Janus (moon)

Janus
Janus, taken by Cassini-Huygens on April 7, 2010 from a distance of approx. 75,000 km.
Provisional or systematic name	S / 1980 S 1
Central body	Saturn
Properties of the orbit
Major semi-axis	151,460 ± 10 km
Periapsis	150,430 km
Apoapsis	152,490 km
eccentricity	0.0068
Orbit inclination	0.163 ± 0.004 °
Orbital time	0.694660342 d
Mean orbital velocity	15.8 km / s
Physical Properties
Albedo	0.71 ± 0.02
Apparent brightness	14.0 mag
Medium diameter	178.8 ± 6.0 (193 × 173 × 137) km
Dimensions	1.912 ± 0.005 x 10 18 kg
surface	≈ 100,400 km 2
Medium density	0.64 ± 0.06 g / cm 3
Sidereal rotation	0.694660342 days
Axis inclination	0.015 °
Acceleration of gravity on the surface	0.0137 m / s 2
Escape speed	55 m / s
discovery
Explorer	Audouin Dollfus
Date of discovery	December 15, 1966
Remarks	Coordinate moon with Epimetheus
The positions of the inner moons of Saturn in Saturn's ring system, from inside to outside Pan, Atlas, Prometheus, Pandora, Janus and Epimetheus, Mimas, Enceladus, Tethys, Dione and Rhea

Janus (also Saturn X ) is the seventh through eighth and ninth largest of the 82 known moons of the planet Saturn . The co-ordinate moon shares its orbit with Epimetheus , with whom it changes orbits every four years.

Discovery and naming

Janus was discovered on December 15, 1966 by the astronomer Audouin Dollfus . It is not clear, however, whether he saw Janus or Epimetheus. The discovery is attributed to Dollfus, although shortly before, on October 29, 1966, Jean Texereau had captured the object photographically, but without realizing its meaning.

On December 18, 1966, Richard L. Walker observed a similar object, the moon Epimetheus . It was believed, however, that Walker had also been watching Janus.

However, the property had unusual track characteristics. In October 1978, Stephen M. Larson and John W. Fountain found that the observations could best be explained by the presence of two separate bodies sharing the same orbit. However, it turned out to be very difficult to break down the orbits of the moons from the observation data.

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

Although the name Janus was proposed as early as 1966 and has been used for the moon since then, the official designation by the International Astronomical Union (IAU) did not take place until September 30, 1983. Until then it had the provisional designation S / 1966 S 2 (→ below ) . Janus is the 10th moon discovered by Saturn .

The moon was named after Janus , the two-faced god from Roman mythology . His name belongs to the same family of words as ianua , the Latin name for door and janus for any unlocked arched passage.

According to the naming conventions of the IAU, Janus names with a connection to the mythical twin pair Castor and Pollux are used for surface structures.

Track properties

Image taken two months after the orbits were replaced, from a distance of 492,000 km from Janus (right) and 452,000 km from Epimetheus. (Cassini, March 20, 2006)

Orbit

Janus orbits Saturn on a prograde , almost perfectly circular orbit at an average distance of 151,410 km to 151,460 km (approx. 2.512 to 2.513 Saturn radii) from its center, i.e. 91,142 km to 91,192 km above its cloud ceiling. The orbital eccentricity is 0.0068, the web is 0.163 ° relative to the equator of Saturn inclined , that is almost exactly in the equatorial plane of the planet. Due to the low eccentricity, the orbit varies in distance to Saturn by only around 2,000 km.

The orbit of the next inner moon Pandora is on average 9,690 and 9,740 km from the orbits of Epimetheus and Janus, the distance of the orbit of the next outer moon Aegaeon is on average about 16,000 km.

Janus orbits Saturn in 16 hours, 40 minutes and 18.7 seconds. This corresponds to a little more than the orbital period of Jupiter's moon Thebe and lies between the Uranus moons Perdita and Puck . The orbital times of Janus and Epimetheus differ by only 28.1 seconds. Janus and Epimetheus need about 1 hour and 34 to 35 minutes longer than their inner neighbor Pandora to complete one cycle.

Orbit behavior of Janus and Epimetheus

This section is not adequately provided with supporting documents ( e.g. individual evidence ). Information without sufficient evidence could be removed soon. Please help Wikipedia by researching the information and including good evidence.

See discussion: Janus (Moon) #Orbit data .

The position of Janus and Epimetheus in the course of a pass of the horseshoe orbit

Janus is co-ordinate with the moon Epimetheus , that is, the two moons run on almost the same orbits around Saturn. Their mean distances from the planet differ only 50 km, which is less than the diameters of both moons. About every four years there is a close encounter between the two moons, which then influence each other through their gravity . According to Kepler's laws , the inner moon, whose orbit is a total of 28.1 seconds (1/4 degree per day) faster, is accelerated and moves to a higher orbit, which in turn slows it down. The outer one is slowed down, moves to a lower orbit and is thereby accelerated. In this way, Janus and Epimetheus swap their orbits during this 100-day process, but do not overtake each other and never approach each other by more than 15,000 km. Since Janus has four times more mass than Epimetheus, he always has to carry around 20% of the total change in orbit. The orbital relationship of the two moons can be understood in the context of the three-body problem , in which case the two moons are of similar size; the third body is Saturn. As far as is known, this behavior of the two moons is unique in the solar system .

From the reference system that moves with the movement of the larger body around the central star, the co-ordinate companions describe a so-called horseshoe orbit , after which they make a large arc along the orbit, which they periodically swing back and forth. Viewed from the stationary reference system ( inertial system ), however, they still describe “normal” orbits.

Image of the Janus / Epimetheus ring and other nearby rings (Cassini, September 15, 2006)

Janus is currently the outer moon of the two. Orbit changes took place on January 21, 2006 (which has been well documented by the Cassini spacecraft ), January 2010, January 2014 and January 2018; the next one will take place in January 2022.

Janus / Epimetheus ring

In 2006, a diffuse dust ring was discovered that could be made visible in backlighting and that runs around Saturn along the orbits of Epimetheus and Janus. The so-called Janus / Epimetheus ring has a width of around 5,000 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 to that of Enceladus .

rotation

The rotation time is the same as the orbital time and Janus shows, like the earth's moon , a synchronous rotation , which also takes place within 16 hours, 40 minutes and 18.7 seconds. Its axis of rotation is exactly perpendicular to its plane . Its axis of rotation is inclined at 0.015 ° to the orbit.

Photo of the south pole region by Janus (Cassini, January 20, 2008)

Physical Properties

size

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

In terms of size, Janus can best be compared with the Jupiter moon Himalia , the Uranus moons Puck and Sycorax or the Neptune moons Despina and Galatea .

The total area of ​​Janus is estimated to be 100,400 km², which is roughly the area of Iceland .

Image of Janus in front of Saturn's clouds (Cassini, January 2006)

internal structure

The mean density of Janus, at 0.64 g / cm³, is much lower than that of Earth and is slightly lower than the density of Saturn; it is so low that Janus would swim on water. This indicates that the moon is predominantly composed of water ice .

The low density of Janus indicates that it may be one of the porous so-called rubble piles that have cavities inside due to the comparatively weak gravity.

surface

Named crater on Janus

Crater name	Diameter in km	coordination naten	Origin of name
Castor	?		Kastor , twin brother of Pollux
Idas	?		Idas , son of Aphareus
Lynceus	?		Lynkeus , son of Aegyptus
Phoibe	?		Phoibe , daughter of Leucippos

High-resolution image from a distance of 33,000 km (Cassini, January 20, 2008)

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

The slopes of some of the craters show evidence of darker material that could be better observed and documented on Epimetheus . Signs of fine lines (possibly ray systems ) have also been discovered on the wall of at least one crater.

So far in 1982, only four craters were officially named Janus, which according to the USGS - nomenclature as Epimetheus the legend of Castor and Pollux of the ( Greek mythology are removed).

Janus has a very high albedo of around 0.71, which means that it has a very bright surface that reflects 71 percent of the incident sunlight . On its surface, the acceleration due to gravity is 0.0137 m / s², which corresponds to about 0.1% of that on earth.

Emergence

Based on the crater density of its surface, it is concluded that it is a relatively old celestial body. Since Epimetheus and Janus are co-ordinate moons, it is assumed that they may have descended from a common original body that broke into two bodies due to a disturbance, for example a collision with another object. If this is the case, it must have happened in an early phase of Saturn's satellite system due to the crater density still observed today.

exploration

Best recording by Janus on Voyager 2 (August 25, 1981).

Janus has an apparent magnitude of 15.5 ^m , which is 1: 302000 of the central planet. Since its discovery in 1966 and confirmation in 1980 and the Voyager flybys, Janus has been studied by terrestrial telescopes and the Hubble Space Telescope , and its orbit parameters have been refined.

Janus has so far been visited by three space probes , namely the flyby probes Voyager 1 on November 12, 1980 and Voyager 2 on August 25, 1981 and the Saturn orbiter Cassini , which orbited Saturn from July 1, 2004 to September 15, 2017. Janus has been targeted several times by Cassini, so its size and shape as well as its orbital parameters are now fairly well known. Cassini's next flyby occurred during its 74th orbit around Saturn on June 1, 2008, when the probe Epimetheus passed at a distance of 14,363 km. Some well-resolved recordings could be made.

On April 2, 2010, Cassini was able to record an occultation (from the probe's point of view) of the leading hemispheres of Janus by Epimetheus from a distance of 2.1 million km.

Provisional numbering

Preliminary designations by Janus due to multiple discoveries and losses

designation	date	Reported by	IAUC
S / 1966 S 2	March 31, 1980 / April 28, 1980	Audouin Dollfus	* 3470
S / 1979 S 2	October 25, 1979	T. Gehrels / JA Van Allen et al.	* 3417
S / 1980 S 1	February 25, 1980	Dan Pascu	* 3454
S / 1980 S 2	February 29, 1980	Bradford A. Smith / Harold J. Reitsema / Stephen M. Larson	* 3456
S / 1980 S 9	March 31, 1980	Harris, AW / Gibson, J	* 3463

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

media

Commons : Janus (moon)  - Collection of images, videos and audio files

Web links

• Emily Lakdawalla: The Orbital Dance of Epimetheus and Janus , description of the orbit exchange between Epimetheus and Janus on January 21, 2006 at www.planetary.org
• USGS list of named structures on Janus
• IAUC 1987: Probable new Satellite of Saturn January 3, 1967 (discovery by A. Dollfus)
• IAUC 1991: Possible new Satellite of Saturn January 6, 1967 (discovery by R. Walker)
• IAUC 1995: Saturn X (Janus) February 1, 1967 (proposed name Janus)
• IAUC 3872: Satellites of Jupiter and Saturn September 30, 1983 (named Epimetheus, confirmation Janus)
• Star stories podcast episode 30: Alternating planets from June 21, 2013

Individual evidence

1. ↑ Categories for Naming Features on Planets and Satellites - IAU Working Group for Planetary System Nomenclature (WGPSN)
2. ↑ ^{a b} Emily Lakdawalla: The Orbital Dance of Epimetheus and Janus. The Planetary Society , February 7, 2006, accessed August 16, 2015 . 
3. ^ List of IAU Preliminary Designations of Natural Satellites. Retrieved February 20, 2011 . 

further inside	Saturn moons	further outside
Epimetheus	Semi- major axis  (km) Janus 151.500	Aegaeon