Pan (moon)

Pan (pan, Greek Πάν) is a moon of Saturn, named after the god Pan. It was discovered by Mark R. Showalter in 1990 from analysis of old Voyager probe photos. Pan is within the Encke division in Saturn's A Ring; it acts as a shepherd and is responsible for keeping the Encke gap open. Its gravity produces wave patterns in the rings that indicated Pan's presence and led to the reexamination of Voyager photographs of its predicted location. Other undiscovered moons may exist within Saturn's rings.

Discovery
Discovered by	M. R. Showalter
Discovered in	July 16, 1990
Orbital characteristics
Semimajor axis	133,583 km [1]
Eccentricity	0.00021±0.00008
Orbital period	0.5764 d
Inclination	0.007±0.002° (to Saturn's equator)
Is a satellite of	Saturn
Physical characteristics
Equatorial diameter	20 km
Mass	2.7×1015 kg
Mean density	0.6 g/cm3
Surface gravity	0.002 m/s2
Rotation period	synchronous
Axial tilt	zero
Albedo	0.5
Surface temp.	min mean max  ? K ~78 K  ? K
Atmosphere	none

The existence of such a moon in the Encke division was first predicted by Jeffrey N. Cuzzi and Jeffrey N. Scargle in 1985. Then Showalter et al. inferred its orbit and mass in 1986 by modeling its "wake", or gravitational disturbance. 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. [2] [3] The actual semi-major axis is 133,583 km and the mass is 2.7×1015 kg, or 4.7×10⁻¹² of Saturn's mass of 5.688×10_²⁶ kg.

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. [4]

After the discovery it received a temporary designation S/1981 S 13. It is also designated as Saturn XVIII.

There is also an asteroid called 4450 Pan.

References

• Cuzzi, J. N. & Scargle, J. D., "Wavy edges suggest moonlet in Encke's gap", ApJ 292 (1985) 276
  
• Showalter, M. R., Cuzzi J. N. et al., "Satellite 'wakes' and the orbit of the Encke Gap moonlet", Icarus 66 (1986) 297
  
• Showalter, M. R., "Visual detection of 1981 S13, Saturn's eighteenth satellite", Nature 351 (June 27, 1991) 709–713
  

Links

The Planetary Society: Pan

Saturn | Pan | Daphnis | ...

Saturn's natural satellites

Pan | Daphnis | Atlas | Prometheus | S/2004 S 6 | S/2004 S 4 | S/2004 S 3 | Pandora | Epimetheus and Janus | Mimas | Methone | Pallene | Enceladus | Telesto, Tethys, and Calypso | Polydeuces, Dione, and Helene | Rhea | Titan | Hyperion | Iapetus | Kiviuq | Ijiraq | Phoebe | Paaliaq | Skathi | Albiorix | S/2004 S 11 | Erriapo | Siarnaq | S/2004 S 13 | Tarvos | Mundilfari | S/2004 S 17 | Narvi | S/2004 S 15 | S/2004 S 10 | Suttungr | S/2004 S 12 | S/2004 S 18 | S/2004 S 9 | S/2004 S 14 | S/2004 S 7 | Thrymr | S/2004 S 16 | Ymir | S/2004 S 8

see also: Rings of Saturn | Cassini-Huygens | Themis