Janus (moon)

Audouin Dollfus

Janus (also Saturn X ) is the seventh-eighth and ninth largest of the 62 known moons of the planet Saturn. The co-orbital moon shares its orbit with Epimetheus, 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

Janus was discovered on 15 December 1966 by the astronomer Audouin Dollfus. However, it is not clear whether he has seen Janus or Epimetheus. The discovery is attributed Dollfus, although recently, had been photographically documented the object on October 29, 1966 Jean Texereau, without, however, to recognize its importance.

On December 18, 1966 Richard L. Walker observed a similar object, the moon Epimetheus. However, it was felt that Walker had also observed Janus.

However, the object pointed to unusual web properties. 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.

Janus was registered by the space probe Pioneer 11, which flew past Saturn on 1 September 1979. Three detectors for the detection of energetic particles took on his shadow. Janus was clearly identified on 1 March 1980 by the Voyager 1 spacecraft.

Although the name Janus was proposed in 1966 and has since been used for the moon, there was an official designation by the International Astronomical Union ( IAU) until September 30, 1983. Till then he had the provisional designation S/1966 S 2 ( → below ). Janus is the 10th discovered moon of Saturn.

It is named after the moon Janus, the two-faced god of Roman mythology. He was originally a light - and sun-god, the male counterpart of Jana, or Diana, and was only gradually to the God of all origin, the beginning and the end, the inputs and outputs of doors and gates, the father of all things and all gods. Its origin is uncertain, different legends describe him as a child of Saturn and Entoria. He had two faces, to see into the past and into the future. A legend says that Janus settled during the Golden Age in the Ianiculumforum to Lazio ruled as king from there. It goes on to say that he had taken in his flight from Saturn Jupiter on the Capitoline Hill. Since he acted as mediator between men and gods, was started in sacrifices with the invocation of Janus.

His name belongs to the same word family as ianua, the Latin word for door and janus for each unsealed vaulted passage. According to him, also the month of January is named. The Kalends, symbolizing beginnings were dedicated to him.

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

Janus 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 0.0068, the orbit is inclined 0.163 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 around 2,000 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.

Janus orbits Saturn in 16 hours, 40 minutes and 18.7 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 Janus and Epimetheus differ by only 28.1 seconds. Janus and Epimetheus need for a round about 1 hour and 34 to 35 minutes longer than the inner neighbor Pandora.

Path behavior of Janus and Epimetheus

Janus is co-orbital with the moon Epimetheus, 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, Janus and Epimetheus swap during this 100 -day process their orbits, but not overtake it and come close to to never more than about 10,000 km. Since Janus has four times more massive than Epimetheus, he has to always carry about 25 % of the total web change. The orbital relationship of the two moons can be understood in the context of the three- body problem, there have in this case, the two moons of similar size on; the third body is Saturn. 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, Janus is the outer 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 Janus has with how the Earth's moon, a synchronous rotation, which therefore also takes place within 16 hours, 40 minutes and 18.7 seconds. Its rotation axis is exactly perpendicular to its orbital plane. Its axis of rotation is inclined 0.015 degrees from the orbit.

Physical Properties

Size

Janus has an average diameter of 178.8 km. Janus appears as an irregularly shaped, elongated object with dimensions of 193 × 173 × 137 km, the longitudinal axis is aligned with Saturn on the images of the Cassini and Voyager spacecraft. The size is specified with 179.2 ± 8.0 (195 × 194 × 152) km;

From the size of Janus is most closely associated with Jupiter's moon Himalia to compare the Uranian moons Puck and Sycorax or the moons of Neptune Despina and Galatea.

The total area of ​​100.4 thousand km ² Janus is estimated, which corresponds approximately to the area of ​​Iceland.

Internal construction

The mean density of Janus is 0.64 g / cm ³ by far lower than that of the earth, and is slightly lower than the density of Saturn; she is so low that Janus would float on water. This indicates that the moon is composed mainly of water ice.

The low density of Janus points out 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 Janus is heavily cratered and has several large impact craters with diameters of 30 km along. Its surface appears older than that of the neighboring moon Prometheus, but younger than that of Pandora.

The slopes of some craters show evidence of darker material, which could be on Epimetheus better observed and documented. There are also signs of fine lines (possibly beam systems ) have been discovered on the wall of a crater at least.

So far in 1982, only officially named Janus four craters that are taken according to the USGS nomenclature as Epimetheus the legend of Castor and Pollux.

Janus has a very high albedo of about 0.71, which means that he has a very bright surface that reflects 71 % of the incident sunlight. On its surface is the acceleration of gravity 0.0137 m / s ², representing about 1% of the earth.

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

Janus has an apparent magnitude of 15.5 m, which is 1:302000 of the central planet. Since the discovery in 1966 and 1980 and the confirmation of the Voyager flybys, Janus has been studied by ground-based telescopes and the Hubble Space Telescope and its Bahnpararameter could be clarified.

Janus 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. Janus 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 74th orbit around Saturn on 1 June 2008, when the spacecraft Epimetheus happened at a distance of 14,363 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 Janus and Epimetheus Janus received by the IAU at least five systematic numbering, which could be proven later that it was Janus. This contributed significantly to the currently highly suspected number of Saturn's moons.

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