Pan (moon)

Mark R. Showalter

Pan ( Saturn XVIII) is the second and one of the smaller of the 62 known moons of the planet Saturn. He is the shepherd moon of the Encke division and the inner of two moons in the A-ring of Saturn's rings.

  • 3.1 size
  • 3.2 Internal structure
  • 3.3 Surface

Discovery and designation

Pan was discovered in Saturn's rings on 16 July 1990 by the astronomer Mark R. Showalter in the analysis of images of the Voyager 2 spacecraft in 1981.

The existence of a moon within the Encke division had already been predicted by Jeffrey N. Cuzzi and Jeffrey D. Scargle in 1985. In the shots of the Voyager 2 spacecraft to wave pattern showed in the ring system, which pointed to gravitational disturbances. Showalter and other astronomers calculated in 1986 that celestial bodies could be responsible for the disturbances. They met a fairly accurate prediction of the trajectory of ( 133,603 km ± 10) and mass ( 5-10 · 10-12 Saturn mass) and concluded that only a single moon within the division could move. Consequently, the details of the orbit deviated by only 19 km from the later observed, the actual mass is 8.6 · 10-12 Saturn mass.

The moon was later found within 1 ° of the predicted position. It all Voyager -2 photos were evaluated, was again being calculated by computer, under what conditions would the moon be visible on each image best. Pan was finally visible on eleven Voyager recordings that were made in the 48 hours before closest approach to Saturn. Each recording with a resolution of better than 50 kilometers per pixel was the moon clearly visible.

The discovery was announced on 16 July 1990 by the International Astronomical Union ( IAU); the moon first received the provisional designation S/1981 S 13, which was backdated to 1981 due to the Voyager flyby. Pan was the 18th discovered moon of Saturn, which also corresponds to his Roman numbering XVIII. Until the discovery of S/2009 S 1 26 July 2009 Pan was the innermost known moon of Saturn.

On September 16, 1991, he was then named after Pan from Greek mythology. Pan was the son of Hermes, the father of the satyrs and the shepherd and herd God. The name alludes to the function of the moon as a shepherd moon. Its shape after it is a chimera of human upper body with a horned head with beard and the lower body of a goat. Frightened by his figure, he was exposed by his mother Dryops. Hermes took him to Olympus, but where Pan was no place and was brought by Hermes to the island of Crete.

According to other sources Pan was a son of Zeus and Callisto or of Zeus and the nymph hubris. According to another narration Pan is a son of Cronus (Saturn ) and the Amaltheia, thus a half-brother of Zeus. The Amalthea, which is also described as goat figure was, at the same time the nurse of Zeus.

After Pan pan flute is named, and the word panic is also derived from his name. The shepherd god Pan was negatively reinterpreted from the middle ages of Christianity, and often regarded as the personification of the devil by its shape and its existing laws as lustful behavior. The equivalent in Roman mythology is Faunus, who was the grandson of Saturn.

From 1955 to 1975, in 1938 discovered and previously known as Jupiter Moon Jupiter Moon XI Carme also sometimes called Pan.

Web properties


Pan orbiting Saturn on a prograde, nearly perfectly circular orbit at an average distance of about 133,584 kilometers (about 2.216 Saturn radii ) from its center, ie 73,316 km above the cloud tops. The orbital eccentricity of 0.0000144, the orbit is inclined 0.001 degrees from the equator of Saturn, that is exactly in the equatorial plane of the planet. Due to the low eccentricity of the track varies by only about 4 km.

The orbit of the next inner moon S/2009 S 1 is in the middle removed about 16,500 km from Pan's orbit, the distance of the path of the next outer moon Daphnis is an average of 2921 km.

Pan orbits Saturn in 13 hours, 48 minutes and 4.4 seconds. This roughly corresponds to the orbital periods of Uranus and Neptune's moon Rosalind moon Larissa. Pan required for one revolution about 2 hours and 28 minutes longer than the inner adjacent S/2009 S 1

The moon orbits Saturn within a critical distance, the so-called Roche limit, which would bring a larger moon in this field to shatter. Probably Pan is preserved only because of its small size or a loose internal structure of this fate.

Pan - Ringlets

The moon runs in the 325 km wide, named after Johann Franz Encke Encke division of the A ring around the planet. It is effective as a shepherd moon and causes with his gravity that the Encke division remains largely free of ring particles.

The Encke pitch, which is about 3000 km from the outer edge of the A- ring contains a small ring ( " ringlet "), which coincides with the orbit of pan, indicating that pan maintains the ring particles in a horseshoe orbit. A second, weaker narrow ring is periodically interrupted by Pan, like disorders of the more outer ring of the F- moon Prometheus. Scientists have not observed perturbations of the Pan- orbits by more outward current moons.


The rotation period is equal to the orbital period and has Pan with how the Earth's moon, a synchronous rotation, which therefore also takes place within 13 hours, 48 minutes and 4.4 seconds. Its rotation axis is exactly perpendicular to its orbital plane.

Physical Properties


Pan has an average diameter of 28.4 km. Pan appears as an irregularly shaped elongated object with dimensions of 35 × 32 × 21 km, wherein the longitudinal axis is aligned with Saturn on the images of the Cassini-Huygens probe. Scientists from the Cassini mission described Pan as a walnut- shaped object that has a bulge at the equator. This equatorial edge has come through the collecting ring material of the Encke division into existence. Other previously known moons with such comparable bulges are Atlas and Iapetus.

The total area of ​​2500 km ² Pan is estimated, which corresponds approximately to the area of ​​the Grand Duchy of Luxembourg.

Internal construction

Its mean density is 0.56 g / cm ³ by far lower than that of the earth and even much lower than the density of Saturn; she is so low that Pan would float on water. This indicates that the moon is composed mainly of water ice.

Since Pan Saturn rotates within the Roche limit, points out that he has either a very strong internal structure, or that he belongs to the so-called Rubble Piles, which, due to the relatively weak gravitational inside cavities. Due to the extremely low mean density, the latter hypothesis is more likely.


Pan has a high albedo of 0.5, which means that it has a bright surface, which reflects 50 % of the incident sunlight. On its surface is the gravitational acceleration 0.002 m / s ², representing about 0.2 ‰ of the earth. The average surface temperature of pan is estimated to be about -195 ° C ( 78 K).


Due to the small size and apparent magnitude of 19.4 m, which is 1:30200000 of the central planet, as well as the close proximity to Saturn and the fact that he is outshone by this, Pan is barely visible with ground-based telescopes.

Pan 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. Pan was repeatedly taken by Cassini targeted so that its size and shape as well as the orbital parameters are now fairly well known.