Pan (moon)

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For other uses, see Pan.
For the moon of Jupiter known by this name 1955-1975, see Carme (moon).
Pan
Image:Pan_side_view.jpg
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.
Discovery
Discovered by M. R. Showalter
Discovery date July 16, 1990
Semi-major axis 133,584.0±0.1 km
Eccentricity 0.000035±0.000007
Orbital period 0.575050718 d
Inclination 0.0010±0.0006°
Satellite of Saturn
Physical characteristics
Dimensions 35 × 35 × 23 km[2]
Mean radius 15 ± 2 km[2]
Mass 4.9 ± 0.7 ×1015 kg[2]
Mean density 0.36 ± 0.16 g/cm³
Equatorial surface gravity 0.001 m/s2
Escape velocity ~0.006 km/s
Rotation period synchronous
Axial tilt zero
Albedo 0.5
Temperature ~78 K

Pan (pronounced /ˈpæn/ PAN, 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.

Contents

[edit] Prediction

The image shows Pan orbiting in the Encke Gap.
The image shows Pan orbiting in the Encke Gap.

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.[3] 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 5–10×10−12Saturn masses, and inferred that there was only a single moon within the Encke gap.[4] 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.[5][6][5]

[edit] Geography

Pan in the center of the image, occupying the Encke Division in Saturn's rings. Its walnut-like shape is clearly visible
Pan in the center of the image, occupying the Encke Division in Saturn's rings. Its walnut-like shape is clearly visible

Cassini scientists have described Pan as "walnut-shaped"[7] 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.

[edit] Pan ringlet

Closeup of the Encke Division. The central ringlet is coincident with Pan's orbit.
Closeup of the Encke Division. The central ringlet is coincident with Pan's orbit.

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

[edit] Name

The moon was named after the god Pan on 16 September 1991,[8] and is also designated Saturn XVIII.[9] 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.[10]

There is also an asteroid called 4450 Pan.

[edit] References

  1. ^ Spitale, J. N.; et al. (2006). "The orbits of Saturn's small satellites derived from combined historic and Cassini imaging observations". The Astronomical Journal 132: 692. doi:10.1086/505206. 
  2. ^ a b c Porco, C. C.; et al. (2006). "Physical Characteristics and Possible Accretionary Origins for Saturn's Small Satellites". Bulletin of the American Astronomical Society 37: 768. 
  3. ^ 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
  4. ^ Showalter, M. R.; et al. (1986). "Satellite "wakes" and the orbit of the Encke Gap moonlet". Icarus 66: 297. doi:10.1016/0019-1035(86)90160-0. 
  5. ^ a b Showalter, M. R. (1990). "Visual Detection of 1981 S 13, the Encke Gap Moonlet". Bulletin of the American Astronomical Society 22: 1031. 
  6. ^ Showalter, M. R. (1991). "Visual detection of 1981 S 13, Saturn's eighteenth satellite, and its role in the Encke gap". Nature 351: 709. doi:10.1038/351709a0. 
  7. ^ "PIA08320: Cruising with Pan", Planetary Photojournal.
  8. ^ IAUC 5347: Satellites of Saturn and Neptune 1991 September 16 (naming the moon)
  9. ^ Planet and Satellite Names and Discoverers. Gazetteer of Planetary Nomenclature. USGS Astrogeology (July 21, 2006). Retrieved on 2006-08-07.
  10. ^ IAUC 5052: Saturn 1990 July 16 (discovery)

[edit] External links

v • d • e
Moons of Saturn
Generally listed in increasing distance from Saturn. Temporary names in italics.
Ring shepherds
moonlets · Pan · Daphnis · Atlas · Prometheus ·  ?S/2004 S 6 · ?S/2004 S 4 · ?S/2004 S 3 · Pandora
Co-orbitals
Inner large (and Trojan)
Mimas · Methone · Anthe · Pallene · Enceladus · Tethys (Telesto • Calypso) · Dione (Helene • Polydeuces)
Outer large
Inuit group
Norse group
Gallic group
Rings of Saturn · Cassini-Huygens · Themis · Chiron