Epimetheus (moon)

Richard L. Walker Stephen M. Larson John W. Fountain

Epimetheus ( Saturn XI ) is the seventh-eighth and tenth largest of the 62 known moons of the planet Saturn. The co-orbital moon shares its orbit with Janus, with whom he exchanges the orbit every four years.

  • 3.1 size
  • 3.2 Internal structure
  • 3.3 Surface
  • 5.1 Preliminary numbering

Discovery and designation

Epimetheus was discovered on 18 December 1966 by the astronomer Richard L. Walker. Already on December 15, 1966 Audouin Dollfus had discovered a new moon with Saturn, for the designation of which he proposed Janus. In the three days later discovered by Richard L. Walker Moon was thought at first that it was the same object.

However, the object showed an unusual path performance. In October 1978 Stephen M. Larson and John W. Fountain found that the observations could best be explained with the presence of two separate bodies, which shared the same orbit. However, it proved very difficult to break down the orbits of the moons from the observational data.

In 1980, this assumption will be confirmed at the flyby of the Voyager 1 spacecraft. Officially, therefore, divides Walker his discovery with Larson and Fountain. The current consumed by Voyager body was first by the International Astronomical Union ( IAU) in 1980 called S 3 (→ below). Epimetheus is the 11th discovered moon of Saturn.

The name Epimetheus was officially awarded on September 30, 1983. At this time also the name Janus was officially recognized, although it has been used since 1966.

The name was the moon after the Titan Epimetheus, son of Iapetus and Clymene, brother of Prometheus, Menoitios and Atlas, and husband of Pandora, as well as by later traditions the father of Pyrrha (wife and cousin Prometheus ' son Deucalion ) from Greek mythology. Since Zeus wanted to punish Prometheus for stealing fire and its return to the people, he had created Pandora by Hephaestus out of clay and equip with many special features to make them more particularly seductive. Zeus went down with Pandora to earth and gave it as a gift to Epimetheus, who had been indeed previously warned by Prometheus, to accept gifts of the Olympian gods, but Pandora charm died and she married. Thus came the previously equipped with many perishable gifts Pandora to Earth. Pandora, for other versions of Epimetheus himself, opens the box, and the stored therein diseases, plagues vices and vices were let loose on the people. Before even the only positive response - hope - can escape from the bush, it is closed from Pandora or Epimetheus again. So the world is a desolate place until Pandora opens the box again and also hope can in the world. But the golden age in which mankind was spared from work, sickness and death, is finally over.

Since the early Christianity, the story of Pandora and Epimetheus is often seen as an allegory of the biblical Fall. Pandora is for seducing Eve and Epimetheus for transmitting seduced Adam.

The name means the later thinker, what is to be understood as antonym to the name of Prometheus ( the previously Thinking ).

After the naming conventions of the IAU are used for surface structures on Janus names with connection to the mythical twins Castor and Pollux.

Web properties

Orbit

Epimetheus orbits Saturn on a prograde, nearly perfectly circular orbit at an average distance of 151,410 km to 151,460 km (about 2.512 to 2.513 Saturn radii ) from its center, ie 91,142 km to 91,192 km above the cloud tops. The orbital eccentricity is 0.0098, the orbit is inclined 0.351 degrees from the equator of Saturn, that is almost exactly in the equatorial plane of the planet. Due to the low eccentricity of the track varies in distance from Saturn to only about 3,000 km.

The orbit of the next inner moon Pandora is located in the middle 9690 or 9740 km from the orbits of Janus and Epimetheus, the distance of the path of the next outer moon Aegaeon averages about 16,000 km.

Epimetheus orbits Saturn in 16 hours, 39 minutes and 50.6 seconds. This is slightly more than the orbital period of Jupiter's moon Thebe and lies between the Uranian moons Perdita and Puck. The orbital periods of Epimetheus and Janus differ by only 28.1 seconds. Epimetheus and Janus need for a round about 1 hour and 34 to 35 minutes longer than the inner neighbor Pandora.

Path behavior of Epimetheus and Janus

Epimetheus is co-orbital with the moon Janus, that is, the two moons run on almost the same orbits around Saturn. Your average distances of the planets differ by only 50 km, which is less than the diameters of the two moons. Approximately every four years there is a close encounter of the two moons, which then interact with each other by gravity. According to Kepler's laws, the inner moon, whose orbit by a total of 28.1 seconds ( daily 1 /4 degree ) is faster, it speeds up and moves to a higher orbit, so it is again slowed down. The outer is braked, migrates to a lower orbit and is thereby accelerated. In this way, Epimetheus and Janus exchange during this 100 -day process their orbits, but not overtake it and come close to to never more than about 10,000 km. As Epimetheus has four times less mass than Janus, he has to always carry about 75 % of the total web change. The orbital relationship of the two moons can be understood in the context of the three- body problem, in which case two moons (the third Saturn ) are similar in size. This behavior of the two moons is, as far as known, unique in the solar system.

The co-orbital companions described by the movement of the larger body around the central star comoving system as seen from a so-called horseshoe orbit, after it along the orbit of a large sheet, which they oscillate periodically back and forth. However, viewed from the stationary reference frame ( inertial frame ) from describing still "normal" orbits.

Currently, Epimetheus is the inner moon of the two. The last track change took place on January 21, 2006 instead (which has been well documented by the Cassini spacecraft ) and in January 2010, the next is scheduled for early 2014.

Janus / Epimetheus Ring

In 2006, a diffuse dust ring was discovered, which could be made ​​visible against the light and runs along the orbits of Janus and Epimetheus by Saturn. The previously so-called Janus / Epimetheus Ring has a width of about 5000 km and is similar in intensity to the rings of Jupiter. The ring is fed by the impact of micrometeorites on the two moons, similar as is the case with Enceladus.

Rotation

The rotation period is equal to the orbital period and Epimetheus has with how the Earth's moon, a synchronous rotation, which therefore also takes place within 16 hours, 39 minutes and 50.6 seconds. Its rotation axis is exactly perpendicular to its orbital plane.

Physical Properties

Size

Epimetheus has a mean diameter of 113.4 kilometers. Epimetheus appears as a very irregularly shaped, elongated object with dimensions of 135 × 108 × 105 km, the longitudinal axis is aligned with Saturn on the images of the Cassini and Voyager spacecraft. Epimetheus has a very angular shape that somewhat resembles the Moon Neptune Proteus.

From the size Epimetheus is best compared with the Uranus moon Portia.

The total area of ​​40,400 km ² is estimated to Epimetheus, this is slightly less than that of Switzerland or the Netherlands area.

Internal construction

The average density of Epimetheus is 0.69 g / cm ³ far lower than that of the Earth and is almost exactly the density of Saturn; she is so low that Epimetheus would float on water. This indicates that the moon is composed mainly of water ice.

The low density of Epimetheus indicates that he may have one of the so-called porous Rubble Piles, which, due to the relatively weak gravitational inside cavities.

Surface

The surface of Epimetheus is heavily cratered; on its surface there are several large impact craters with diameters of up to 30 km, as well as small and larger ridges and depressions. The south polar region shows the remains of a large strike, which covers most of the area. This could be responsible for the conspicuous flattening of the southern hemisphere.

A distinction is made between two types of terrain: Dark, gentler terrain, and brighter, slightly yellowish rugged terrain. One interpretation is that the darker material proven to slide down slopes and possibly a lower ice content than the lighter material that may be considered rather than bedrock. In any case, both types of terrain are very rich in ice.

So far, only two craters were officially named on Epimetheus in 1982, which are taken according to the USGS nomenclature as for Janus the legend of Castor and Pollux.

Epimetheus has a very high albedo of about 0.73, which means that he has a very bright surface that reflects 73 % of the incident sunlight. On its surface is the acceleration of gravity 0.0078 m / s ², equivalent to about 8 ‰ of the earth. The average surface temperature of Epimetheus is estimated to be about -195 ° C ( 78 K).

Formation

On the basis of crater density of its surface is concluded that there is a relatively old celestial bodies. As Epimetheus and Janus co-orbital moons are, it is believed that they may have descended from a common parent body, the body broke in two by a disturbance, such as a collision with another object. If this is the case, it would have to be done by today observed crater density at an early stage of Saturn's satellite system.

Research

Epimetheus has an apparent magnitude of 14.5 m, which is 1:912000 of the central planet. Since the discovery and confirmation of 1980, and the Voyager flybys Epimetheus was studied by ground-based telescopes and the Hubble Space Telescope and its orbit could be clarified.

Epimetheus has so far been visited by three spacecraft, notably by the flyby probes Voyager 1 on 12 November 1980 and Voyager 2 on August 1981 and the Saturn orbiter Cassini, orbiting Saturn since July 1, 2004 25. Epimetheus was repeatedly taken by Cassini targeted so that its size and shape as well as its orbital parameters are now fairly well known. The next flyby of Cassini occurred during the 53rd orbit around Saturn on 3 December 2007, when the spacecraft Epimetheus happened at a distance of 15,779 km. It was some well-resolved images are taken.

On 2 April 2010 Cassini was a cover (from the perspective of the probe ) take the leading hemisphere of Janus by Epimetheus from a distance of 2.1 million km.

Preliminary numbering

Due to the complicated history of discovery of Epimetheus and Janus, Epimetheus received by the IAU at least nine systematic numbering, which could be proven later that it was Epimetheus. This contributed significantly to the currently too high to suspected number of Saturn's moons.

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