Epimetheus (moon)

Epimetheus
Epimetheus' south polar region, taken by the NASA -Raumsonde Cassini on 3 December 2007 from 37,400 kilometers away.
Provisional or systematic name	S / 1980 S 3
Central body	Saturn
Properties of the orbit
Major semi-axis	151.410 ± 10 km
Periapsis	149,926 km
Apoapsis	152,894 km
eccentricity	0.0098
Orbit inclination	0.351 ± 0.004 °
Orbital time	0.694333517 d
Mean orbital velocity	15.8 km / s
Physical Properties
Albedo	0.73 ± 0.03
Apparent brightness	15.6 mag
Medium diameter	113.4 ± 6.2 (135 × 108 × 105) km
Dimensions	5.304 ± 0.013 x 10 17 kg
surface	≈ 40,400 km 2
Medium density	0.69 ± 0.11 g / cm 3
Sidereal rotation	0.694333517 days
Axis inclination	0.0 °
Acceleration of gravity on the surface	0.0078 m / s 2
Escape speed	32.0 m / s
Surface temperature	≈ −195 ° C (78) K
discovery
Explorer	Richard L. Walker Stephen M. Larson John W. Fountain
Date of discovery	18th December 1966
Remarks	Co-ordinate moon with Janus
The positions of the inner moons of Saturn in Saturn's ring system, from inside to outside Pan, Atlas, Prometheus, Pandora, Janus & Epimetheus, Mimas, Enceladus, Tethys, Dione and Rhea

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

Discovery and naming

Epimetheus was discovered on December 18, 1966 by the astronomer Richard L. Walker . On December 15, 1966, Audouin Dollfus had already discovered a new moon near Saturn, and suggested that Janus name it . The moon, discovered by Richard L. Walker three days later, was initially believed to be the same object.

However, the object showed unusual behavior on the path. 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.

In 1980 this assumption was confirmed when the Voyager 1 space probe flew by. Walker therefore officially shares his discovery with Larson and Fountain. The body recorded by Voyager was initially named S 3 by the International Astronomical Union (IAU) in 1980 (→ below ). Epimetheus is the 11th moon discovered by Saturn .

The name Epimetheus was officially given on September 30, 1983. It was also at this point that the name Janus was officially recognized, although it has been in use since 1966.

The moon was named after the titan Epimetheus , son of Iapetus and Clymene , brother of Prometheus , Menoitius and Atlas , and husband of Pandora and, according to later traditions, the father of Pyrrha (wife and cousin of Prometheus' son Deucalion ) from Greek mythology .

The name means the one who thinks afterwards , which is to be understood as an antonym to the name of Prometheus ( the one who thinks before ).

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

Cassini image taken on March 20, 2006, two months after the orbit replacement, at 452,000 km from Epimetheus (left) and 492,000 km from Janus. Although the positions of the two moons appear to be close, they are still approximately 15,000 km apart.

Orbit

Epimetheus 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 0.0098, the web is 0.351 ° 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 3,000 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.

Epimetheus orbits Saturn in 16 hours, 39 minutes and 50.6 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 Epimetheus and Janus differ by only 28.1 seconds. Epimetheus and Janus need about 1 hour and 34 to 35 minutes longer than their inner neighbor Pandora to complete one cycle.

Orbit behavior of Epimetheus and Janus

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: Epimetheus (Moon) #Orbit Dates .

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

Epimetheus is co-ordinate with the moon Janus , 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, Epimetheus and Janus 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 Epimetheus has only a quarter as much mass as Janus, he always has to carry around 80% of the total change in orbit. The orbital relationship of the two moons can be understood in terms of the three-body problem , in which case two moons (the third is Saturn) are similar in size. 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.

Cassini image from September 15, 2006 of the Janus / Epimetheus ring and other nearby rings.

Epimetheus is currently the inner moon of the two. The last 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 is therefore expected to 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 Epimetheus shows, like the earth's moon , a synchronous rotation , which also takes place within 16 hours, 39 minutes and 50.6 seconds. Its axis of rotation is exactly perpendicular to its plane .

Physical Properties

Epimetheus before Saturn, recorded by Cassini in May 2016.

size

Epimetheus has a mean diameter of 113.4 km. On the images of the Cassini and Voyager probes , Epimetheus appears as a very irregularly shaped, elongated object with dimensions of 135 × 108 × 105 km, with the longitudinal axis aligned with Saturn. Epimetheus has a very angular shape that is somewhat similar to the Neptune moon Proteus .

In terms of size, Epimetheus can best be compared with the Uranus moon Portia .

The total area of ​​Epimetheus is estimated to be 40,400 km², which is slightly less than the area of Switzerland or the Netherlands .

internal structure

At 0.69 g / cm³, the mean density of Epimetheus is far lower than that of Earth and corresponds almost exactly to the density of Saturn; it is so low that Epimetheus would swim on water. This indicates that the moon is predominantly composed of water ice .

Cassini recording of Epimetheus, July 2005.

The low density of Epimetheus indicates that it may belong to the porous so-called rubble piles , which have cavities inside due to the comparatively weak gravity.

surface

The surface of Epimetheus is badly cratered; on its surface there are several larger impact craters with diameters of up to 30 km as well as small and larger ridges and depressions. The South Pole region shows the remains of a large impact that covered most of this area. This could be responsible for the noticeable flattening of the southern hemisphere.

A distinction is made between two types of terrain: dark, gentler terrain, and lighter, slightly yellowish rugged terrain. One interpretation is that the darker material has been shown to slide down slopes and possibly have a lower ice content than the lighter material, which can be viewed more as 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 according to the USGS - nomenclature as Janus the legend of Castor and Pollux are taken.

List of named craters on Epimetheus

Crater name	Diameter (km)	Coordinates	Origin of name
Hilairea	33		Hilairea , Phoibes sister ( Greek mythology )
Pollux	?		Pollux , ( lat.Polydeukes ) Castor's brother ( Greek mythology )

Cassini image of Epimetheus and Titan , from distances of 667,000 and 1.8 million km, respectively, which documents the proportions of the two well.

Epimetheus has a very high albedo of around 0.73, which means that it has a very bright surface that reflects 73% of the incident sunlight . On its surface, the acceleration due to gravity is 0.0078 m / s², which corresponds to about 8 ‰ of that on earth. The mean surface temperature of Epimetheus is estimated to be around −195 ° C (78 K ).

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 Voyager 1 recording of Epimetheus one hour before the closest approach to Saturn on November 12, 1980. The dark line is not a surface structure, but the shadow of the F-ring.

Epimetheus has an apparent magnitude of 14.5 ^m , which is 1: 912000 of the central planet. Since its discovery and confirmation in 1980 and the Voyager flyby flights, Epimetheus has been studied by earth-based telescopes and the Hubble space telescope , and its orbit parameters have thus been refined.

Epimetheus has so far been visited by three space probes , namely by the flyby probes Voyager 1 on November 12, 1980 and Voyager 2 on August 25, 1981 and the Saturn orbiter Cassini , which has orbited Saturn since July 1, 2004. Epimetheus was targeted by Cassini several times, so its size and shape, as well as its orbital parameters, are now fairly well known. Cassini's next flyby occurred during its 53rd orbit around Saturn on December 3, 2007, when the probe Epimetheus passed at a distance of 15,779 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

Due to the complicated history of the discovery of Epimetheus and Janus, Epimetheus received at least nine systematic numbering from the IAU , which could later be proven to be Epimetheus. At the time, this contributed significantly to the presumed too high number of Saturn's moons.

List of the provisional names of Epimetheus

designation	date	Reported by	IAUC
S / 1980 S 3	March 6, 1980	Dale P. Cruikshank	* 3457
S / 1980 S 4	March 6, 1980	Dale P. Cruikshank	* 3457
S / 1980 S 5	March 6, 1980	Bradford A. Smith / Harold J. Reitsema / Stephen M. Larson	* 3457
S / 1980 S 8	March 31, 1980	Harris, AW / Gibson, J	* 3463
S / 1980 S 11	April 10, 1980	Harold J. Reitsema / Bradford A. Smith / Stephen M. Larson	* 3466
S / 1980 S 15	April 28, 1980	Christian Veillet	* 3470
S / 1980 p 16	April 28, 1980	Christian Veillet	* 3470
S / 1980 S 17	April 28, 1980	Christian Veillet	* 3470
S / 1980 S 19	May 9, 1980	Audouin Dollfus / Serge Brunier	* 3474

media

Commons : Epimetheus (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 Epimetheus
• 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)
• astronews: Picture of the day December 28, 2015
• astronews.com: Picture of the day: July 6, 2017
• Star stories podcast episode 30: Alternating planets from June 21, 2013

Individual evidence

1. ↑ Paul Rincon: Saturn overtakes Jupiter as planet with most moons. BBC , October 7, 2019, accessed March 20, 2020 . 
2. ↑ Categories for Naming Features on Planets and Satellites - IAU Working Group for Planetary System Nomenclature (WGPSN)
3. ↑ ^{a b} Emily Lakdawalla: The Orbital Dance of Epimetheus and Janus. The Planetary Society , February 7, 2006, accessed August 16, 2015 . 
4. ^ List of IAU Preliminary Designations of Natural Satellites. Retrieved February 20, 2011 . 

further inside	Saturn moons	further outside
Pandora	Semi- major axis  (km) Epimetheus 151.400	Janus