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Pan (moon)

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Color photo of Pan by Cassini in March 2017[a]
Discovered byM. R. Showalter
Discovery dateJuly 16, 1990
Saturn XVIII
Named after
Πάν Pān
S/1981 S 13
AdjectivesPandean /pænˈdən/[1]
Orbital characteristics[2]
133584.0±0.1 km
0.575050718 d (13.801217 h)
Satellite ofSaturn
GroupShepherd moon of the Encke Gap
Physical characteristics
Dimensions34.6 × 28.2 × 21.0 km
(± 0.4 × 0.4 × 1.0 km)[3]: 2 
Mean diameter
27.4±0.6 km[3]: 2 
Volume10748 km3[3]: 8 
Mass(4.30±0.22)×1015 kg[3]: 3 
Mean density
0.400±0.031 g/cm3[3]: 3 
0.0111–0.0169 m/s2[3]: 3 
0.006 km/s at longest axis
to 0.007 km/s at poles
Temperature≈ 78 K

Pan is the innermost named moon of Saturn.[4] It is a small, ravioli[5]-shaped moon approximately 35 kilometres across and 23 km wide that orbits within the Encke Gap 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 walnut.

It was discovered by Mark R. Showalter in 1990 from analysis of old Voyager 2 probe photos and received the provisional designation S/1981 S 13 because the discovery images dated back to 1981.[6]

Prediction and discovery[edit]

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.[7] In 1986, Showalter et al. inferred its orbit and mass by modeling its gravitational wake. They arrived at a precise prediction of 133,603 ± 10 km for the semi-major axis and a mass of 5–10×10−12 Saturn masses, and inferred that there was only a single moon within the Encke gap.[8] The actual semi-major axis differs by 19 km, and the actual mass is 8.6×10−12 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 a resolution better than ~50 km/pixel shows Pan clearly. In all, it appears in eleven Voyager 2 images.[9][10]


The moon was named on 16 September 1991[11] after the mythological Greek god named Pan, who was (among other things) the god of shepherds. This is a reference to Pan's role as a shepherd moon. It is also designated Saturn XVIII.[12]


The eccentricity of Pan's orbit causes its distance from Saturn to vary by ~4 km. Its inclination, which would cause it to move up and down, is not distinguishable from zero with present data. The Encke Gap, within which Pan orbits, is about 322 km wide.[13]


Pan, photographed by Cassini on March 7, 2017. The thin equatorial ridge is clearly visible.

Cassini scientists have described Pan as "walnut-shaped"[14] 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. It has been referred to by journalists as a space empanada, a form of stuffed bread or pastry, as well as a ravioli.[15][16] A new study suggests that the bizarre shape of Pan could also be due to collisions between tiny moonlets, thus causing them to merge and form Pan (known as the pyramidal regime formation scenario).[17]

Pandean ringlet[edit]

The Encke Gap contains a ringlet that is coincident with Pan's orbit, indicating that Pan maintains the particles in horseshoe orbits.[18] A second ringlet is periodically disrupted by Pan, similarly to how the F Ring is disturbed by Prometheus.[19]


See also[edit]


  1. ^ This color photo of Pan was created by combining separate photos taken in infrared, green, and ultraviolet spectral filters of the Cassini spacecraft's narrow-angle Imaging Science Subsystem (ISS) camera on 7 March 2017. This view of the moon's northern hemisphere shows its walnut-like appearance, with a highly inclined equatorial ridge almost eclipsing the moon's southern hemisphere from view.


  1. ^ "Pandean". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
  2. ^ Jacobson, R. A.; et al. (2008). "Revised orbits of Saturn's small inner satellites". Astronomical Journal. 135 (1): 261–263. Bibcode:2008AJ....135..261J. CiteSeerX doi:10.1088/0004-6256/135/1/261. S2CID 122998668.
  3. ^ a b c d e f Thomas, P. C.; Helfenstein, P. (July 2020). "The small inner satellites of Saturn: Shapes, structures and some implications". Icarus. 344: 20. Bibcode:2020Icar..34413355T. doi:10.1016/j.icarus.2019.06.016. S2CID 197474587. 113355.
  4. ^ "Saturn - Moons". NASA. Archived from the original on 7 September 2015. Retrieved 5 October 2017.
  5. ^ "Saturn's Tiny Moon Pan Looks Like a Ravioli". CNET. Retrieved 8 January 2020.
  6. ^ IAUC 5052: Saturn July 16, 1990 (discovery)
  7. ^ 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
  8. ^ Showalter, M. R.; et al. (1986). "Satellite "wakes" and the orbit of the Encke Gap moonlet". Icarus. 66 (2): 297–323. Bibcode:1986Icar...66..297S. doi:10.1016/0019-1035(86)90160-0.
  9. ^ Showalter, M. R. (1990). "Visual Detection of 1981 S 13, the Encke Gap Moonlet". Bulletin of the American Astronomical Society. 22: 1031.
  10. ^ Showalter, M. R. (1991). "Visual detection of 1981 S 13, Saturn's eighteenth satellite, and its role in the Encke gap". Nature. 351 (6329): 709–713. Bibcode:1991Natur.351..709S. doi:10.1038/351709a0. S2CID 4317496.
  11. ^ IAUC 5347: Satellites of Saturn and Neptune 1991 September 16 (naming the moon)
  12. ^ "Planet and Satellite Names and Discoverers". Gazetteer of Planetary Nomenclature. USGS Astrogeology. 21 July 2006. Retrieved 7 August 2006.
  13. ^ "Vital Statistics for Saturn's Rings and Inner Satellites". NASA Planetary Data System.
  14. ^ "PIA08320: Cruising with Pan", Planetary Photojournal.
  15. ^ Chang, Kenneth (10 March 2017). "Pan, Moon of Saturn, Looks Like a Cosmic Ravioli (or Maybe a Walnut)". The New York Times. Retrieved 11 March 2017.
  16. ^ Perkins, Sid (9 March 2017). "Stunning close-up of Saturn's moon, Pan, reveals a space empanada". Science.
  17. ^ "Here's Why Saturn's Inner Moons Are Shaped Like Ravioli and Potatoes". Space.com. 22 May 2018. Retrieved 8 February 2021.
  18. ^ Hedman, M.M.; Burns, J.A.; Hamilton, D.P.; Showalter, M.R. (2013). "Of horseshoes and heliotropes: Dynamics of dust in the Encke Gap". Icarus. 223 (1): 252–276. arXiv:1211.4762. Bibcode:2013Icar..223..252H. doi:10.1016/j.icarus.2012.11.036. S2CID 974916.
  19. ^ Porco, C.C.; Baker, E.; Barbara, John; Beurle, K.; Brahic, A.; Burns, J.A.; Charnoz, S.; Cooper, N.; Dawson, Douglas; Delgenio, Anthony; Denk, T.; Dones, Luke; Dyudina, Ulyana; Evans, M.W.; Giese, B.; Grazier, Kim; Helfenstein, Paul; Ingersoll, A.P.; Jacobson, R.A.; West, Robert (2005). "Cassini Imaging Science: Initial Results on Saturn's Rings and Small Satellites". Science. 307 (5713): 1226–1236. Bibcode:2005Sci...307.1226P. doi:10.1126/science.1108056. PMID 15731439. S2CID 1058405.

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