Pan (moon)

Pan is the innermost[4] named moon of Saturn. It is a small, walnut-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 raviolo.[5]

For the moon of Jupiter formerly known by this name, see Carme (moon). For other uses, see Pan (disambiguation).

Pan
View of Pan by Cassini in 2017.[lower-alpha 1]
Discovery
Discovered byM. R. Showalter
Discovery dateJuly 16, 1990
Designations
Designation
Saturn XVIII
Pronunciation/ˈpæn/
Named after
Πάν Pān
S/1981 S 13
AdjectivesPandean /pænˈdiːən/[1]
Orbital characteristics[2]
133584.0±0.1 km
Eccentricity0.0000144±0.0000054
0.575050718 d (13.801217 h)
Inclination0.0001°±0.0004°
Satellite ofSaturn
GroupShepherd moon of the Encke Gap
Physical characteristics
Dimensions34.4 × 31.4 × 20.8 km
Mean radius
14.1±1.3 km[3]
Mass(4.95±0.75)×1015 kg[3]
Mean density
0.42±0.15 g/cm³[3]
0.0001–0.0018 m/s2
≈ 0.006 km/s
synchronous
zero
Albedo0.5
Temperature≈ 78 K

    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

    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 very 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 resolution better than ~50 km/pixel shows Pan clearly. In all, it appears in eleven Voyager 2 images.[9][10]

    Name

    The moon was named on 16 September 1991,[11] 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. It is also designated Saturn XVIII.[12]

    Orbit

    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 325 km wide.

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

    Cassini scientists have described Pan as "walnut-shaped"[13] 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 also been referred to by journalists as a space empanada, a form of stuffed bread or pastry, as well as a ravioli.[14][15]

    Pandean ringlet

    The Encke Gap contains a ringlet that is coincident with Pan's orbit, indicating that Pan maintains the particles in horseshoe orbits.[16] A second ringlet is periodically disrupted by Pan, similarly to how the F Ring is disturbed by Prometheus.[17]

    • Closeup of the Encke Gap, showing the central ringlet that is coincident with Pan's orbit.
    • Cassini image showing Pan orbiting in the Encke Gap.
    • Equatorial view of Pan from Cassini, with the rings of Saturn surrounding the moon.
    • Pan in the center of the image, occupying the Encke Gap in Saturn's rings. Its walnut-like shape is clearly visible.
    • Stretched (4x) images, processed in various ways.

    References
    Notes
    1. This photo of Pan was acquired by 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.
    Citations
    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 10.1.1.653.3917. doi:10.1088/0004-6256/135/1/261.
    3. Thomas, P. C. (July 2010). "Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission" (PDF). Icarus. 208 (1): 395–401. Bibcode:2010Icar..208..395T. doi:10.1016/j.icarus.2010.01.025.
    4. "Saturn - Moons". NASA. 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.
    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. "PIA08320: Cruising with Pan", Planetary Photojournal.
    14. Chang, Kenneth (10 March 2017). "Pan, Moon of Saturn, Looks Like a Cosmic Ravioli (or Maybe a Walnut)". The New York Times. The New York Times. Retrieved 11 March 2017.
    15. Perkins, Sid (9 March 2017). "Stunning close-up of Saturn's moon, Pan, reveals a space empanada". Science.
    16. 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.
    17. 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): 1226–1236.
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