Rhea ( , or as
Greek
) 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.More precisely,
0.3911 ± 0.0045 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 crater, 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 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 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. 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
Image:Rhea true color.jpg|Cassini color image of
Rhea, showing the wispy trailing hemisphereImage:Rhea ice
cliffs.jpg|Higher-resolution image of the wispy hemisphere, showing
ice cliffsImage:Rhean rings PIA10246 Full res.jpg|An artist
impression of Rhea's ringsImage:Rhea 2006 PIA08343.jpg|Composite
image map of Rhea's surface
See also
References
- In US dictionary transcription, .
- As reported by William Lassell, Monthly Notices of the Royal Astronomical Society,
Vol. 8, No. 3, pp. 42–43 (January 14, 1848)
- 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.
-
http://www.nasa.gov/mission_pages/cassini/media/rhea20080306.html
NASA - Saturn's Moon Rhea Also May Have Rings
External links