Pan (moon)

:"For the moon of Jupiter known by this name 1955-1975, see Carme (moon)." Infobox Planet
name = Pan

caption = Pan amid the rings of Saturn. The 'side' view gives Pan the appearance of being embedded in the rings, although it actually travels in the empty Encke Division.
bgcolour = #a0ffa0
discovery = yes
discoverer = M. R. Showalter
discovered = July 16, 1990
orbit_ref =cite journal | author= Spitale, J. N.; "et al."| title= "The orbits of Saturn's small satellites derived from combined historic and "Cassini" imaging observations"| journal= The Astronomical Journal| year= 2006| volume= 132 | issue=2 | pages= 692–710 | url= | doi = 10.1086/505206]
semimajor = 133,584.0±0.1 km
eccentricity = 0.000035±0.000007
period = 0.575050718 d
inclination = 0.0010±0.0006°
satellite_of = Saturn
physical_characteristics = yes
dimensions = 35 × 35 × 23 kmcite journal | author= Porco, C. C.; "et al."| title= "Physical Characteristics and Possible Accretionary Origins for Saturn's Small Satellites"| journal= Bulletin of the American Astronomical Society| year= 2006| volume= 37| pages= 768| url=]
mean_radius = 15 ± 2 km
mass = 4.9 ± 0.7 e|15 kg
density = 0.36 ± 0.16 g/cm³
surface_grav = 0.001 m/s2
escape_velocity = ~0.006 km/s
rotation = synchronous
axial_tilt = zero
albedo = 0.5
single_temperature = ~78 K

Pan (pronEng|ˈpæn respell|PAN, or as in Greek "Πάν)" is the innermost moon of Saturn. Pan is a "walnut-shaped" small moon about 35 kilometres across and 23 km high that orbits within the Encke gap in Saturn's A Ring. It acts as a ring shepherd and is responsible for keeping the Encke gap open.


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. [Cuzzi, J. N.; and Scargle, J. D.; [ "Wavy Edges Suggest Moonlet in Encke's Gap"] , Astrophysical Journal, Vol. 292 (May 1, 1985), pp. 276–290] 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 5–10e|−12Saturn masses, and inferred that there was only a single moon within the Encke gap.cite journal | author= Showalter, M. R.; "et al."| title= "Satellite "wakes" and the orbit of the Encke Gap moonlet"| journal= Icarus| year= 1986| volume= 66| pages= 297| url= | doi = 10.1016/0019-1035(86)90160-0] The actual semi-major axis differs by 19 km and the actual mass is 8.6e|−12 of Saturn's.

The moon was later found within 1° of the predicted position. The search was undertaken by considering all "Voyager 2" 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.cite journal | author=Showalter, M. R.| title= "Visual Detection of 1981 S 13, the Encke Gap Moonlet"| journal= Bulletin of the American Astronomical Society| year= 1990| volume= 22| pages= 1031| url=] cite journal | author= Showalter, M. R.| title= "Visual detection of 1981 S 13, Saturn's eighteenth satellite, and its role in the Encke gap"| journal= Nature| year= 1991| volume= 351| pages= 709| doi= 10.1038/351709a0]


"Cassini" scientists have described Pan as "walnut-shaped" [ [ "PIA08320: Cruising with Pan"] , "Planetary Photojournal".] owing to the equatorial ridge, similar to that on Atlas, that is visible in images. The ridge is due to ring material that Pan has swept up from the Encke gap.

Pan ringlet

The Encke Division contains a ringlet that is coincident with Pan's orbit, indicating that Pan maintains the particles in horseshoe orbits.


The moon was named after the god Pan on 16 September 1991, [ [ IAUC 5347: "Satellites of Saturn and Neptune"] 1991 September 16 (naming the moon)] and is also designated nowrap|Saturn XVIII.cite web
title = Planet and Satellite Names and Discoverers
work = Gazetteer of Planetary Nomenclature
publisher = USGS Astrogeology
date = July 21, 2006
url =
accessdate = 2006-08-07
] It was discovered by Mark R. Showalter in 1990 from analysis of old "Voyager 2" probe photos and received the provisional designation nowrap|S/1981 S 13 because the discovery images dated back to 1981. [ IAUC 5052: "Saturn"] 1990 July 16 (discovery)]

There is also an asteroid called 4450 Pan.


External links

* [ Pan Profile] by [ NASA's Solar System Exploration]
* [ The Planetary Society: Pan]

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