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Rhea (moon)
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:''There is also an
asteroid named
577 Rhea.''
{| style="margin-left: 1em; float: right; border: 1px solid #CCC;"
|+ '''Rhea'''
|-
| colspan="2" bgcolor="#080000" align="center" |
Image:Rhea hi-res PIA07763.jpg 270px|Rhea, taken by Cassini (NASA). The large crater near the top of the image is Tirawa.
|-
! bgcolor="#a0ffa0" colspan="2" | Discovery
|-
! align="left" | Discovered by
|
Giovanni Domenico Cassini
|-
! align="left" | Discovered in
|
December 23,
1672
|-
! bgcolor="#a0ffa0" colspan="2" |
Orbital characteristics
[http://cfa-www.harvard.edu/iau/NatSats/NaturalSatellites.html]
|-
! align="left" |
Semimajor axis
| 527,108
kilometre km
|-
! align="left" |
Eccentricity (orbit) Eccentricity
| 0.0012583
|-
! align="left" | Revolution period
| 4.518212 d
|-
! align="left" |
Inclination
| 0.345° (to Saturn's equator)
|-
! align="left" | Is a
natural satellite satellite of
|
Saturn (planet) Saturn
|-
! bgcolor="#a0ffa0" colspan="2" | Physical characteristics
|-
! align="left" | Mean
diameter
| 1528 km
|-
! align="left" | Surface
area
| 7,300,000
square kilometre km2
|-
! align="left" |
Mass
| 2.3166{{e|21}}
kilogram kg
|-
! align="left" | Mean
density
| 1.24 g/cm
3
|-
! align="left" | Surface
gravity
| 0.27
Acceleration m/s2
|-
! align="left" |
Escape velocity
| 0.64 km/s
|-
! align="left" | Rotation period
| 4.518212 d
(
Synchronous rotation synchronous)
|-
! align="left" |
Axial tilt
| zero
|-
! align="left" |
Albedo
| 0.65
|-
! align="left" | Surface
Temperature temp.
|
{| cellspacing="0" cellpadding="2" border="0"
|-
! min !! mean !! max
|-
| 53
Kelvin K
| || 99 K
|}
|-
! align="left" |
celestial body's atmosphere Atmosphere
| none
|}
'''Rhea''' ''(ree'-a,''
IPA {{IPA.html">natural satellite
moon of
Saturn (planet) Saturn and was discovered in
1672 by
Giovanni Domenico Cassini.
Name
Rhea is named after the
titan (mythology) titan Rhea (mythology) Rhea of
Greek mythology. It is also designated '''Saturn V'''.
Cassini named the four moons he discovered (
Tethys (moon) Tethys,
Dione (moon) Dione, Rhea and
Iapetus (moon) Iapetus) ''Lodicea Sidera'' ("the stars of Louis") to honour king
Louis XIV of France Louis XIV. Astronomers fell into the habit of referring to them and
Titan (moon) Titan as ''Saturn I'' through ''Saturn V''. Once
Mimas (moon) Mimas and
Enceladus (moon) 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'' ([http://adsabs.harvard.edu//full/seri/MNRAS/0008//0000042.000.html]), wherein he suggested the names of the
Titans, sisters and brothers of Cronos (the Greek Saturn), be used.
Physical characteristics
Image:Rhea true color.jpg thumb|left|250px|''Cassini'' color image of Rhea, showing the wispy trailing hemisphereRhea is an icy body with a density of about 1,240 kg/m
3. This low density indicates that it has a rocky core taking up less than one-third of the moon's mass with the rest composed of water-ice. Rhean features resemble those of
Dione (moon) 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
Impact crater 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 (moon) Callisto, the craters lack the high relief features seen on the
Moon and
Mercury (planet) 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.
Image:Rhea ice cliffs.jpg thumb|left|250px|Higher-resolution image of the wispy hemisphere, showing ice cliffsThe
January 17,
2006 distant flyby by the
Cassini-Huygens 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.
'''See also:'''
*
List of geological features on Rhea
The ''Cassini'' orbiter performed a targeted flyby of Rhea on
November 25,
2005 with an altitude of 500
kilometers.
External links
-
The Planetary Society: Rhea
{{Saturn Full Footer}}
Category:Saturn's moons
bg:РеÑ? (Ñ?пътник)
ca:Rea (satèl·lit)
da:Rhea (måne)
de:Rhea (Mond)
el:ΡÎα (δοÏ?υφόÏ?ος)
es:Rea (luna)
fr:Rhéa (lune)
hr:Reja (mjesec)
it:Rea (astronomia)
he:רי×?×” (ירח)
la:Rhea (satelles)
nl:Rhea (maan)
ja:レア (衛星)
no:Rhea
nn:Saturnmånen Rhea
pl:Rea (księżyc)
pt:Reia (satélite)
ru:РеÑ? (Ñ?путник)
sk:Rhea (mesiac)
fi:Rhea (kuu)
sv:Rhea (måne)
zh:土�五
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