Rhea (moon)

Not to be confused with the asteroid named 577 Rhea.
Rhea
PIA07763 Rhea full globe5.jpg
Cassini view of Rhea's anti-Saturn hemisphere, showing the moon's two largest impact basins (Mamaldi at lower left, and Tirawa adjacent to it at upper right)
Discovery
Discovered by G. D. Cassini
Discovery date December 23, 1672
Designations
Alternate name(s) Saturn V
Adjective Rhean
Semi-major axis 527 108 km
Eccentricity 0.001 258 3
Orbital period 4.518 212 d
Inclination 0.345° (to Saturn's equator)
Satellite of Saturn
Physical characteristics
Dimensions 1535.2 × 1525 × 1526.4 km[2]
Mean radius 764.30 ± 1.10 km[3]
Surface area 7 337 000 km2
Mass (2.306 518 ± 0.000 353) × 1021 kg[3] (~3.9 × 10−4 Earths)
Mean density 1.233 3 ± 0.005 3 g/cm3[3]
Equatorial surface gravity 0.264 m/s2
Escape velocity 0.635 km/s
Rotation period 4.518 212 d
(synchronous)
Axial tilt zero
Albedo 0.949 ± 0.003 (geometric)[4]
Surface temp.
   Kelvin
min mean max
53 K   99 K
Apparent magnitude 10 [5]
Giovanni Domenico Cassini, discoverer of Rhea

Rhea (pronounced /ˈriːə/,[6] 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.

Contents

Name

Rhea is named after the Titan Rhea of Greek mythology, "mother of the gods". It is also designated Saturn V (being the fifth major moon going outward from the planet).

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 Cronian 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.[7]

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% rock (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.[8] Earlier it was assumed that Rhea had a rocky core in the center.[9] 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.[10][11] 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.[9] 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 has rather a typical heavily cratered surface,[12] with the exceptions of a few large Dione-type fractures (whispy terrain) on the trailing hemisphere(the side facing away from the direction of motion along Rhea's orbit)[13] and a very faint "line" of material at Rhea's equator that may have been deposited by material deorbiting from its rings.[14] Rhea has two very large impact basins on its anti-Cronian hemisphere, which are about 400 and 500 km across.[13] The more northerly and less degraded of the two, called Tirawa, is roughly comparable to the basin Odysseus on Tethys.[12] There is a 48 km-diameter impact crater at 112°W that is prominent because of an extended system of bright rays.[13] This crater, Inktomi, is nick-named "The Splat", and may be one of the youngest craters on the inner moons of Saturn.[13] No evidence of any endogenic activity has been discovered.[13]

Size comparison of Earth, Moon and Rhea

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 prominent bright streaks, show that the streaks are in fact ice cliffs resulting from extensive fracturing of the moon's surface. It is plausible 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

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 was inferred by observed changes in the flow of electrons trapped by Saturn's magnetic field as Cassini passed by Rhea.[15][16][17] Dust and debris could to 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).[18] However, when Cassini made targeted observations of the putative ring plane from several angles, no evidence of ring material was found, suggesting that another explanation for the earlier observations is needed.[19][20]

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, with another flyby at a distance of 5,750 km on August 30, 2007. During the extended mission there was a second targeted encounter of 100 km pass on March 2, 2010. The viewing angle of that flyby was not good for attempting to spot the rings, something that is planned for a later non-targeted flyby at a better angle.

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

See also

References

  1. ↑ Natural Satellites Ephemeris Service 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. ↑ 3.0 3.1 3.2 Jacobson, R. A.; Antreasian, P. G.; Bordi, J. J.; Criddle, K. E.; et al. (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. 
  4. ↑ Verbiscer, A.; French, R.; Showalter, M.; Helfenstein, P. (2007). "Enceladus: Cosmic Graffiti Artist Caught in the Act". Science 315 (5813): 815. doi:10.1126/science.1134681. PMID 17289992.  p. 815 (supporting online material, table S1)
  5. ↑ "Classic Satellites of the Solar System". Observatorio ARVAL. http://www.oarval.org/ClasSaten.htm. Retrieved 2007-09-28. 
  6. ↑ In US dictionary transcription, us dict: rē′·ə.
  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. ↑ 9.0 9.1 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. doi:10.1016/S0031-9201(03)00035-9. 
  10. ↑ More precisely, 0.3911 ± 0.0045 kg·m2.
    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-06 319 (5868): 1380. doi:10.1126/science.1151524. PMID 18323452. 
  11. ↑ Anderson, J. D.; Schubert, J. (2007). "Saturn's satellite Rhea is a homogeneous mix of rock and ice". Geophysical Research Letters 34: L02202. doi:10.1029/2006GL028100. 
  12. ↑ 12.0 12.1 Moore, Jeffrey M.; Schenk, Paul M.; Bruesch, Lindsey S. et.al. (2004). "Large impact features on middle-sized icy satellites" (pdf). Icarus 171: 421–43. doi:10.1016/j.icarus.2004.05.009. http://lasp.colorado.edu/~espoclass/homework/5830_2008_homework/Ch17.pdf. 
  13. ↑ 13.0 13.1 13.2 13.3 13.4 Wagner, R.J.; Neukum, G.; Giese, B. et al. (2008). "Geology of Saturn's Satellite Rhea on the Basis of the High-Resolution Images from the Targeted Flyby 049 on Aug. 30, 2007". Lunar and Planetary Science XXXIX: 1930. http://adsabs.harvard.edu/abs/2008LPI....39.1930W. 
  14. ↑ Schenk, Paul M.; McKinnon, W. B. (2009). "Global Color Variations on Saturn's Icy Satellites, and New Evidence for Rhea's Ring". American Astronomical Society, DPS meeting #41, #3.03. http://adsabs.harvard.edu/abs/2009DPS....41.0303S. Retrieved January 6, 2010. 
  15. ↑ Saturn's Moon Rhea Also May Have Rings, NASA, June 3, 2006
  16. ↑ Jones, G. H.; et al. (2008-03-07). "The Dust Halo of Saturn's Largest Icy Moon, Rhea". Science (AAAS) 319 (5868): 1380–1384. doi:10.1126/science.1151524. PMID 18323452. http://www.sciencemag.org/cgi/content/short/319/5868/1380. 
  17. ↑ Lakdawalla, E. (2008-03-06). "A Ringed Moon of Saturn? Cassini Discovers Possible Rings at Rhea". The Planetary Society web site. Planetary Society. http://planetary.org/news/2008/0306_A_Ringed_Moon_of_Saturn_Cassini.html. Retrieved 2008-03-09. 
  18. ↑ Lakdawalla, E. (5 October 2009). "Another possible piece of evidence for a Rhea ring". The Planetary Society Blog. Planetary Society. http://planetary.org/blog/article/00002137/. Retrieved 2009-10-06. 
  19. ↑ Matthew S. Tiscareno, Joseph A. Burns, Jeffrey N. Cuzzi, Matthew M. Hedman (2010). "Cassini imaging search rules out rings around Rhea". Geophysical Research Letters 37 (14): L14205. doi:10.1029/2010GL043663. Bibcode: 2010GeoRL..3714205T. 
  20. ↑ Kerr, Richard A. (2010-06-25). "The Moon Rings That Never Were". ScienceNow. http://news.sciencemag.org/sciencenow/2010/06/the-moon-rings-that-never-were.html. Retrieved 2010-08-05. 

External links

Moons of Saturn
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