Pan is the innermost named

moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...

. It is a small, walnut-shaped moon approximately 35

kilometre The kilometre ( SI symbol: km; or ), spelt kilometer in American English, is a unit of length in the International System of Units (SI), equal to one thousand metres (kilo- being the SI prefix for ). It is now the measurement unit used for ex ...

s across and 23 km wide that orbits within the

Encke Gap The rings of Saturn are the most extensive ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometers to meters, that orbit around Saturn. The ring particles are made almost entirel ...

in Saturn's A Ring. Pan is a ring shepherd and is responsible for keeping the Encke Gap free of ring particles. It is sometimes described as having the appearance of a

ravioli Ravioli (; singular: ''raviolo'', ) are a type of pasta comprising a filling enveloped in thin pasta dough. Usually served in broth or with a sauce, they originated as a traditional food in Italian cuisine. Ravioli are commonly square, though o ...

. It was discovered by

Mark R. Showalter Mark Robert Showalter (born December 5, 1957) is a Senior Research Scientist at the SETI Institute. He is the discoverer of six moons and three planetary rings. He is the Principal Investigator of NASA's Planetary Data System Rings Node, a co-inv ...

in 1990 from analysis of old ''Voyager 2'' probe photos and received the provisional designation because the discovery images dated back to 1981.IAUC 5052: ''Saturn''
July 16, 1990 (discovery)

Prediction and discovery

The existence of a moon in the Encke Gap was first predicted by Jeffrey N. Cuzzi and Jeffrey D. Scargle in 1985, based on wavy edges of the gap which indicated a gravitational disturbance. In 1986 Showalter ''et al.'' inferred its orbit and mass by modeling its gravitational wake. They arrived at a very precise prediction of 133,603 ± 10 km for the semi-major axis and a mass of 5–10 Saturn masses, and inferred that there was only a single moon within the Encke gap. The actual semi-major axis differs by 19 km and the actual mass is 8.6 of Saturn's. The moon was later found within 1° of the predicted position. The search was undertaken by considering all ''

Voyager 2 ''Voyager 2'' is a space probe launched by NASA on August 20, 1977, to study the outer planets and interstellar space beyond the Sun's heliosphere. As a part of the Voyager program, it was launched 16 days before its twin, ''Voyager 1'', on a ...

'' images and using a computer calculation to predict whether the moon would be visible under sufficiently favorable conditions in each one. Every qualifying Voyager 2 image with resolution better than ~50 km/pixel shows Pan clearly. In all, it appears in eleven ''Voyager 2'' images.

Name

The moon was named on 16 September 1991, after the mythological Pan, who was (among other things) the god of shepherds. This is a reference to Pan's role as a

shepherd moon A shepherd moon (also herder moon or watcher moon) is a small natural satellite that clears a gap in planetary-ring material or keeps particles within a ring contained. The name is a result of the fact they limit the "herd" of the ring particle ...

. It is also designated .

Orbit

The eccentricity of Pan's orbit causes its distance from Saturn to vary by ~4 km. Its

inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a Plane of reference, reference plane and the orbital plane or Axis of rotation, axis of direction of the orbiting object ...

, which would cause it to move up and down, is not distinguishable from zero with present data. The

, within which Pan orbits, is about 325 km wide.

Geography

'' Cassini'' scientists have described Pan as " walnut-shaped" owing to the

equatorial ridge Equatorial ridges are a feature of at least three of Saturn's moons: the large moon Iapetus and the tiny moons Atlas and Pan. They are ridges that closely follow the moons' equators. They appear to be unique to the Saturnian system, but it i ...

, similar to that on

Atlas An atlas is a collection of maps; it is typically a bundle of maps of Earth or of a region of Earth. Atlases have traditionally been bound into book form, but today many atlases are in multimedia formats. In addition to presenting geographic ...

, that is visible in images. The ridge is due to ring material that Pan has swept up from the Encke gap. It has been referred to by journalists as a space

empanada An empanada is a type of baked or fried turnover consisting of pastry and filling, common in Spanish, other Southern European, Latin American, and Iberian-influenced cultures around the world. The name comes from the Spanish (to bread, i.e., ...

, a form of stuffed bread or pastry, as well as a

. A new study suggests that the bizarre shape of Pan could also be due to collisions between tiny

moonlets A moonlet, minor moon, minor natural satellite, or minor satellite is a particularly small natural satellite orbiting a planet, dwarf planet, or other minor planet. Up until 1995, moonlets were only hypothetical components of Saturn's F-ring ...

, thus causing them to merge and form Pan (known as the pyramidal regime formation scenario).

Pandean ringlet

The Encke Gap contains a ringlet that is coincident with Pan's orbit, indicating that Pan maintains the particles in

horseshoe orbit In celestial mechanics, a horseshoe orbit is a type of co-orbital configuration, co-orbital motion of a small orbiting body relative to a larger orbiting body. The osculating orbit, osculating (instantaneous) orbital period of the smaller body re ...

s. A second ringlet is periodically disrupted by Pan, similarly to how the F Ring is disturbed by Prometheus.

Gallery

File:PIA06534 Encke Division.jpg, Closeup of the Encke Gap, showing the central ringlet that is coincident with Pan's orbit. File:Pans highway 3-05-07.jpg, ''Cassini'' image showing Pan orbiting in the Encke Gap. File:Pan side view.jpg, Equatorial view of Pan from ''Cassini'', with the rings of Saturn surrounding the moon. File:Pan Rev09.2x.jpg, Pan in the center of the image, occupying the Encke Gap in Saturn's rings. Its walnut-like shape is clearly visible. File:PIA21436 - Pan, 3 versions.jpg, Stretched (4x) images, processed in various ways.

References

;Notes ;Citations

External links

Pan Profile
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NASA's Solar System Exploration

{{DEFAULTSORT:Pan (Moon) Moons of Saturn 19900716 Moons with a prograde orbit