Rhea (moon)

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Rhea
Rhea hi-res PIA07763.jpg
Saturn's moon Rhea, photographed by the Cassini spacecraft
Central body Saturn
Properties of the orbit
Major semi-axis 527,040 km
Periapsis 526,510 km
Apoapsis 527,570 km
eccentricity 0.001
Orbit inclination 0.35 °
Orbital time 4,518 d
Mean orbital velocity 8.48 km / s
Physical Properties
Albedo 0.65
Apparent brightness 9.6 likes
Medium diameter (1528.6 ± 2.2)
1532.4 × 1525.6 × 1524.4 km
Dimensions 2.3166 · 10 21 kg
surface 7,300,000 km 2
Medium density (1.233 ± 0.005) g / cm 3
Sidereal rotation 4.518 days
Axis inclination 0.029 °
Acceleration of gravity on the surface 0.26 m / s 2
Escape speed 636 m / s
Surface temperature 73 K
discovery
Explorer

Giovanni Domenico Cassini

Date of discovery December 23, 1672
Remarks Simply bound rotation, extremely thin atmosphere of oxygen and carbon dioxide
Rhea Earth Moon Comparison.png
Size comparison between Rhea (bottom left), Earth's moon (top left) and Earth (true-to-scale photomontage)

Rhea (also Saturn V ) is the second largest moon on the planet Saturn . She is an ice moon .

discovery

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

The moon was named after the titan Rhea , the daughter of Uranos and Gaia from Greek mythology . The name "Rhea" and another seven moons of Saturn was suggested by the British astronomer John Herschel in the 1847 publication Results of Astronomical Observations made at the Cape of Good Hope .

Track properties

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

Structure and physical properties

Crater on Rhea photographed from Voyager 1

Rhea has a mean diameter of 1,528 km. Its low density of 1.240 g / cm 3 suggests that around 2/3 of it is composed of water ice and a core of silicate rock. Their albedo is 0.65, which means that 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 between −200 and −220 ° C in the shade. Rhea rotates around its own axis in 108 hours and 25 minutes and thus, like the Earth's moon, exhibits a bound rotation . The axis of rotation is inclined from the vertical by 0.029 ° in relation to the plane of the orbit.

Rhea resembles the Saturn moon Dione in its composition, the albedo and the structures of its surface . Both moons have different hemispheres . Obviously the moons went through the same stages of development.

Rhea is heavily cratered and has light structures in places. Based on the distribution and size of the craters, their surface can be divided into two different terrains, one with craters over 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 Rhea's surface were renewed by geological processes during its development. By far the largest named crater called Mamaldi has a diameter of 480 km and is located on the central meridian of the hemisphere facing away from Saturn. The southern part of its barely noticeable edge extends over the equator.

The leading hemisphere is heavily cratered and does not show any major differences in brightness. As with Jupiter's moon Callisto , the impact craters do not have any ring walls or central mountains, as are typical for the Earth's moon or the planet Mercury . The thin ice crust of Rhea has given way over geological time and such structures have been leveled. On the following hemisphere of Rhea, light streaks are visible on a dark surface as well as some impact craters. The stripes were formed in an early development phase by cryovolcanism (cold volcanism), when the interior of the moon was still liquid.

Rhea has an apparent brightness of 9.7 m and is one of the brightest moons of Saturn when viewed from Earth. In order to observe them, however, you need a telescope with an objective aperture of at least 10 cm.

In measurement results from the Cassini space probe from 2005, a group of astronomers found indications of a ring system made up of dust and rocks up to one meter in size around the moon of Saturn. The probe's measuring devices recorded a concentration of dust particles in certain areas close to it, and a strong reduction in the electrons from Saturn's magnetosphere. Rhea would have been the first moon with a ring system. The only thing missing was a photo. It has now been established that Rhea has no rings because none could be found in photos.

Traces of an atmosphere

Part of the surface of Rhea (2013).

During the Rhea overflight of the Cassini space probe in March 2010 at a distance of only 97 km, a mass spectrometer on board the probe registered traces of oxygen and carbon dioxide . This lunar atmosphere has significantly more mass on the day side. Researchers suspect that oxygen is created when electromagnetic radiation hits the water ice on the moon's surface and decomposes it. The carbon dioxide may arise from the oxidation of organic molecules or comes from outgassing of the surface ice.

Web links

Commons : Rhea  - collection of images, videos and audio files

Individual evidence

  1. Roatsch, T .; Jaumann, R .; Stephan, K .; Thomas, PC (2009). "Cartographic Mapping of the Icy Satellites Using ISS and VIMS Data". Saturn from Cassini-Huygens. pp. 763-781. doi : 10.1007 / 978-1-4020-9217-6_24 . ISBN 978-1-4020-9216-9 .
  2. ^ Cassini: A Discovery of Two New Planets about Saturn, Made in the Royal Parisian Observatory by Signor Cassini, Fellow of Both the Royal Societys, of England and France; English isn't out of French. Phil. Trans. January 1, 1673 8: 5178-5185; doi : 10.1098 / rstl.1673.0003 ( full text ).
  3. Rhea in the Gazetteer of Planetary Nomenclature of the IAU (WGPSN) / USGS ; last accessed on September 25, 2016.
  4. Jump up ↑ Rings for Rhea. In: Wissenschaft.de. March 7, 2008, accessed September 8, 2019 .
  5. Astronomy-Today article by Tilmann Althaus: No rings around Saturn's moon Rhea Date: August 2, 2010, this gives as source: "Cornell University: by Lauren Gold: No rings around Saturn's Rhea, astronomers find. Date: July 29, 2010 ". Both Retrieved November 28, 2010.
  6. Spiegel Online: "Cassini" discovers oxygen atmosphere around Rhea
  7. BD Teolis, GH Jones et al .: Cassini finds an oxygen-carbon dioxide atmosphere at saturn's icy moon Rhea , Science-Online, November 25, 2010, doi : 10.1126 / science.1198366 .
further inside Saturn moons further outside
Polydeuces
Semi- major axis  (km) Rhea 527.100
titanium