Rhea (moon)

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Rhea color.jpg
True-color image of Rhea taken by Voyager 1
Date of discovery December 23, 1672[1]
Name of discoverer Giovanni Domenico Cassini[1]
Name origin Titaness and mother of Zeus by Kronos
Orbital characteristics
Primary Saturn
Order from primary 19
Perikrone 526,513 km[2]
Apokrone 527,567 km[2]
Semi-major axis 527,040 km[3]
Orbital eccentricity 0.0010[3]
Sidereal month 4.517500 da[3]
Inclination 0.35°[3] to Saturn's equator
Rotational characteristics
Sidereal day 4.517500 da[3]
Rotational speed 12.3 m/s[2]
Physical characteristics
Mass 2.309 * 1021 kg[3]
Density 1,230 kg/m³[3]
Mean radius 764.5 km[3]
Surface gravity 0.2634 m/s²[2]
Escape speed 0.6346 km/s[2]
Surface area 7,344,544 km²[2]
Minimum temperature 63 K[4]
Mean temperature 76 K[5]
Maximum temperature 99 K[4]
Composition Water ice and rock mixture[4]
Color Pink-gray
Albedo 0.7[3]
Rhea, or Saturn V, is the second largest moon of Saturn (and the largest moon without an atmosphere) and the ninth most massive moon in all the solar system. Recent evidence from the Cassini-Huygens Mission indicates that Rhea, unique among natural satellites, might have a ring system.

Discovery and naming

Giovanni Domenico Cassini discovered Rhea, along with the moons Tethys, Dione, and Iapetus, in the latter half of the seventeenth century, during the reign of "Sun-King" Louis XIV of France.[1] Cassini named these moons the "Sidera Lodoicea" in honor of the king. Later generations (especially after the French Revolution) would not retain such regal flattery.[6]

Sir John Herschel, son of the astronomer William Herschel, suggested the current names of the seven largest satellites of Saturn, including Rhea. Titan received a generic name, and the other six received names of the Titans of mythology. Rhea was the wife of Kronos or Cronus and mother of Zeus, the eventual king of the gods.[7]

Orbital and rotational characteristics

Rhea is in a relatively circular orbit around Saturn, at a mean distance of 527,040 km. It makes one orbit around Saturn in 4.52 days. Rhea is in tidal lock with Saturn and hence its rotation is synchronous with its orbit.

Physical characteristics

Rhea is a low-density body consisting mainly of water ice.


Rhea trailing.jpg
Rhea has multiple craters on its surface. These craters, like those of Callisto, are almost flat, and their walls and central mountains are of relatively low altitude.

The trailing hemisphere of Rhea has multiple wisp-like formations that might be mountain ranges.


The density of Rhea at first led astronomers to believe that Rhea had a rocky core consisting of one-third the total mass of the body, and a mantle of water ice.[8] But recent evidence suggests an undifferentiated interior. Specifically, the moment of inertia for Rhea is 0.3911 ± 0.0045 kg m², which suggests that Rhea's rock and ice are evenly mixed, with compression of the ice from Ice I to Ice II toward the core.[9]

Ring system

Artist's conception of the rings of Rhea
NASA announced on March 6, 2008 their tentative hypothesis that Rhea has its own ring system. In its first rendezvous with Rhea, the Cassini orbiter's instruments, including its Magnetospheric Imaging Instrument (MIMI) detected brief interruptions in the normal flow of electrons from Saturn's magnetic field, before the orbiter passed into Rhea's electronic shadow. The same interruptions, faithfully in reverse, occurred on Rhea's far side. These data, and the previously unexplained depletion of high-energy electrons downstream from Rhea that Voyager 1 had found in 1980, strongly suggest that Rhea has rings.[10][11][12]

Observation and Exploration

The first spacecraft to explore Rhea was Voyager 1. The Cassini orbiter has made two close rendezvous with Rhea during its first four years of operation, on November 26, 2005, and August 30, 2007. Mission planners have fixed a date for another rendezvous with Rhea on March 2, 2010, during the two-year extended mission.


  1. 1.0 1.1 1.2 "Gazetteer of Planetary Nomenclature: Planetary Body Names and Discoverers." US Geological Survey, Jennifer Blue, ed. March 31, 2008. Accessed April 17, 2008.
  2. 2.0 2.1 2.2 2.3 2.4 2.5 Calculated
  3. 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 Williams, David R. "Saturnian Satellite Fact Sheet." National Space Science Data Center, NASA, November 23, 2007. Accessed June 4, 2008.
  4. 4.0 4.1 4.2 Harvey, Samantha. "Entry for Rhea." Solar System Exploration, NASA, October 6, 2003. Accessed June 4, 2008.
  5. "Entry for Rhea." Sea and Sky Tour of the Solar System. Accessed June 4, 2008.
  6. Boulay, J. C. "Saturne: les satellites." Astronomie-astronautique. Accessed June 5, 2008.
  7. Lassell, William. "Satellites of Saturn." Monthly Notices of the Royal Astronomical Society, 8(3):42-43, January 14, 1848. Accessed June 4, 2008.
  8. Anderson, J. D., Rappaport, N. J.; Giampieri, G.; et. al. "Gravity field and interior structure of Rhea" Physics of the Earth and Planetary Interiors, 2003; volume 136, 201-213
  9. Anderson, J. D., and Schubert, G. "Saturn's satellite Rhea is a homogeneous mix of rock and ice." Geophysical Research Letters 34(L02202), January 18, 2007. doi:10.1029/2006GL028100 Accessed June 5, 2008.
  10. Martinez, C., and Brown, D. "Saturn's Moon Rhea Also May Have Rings." Cassini mission pages, NASA, March 6, 2008. Accessed June 5, 2008.
  11. Jones, G. H., Roussos, E., Krupp, N., et al. "The Dust Halo of Saturn's Largest Icy Moon, Rhea." Science 319(5868):1380-1384, March 7, 2008. doi:10.1126/science.1151524 Accessed June 5, 2008.
  12. Lakdawalla, Emily. "A Ringed Moon of Saturn? Cassini Discovers Possible Rings at Rhea." The Planetary Society, March 6, 2008. Accessed June 5, 2008.