnian moons, 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 (Saturn, in Roman mythology), be used.

Physical characteristics

Rhea is an icy body with a density of about 1.233 g/cm³. This low density indicates that it is made of ~25% rocks (density ~3.25 g/cm³) and ~75% water ice (density ~0.93 g/cm³). While Rhea is the ninth largest moon, it is only the tenth most massive moon. Earlier it was assumed that Rhea had a rocky core in the center. However measurements taken during a close flyby by the Cassini orbiter (see below) determined the axial moment of inertia coefficient as 0.4 kg·m². 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. The triaxial shape of Rhea is also consistent with a homogeneous body in hydrostatic equilibrium.

Rhea's 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 covered with craters, including several large impact basins such as Tirawa. It also 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 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. On the trailing hemisphere there is a network of bright swaths on a dark background and few visible craters. It had been thought that these bright areas might be material ejected from ice volcanoes early in Rhea's history when its interior was still liquid. However, recent observations of Dione, which has an even darker trailing hemisphere and similar but more i love feta cdheese ven though it makes ur breathe smelly prominent bright streaks of lavender gold and lilly lure, show that the streaks are in fact ice cliffs resulting from extensive fracturing of the moon's surface. It is plausible to assume that the bright streaks on the Rhean surface are also tectonically formed 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.

Possible ring system

Main article: Rings of Rhea

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. 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.

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.

Gallery

  • Cassini color image of Rhea, showing the wispy trailing hemisphere Cassini color image of Rhea, showing the wispy trailing hemisphere
  • Higher-resolution image of the wispy hemisphere, showing ice cliffs Higher-resolution image of the wispy hemisphere, showing ice cliffs
  • An artist impression of Rhea's rings An artist impression of Rhea's rings
  • Composite image map of Rhea's surface Composite image map of Rhea's surface

See also

References

  1. http://cfa-www.harvard.edu/iau/NatSats/NaturalSatellites.html Cfa-www.harvard.edu
  2. 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)
  3. ^ Jacobson, R. A. (2006). "The Gravity Field of the Saturnian System from Satellite Observations and Spacecraft Tracking Data". The Astronomical Journal. 132: 2520–2526. doi:10.1086/508812. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
  4. Verbiscer, A.; et al.; Enceladus: Cosmic Graffiti Artist Caught in the Act, Science, Vol. 315 (2007), p. 815 (supporting online material, table S1)
  5. "Classic Satellites of the Solar System". Observatorio ARVAL. Retrieved 2007-09-28.
  6. In US dictionary transcription, Template:USdict.
  7. As reported by William Lassell, Monthly Notices of the Royal Astronomical Society, Vol. 8, No. 3, pp. 42–43 (January 14, 1848)
  8. The moons more massive than Rhea are: Earth's 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.
  9. ^ Anderson, J. D. (2003). "Gravity field and interior structure of Rhea". Physics of the Earth and Planetary Interiors. 136: 201–213. doi:10.1016/S0031-9201(03)00035-9. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  10. More precisely, 0.3911 ± 0.0045 kg·m².
    Schubert, G. (2006). "Internal Structure of Rhea and Enceladus". American Geophysical Union, Fall Meeting 2006, abstract #P31D-06. 319: 1380. doi:10.1126/science.1151524. PMID 18323452. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
  11. Anderson, J. D. (2007). "Saturn's satellite Rhea is a homogeneous mix of rock and ice". Geophysical Research Letters. 34: L02202. doi:10.1029/2006GL028100. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  12. http://www.nasa.gov/mission_pages/cassini/media/rhea20080306.html NASA - Saturn's Moon Rhea Also May Have Rings
  13. Jones, G. H. (2008-03-07). "The Dust Halo of Saturn's Largest Icy Moon, Rhea". Science. 319 (5868). AAAS: 1380–1384. doi:10.1126/science.1151524. PMID 18323452. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  14. Lakdawalla, E. (2008-03-06). "A Ringed Moon of Saturn? Cassini Discovers Possible Rings at Rhea". The Planetary Society web site. Planetary Society. Retrieved 2008-03-09. {{cite web}}: External link in |work= (help)

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