Rhea (moon)

Giovanni Cassini

Rhea ( Saturn V) is the second largest moon of the planet Saturn.

Discovery

Rhea was discovered on December 23, 1672 by Giovanni Cassini.

It was named after the moon Titan Rhea, daughter of Uranus and Gaia in Greek mythology. The name " Rhea " and seven other moons of Saturn was proposed by the British astronomer John Herschel in the 1847 publication Results of Astronomical Observations published made ​​at the Cape of Good Hope.

Web properties

Rhea orbits Saturn at an average distance of 527,040 km in 108 hours and 25 minutes. The track has an eccentricity of 0.001 and 0.35 ° relative to the equatorial plane of the Saturn. The moon moves within Saturn's magnetosphere.

Structure and physical properties

Rhea has an average diameter of 1528 km. Their low density of 1.240 g/cm3, suggesting that it is composed to about 2/3 of water ice as well as a core of silicate rock. Your albedo is 0.65, ie 65% of the incident sunlight is reflected. Compared to the moons Tethys and Enceladus, the surface is relatively dark. The temperatures at the surface are -174 ° C in direct sunlight, and -200 to -220 ° C in the shade. Rhea rotates in 108 hours and 25 minutes around its own axis, thus showing how the Earth's moon, a tidal locking on. The rotation axis is inclined relative to the plane of the web to 0,029 ° from the vertical.

Rhea is similar to Saturn 's moon Dione in its composition, the albedo and the structures of their surface. Both moons have different hemispheres. Obviously, the same moons stages of development went through.

Rhea is heavily cratered and has in places, bright structures. Their surface can be divided based on the distribution and size of the crater in two different terrains, one with craters larger than 40 km in diameter and a second, in parts of the polar and equatorial regions, with craters less than 40 km in diameter. This suggests that parts of the surface of Rhea were renewed during their development through geological processes.

The leading hemisphere is heavily cratered and shows no major differences in brightness. As with Jupiter's moon Callisto, the craters on no ringwalls or central mountains, as they are typical of the Earth's moon or the planet Mercury. The thin crust of ice Rhea has given over geological time periods using such structures were leveled. On the trailing hemisphere of Rhea light stripes on a dark surface, as well as some impact craters are visible. The strip originated in an early development phase by Kryovulkanismus ( Kältevulkanismus ), as the interior of the moon was still liquid.

Rhea has an apparent magnitude of 9.7 m and is therefore, seen from Earth, one of the brightest moons of Saturn. To watch them but you need a telescope with a lens aperture of 10 cm.

In the measurement results of the Cassini spacecraft in 2005 a group of astronomers has found evidence of a ring system from dust and up to a meter large rocks around the Saturn moon. The instruments of the probe have in certain vicinities of him a concentration of dust particles and it registered a strong reduction of the electrons from Saturn's magnetosphere. Rhea was thus the first moon with a ring system. As proof, the only thing missing a photo. It is now clear that Rhea has no rings, since neither could be found on photos.

Traces of an atmosphere

When Rhea overflight of the Cassini spacecraft in March 2010 at a distance of only 97 km registered a mass spectrometer on board the probe traces of oxygen and carbon dioxide. This lunar atmosphere has significantly more mass on the day side. Researchers suspect that the oxygen produced when electromagnetic radiation strikes the water ice of the lunar surface and decomposes this. The carbon dioxide may be formed in the oxidation of organic molecules, or originates from the outgassing surface ice.

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