Rhea (satellite)

Rhea is a satellite of the planet Saturn. At 1,529 km in diameter (about half that of the Moon), it is the planet's second-largest satellite after Titan, and closely followed by Iapetus. The satellite occupies the fourteenth position in proximity to the planet, orbiting at a distance of about 527,000 km. Its orbit is nearly circular and slightly tilted relative to Saturn's equator, taking about 4.5 Earth days for the satellite to complete.

Rhea is believed to have a rocky core that is slightly less than a third of its diameter, while its mantle and crust could be composed of water ice and various impurities.

The satellite was discovered in 1672 by Italian-French astronomer Giovanni Cassini, who named it after the Greek mythological figure Rhea, sister and wife of Cronus (Saturn in the Roman pantheon). The moon was photographed by the American probe Voyager 1 in November 1980, when it passed through the Saturn system, and later by the Cassini probe, which studied it closely in November 2005, in March 2010, and in January. of 2011.

Origin

Saturn's moons are thought to have formed by an accretion process similar to that thought to have formed the planets of the solar system. Saturn was surrounded by an accretion disk that gradually merged into different satellites. However, in 2012 it was proposed that Titan had formed after a succession of large impacts between pre-existing satellites (with a structure similar to that of the Jovian system). In this model, Rhea, Iapetus, and the other major satellites would have formed from the debris of these collisions.

Physical characteristics

Rhea is an icy moon with a density of about 1.24 g/cm³ This density indicates that Rhea probably has a rocky core that makes up one-third of its mass, the rest being a combination of water-ice.

Rhea's physical features resemble Dione's, with different hemispheres, suggesting similar compositions and histories. The temperature in Rea oscillates between 53 and 99 K (-220.o and -174. o Celsius).

Rhea's surface is highly cratered, with linear, bright, and diffuse markings; At first it was thought that these lines were produced by material ejected during the formation of large craters, such as the 375 km diameter Tirawa (visible at the top of the Rea image), but later high-resolution images have shown which are actually fault systems similar to those existing on Dione.

Analysis of the Cassini data discovered a very tenuous atmosphere composed of oxygen and carbon dioxide, making it the only celestial body other than Earth where atmospheric oxygen has been detected.

Ring system

On March 6, 2008, NASA announced that Rea might have a faint ring system. This would mean the discovery of the first ring around a satellite. The existence of the ring was deduced from observed changes in the flow of electrons trapped by Saturn's magnetic field as Cassini passed near Rhea. Debris and dust could spread to the Hill sphere of Rhea, but they were expected to be more dense in the vicinity of the satellite. The presence of the ring was reinforced by the subsequent discovery of a set of small spots, bright in the ultraviolet, distributed along Rhea's equator and interpreted as impact sites of material from the ring. However, when the < i>Cassini made specific observations in the putative ring plane from different angles, found no evidence of its existence. This leads to the need to look for another explanation for the first observations.

Map

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  Rea (down to the left) compared to the Moon (up to the left) and the Earth (right).

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  Image of Rea and its craters obtained by the Cassini probe.

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  Rea color image obtained by Cassini probe.

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  Image near Rea obtained by the Cassini probe.