Pan is a moon of Saturn 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.[1] Other undiscovered moons may exist within Saturn's rings.

The existence of such a moon in the Encke division was first predicted by Jeffrey N. Cuzzi and Jeffrey D. 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] 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.[3][1]

The moon named after the god Pan, and is also designated Saturn XVIII.[4] After the discovery it received a temporary designation S/1981 S 13.

There is also an asteroid called 4450 Pan.


  1. 1.0 1.1 M.R. Showalter (1990). "Visual Detection of 1981S13, the Encke Gap Moonlet". Bulletin of the American Astronomical Society 22: 1031.
  2. M.R. Showalter et al (1986). "Satellite "wakes" and the orbit of the Encke Gap moonlet". Icarus 66: 297.
  3. M.R. Showalter (1991). "Visual detection of 1981S13, Saturn's eighteenth satellite, and its role in the Encke gap". Nature 351: 709.
  4. Planet and Satellite Names and Discoverers. Gazetteer of Planetary Nomenclature. USGS Astrogeology (July 21, 2006). Retrieved on 2006-08-07.

External links

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