Rhea (moon)

Infobox Planet
name = Rhea
alt_names = Saturn V
adjectives = Rhean


caption = "Cassini" view of Rhea showing two adjacent impact basins
bgcolour = #a0ffa0
discovery = yes
discoverer = G. D. Cassini
discovered = December 23, 1672
orbit_ref = [ [http://cfa-www.harvard.edu/iau/NatSats/NaturalSatellites.html http://cfa-www.harvard.edu/iau/NatSats/NaturalSatellites.html] "Cfa-www.harvard.edu" ]
semimajor = 527 108 km
eccentricity = 0.001 258 3
period = 4.518 212 d
inclination = 0.345° (to Saturn's equator)
satellite_of = Saturn
physical_characteristics = yes
mean_radius = 764.30 ± 1.10 kmcite journal| last=Jacobson| first=R. A.| coauthors=Antreasian, P. G.; Bordi, J. J.; Criddle, K. E.; et al.| title=The Gravity Field of the Saturnian System from Satellite Observations and Spacecraft Tracking Data| journal=The Astronomical Journal| month=December| year=2006| volume=132| pages=2520–2526| doi=10.1086/508812]
dimensions = 1535.2 × 1525 × 1526.4 km [Thomas, P. C.;Veverka, J.; Helfenstein, P.; Porco, C.; Burns, J. A.; Denk, T.; Turtle, E.; Jacobson, R. A.; and the ISS Science team; [http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1639.pdf "Shapes of the Saturnian Icy Satellites"] , Lunar and Planetary Science XXXVII (2006)]
surface_area = 7 337 000 km²
mass = (2.306 518 ± 0.000 353)e|21 kg (~3.9e|-4 Earths)
density = 1.233 3 ± 0.005 3 g/cm³
surface_grav = 0.264 m/s²
escape_velocity = 0.635 km/s
rotation = 4.518 212 d (synchronous)
axial_tilt = zero
albedo = 0.949 ± 0.003 (geometric)Verbiscer, A.; "et al."; [http://www.sciencemag.org/cgi/content/abstract/315/5813/815 "Enceladus: Cosmic Graffiti Artist Caught in the Act"] , Science, Vol. 315 (2007), p. 815 (supporting online material, table S1)]
magnitude = 10 cite web|title=Classic Satellites of the Solar System|url=http://www.oarval.org/ClasSaten.htm|publisher=Observatorio ARVAL|accessdate=2007-09-28]
temperatures = yes
temp_name1 = Kelvin
min_temp_1 = 53 K
mean_temp_1 =
max_temp_1 = 99 K

Rhea (pronEng|ˈriːə respell|REE|ə, or as in Greek "Polytonic|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.

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 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. [As reported by William Lassell, [http://adsabs.harvard.edu//full/seri/MNRAS/0008//0000042.000.html Monthly Notices of the Royal Astronomical Society, Vol. 8, No. 3, pp. 42–43] (1848 January 14)]

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 3.250 g/cm³) and ~75% water ice (density 1.000 g/cm³). Earlier it was assumed that Rhea had a rocky core in the center.cite journal| last=Anderson| first=J. D.| coauthors=Rappaport, N. J.; Giampieri, G.; et.al.| title=Gravity field and interior structure of Rhea| journal=Physics of the Earth and Planetary Interiors| year=2003| volume=136| pages=201–213| doi=10.1016/S0031-9201(03)00035-9] 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.More precisely, 0.3911 ± 0.0045 kg·m².
cite journal| last=Schubert| first=G.| coauthors=Anderson, J. D.; Palguta, J.; Travis, B. J.| title=Internal Structure of Rhea and Enceladus| journal=American Geophysical Union, Fall Meeting 2006, abstract #P31D-06| month=December| year=2006| doi=10.1126/science.1151524| pmid=18323452| volume=319| pages=1380] cite journal| last=Anderson| first=J. D.| coauthors=Schubert, J.| title=Saturn's satellite Rhea is a homogeneous mix of rock and ice| journal=Geophysical Research Letters| year=2007| volume=34| pages=L02202| doi=10.1029/2006GL028100] 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 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 had been thought that these bright areas may 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, 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.

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. [http://www.nasa.gov/mission_pages/cassini/media/rhea20080306.html NASA - Saturn's Moon Rhea Also May Have Rings] [cite journal
last = Jones
first = G. H.
authorlink =
coauthors = "et al."
title = The Dust Halo of Saturn's Largest Icy Moon, Rhea
journal = Science
volume = 319
issue = 5868
pages = 1380–1384
publisher = AAAS
location =
date = 2008-03-07
url = http://www.sciencemag.org/cgi/content/short/319/5868/1380
doi = 10.1126/science.1151524
id =
pmid = 18323452 ] cite web
last = Lakdawalla
first = E.
title = A Ringed Moon of Saturn? "Cassini" Discovers Possible Rings at Rhea
work = [http://www.planetary.org/home/ The Planetary Society web site]
publisher = Planetary Society
date = 2008-03-06
url = http://planetary.org/news/2008/0306_A_Ringed_Moon_of_Saturn_Cassini.html
accessdate = 2008-03-09] 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

ee also

* List of geological features on Rhea

References

External links

* [http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sat_Rhea Rhea Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
* [http://www.planetary.org/explore/topics/saturn/rhea.html The Planetary Society: Rhea]