Pan (moon)

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. 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 Saturn masses, and inferred that there was only a single moon within the Encke gap. The actual semi-major axis differs by 19 km, and the actual mass is 8.6 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.

Name

The moon was named on 16 September 1991 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.

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 322 km wide.

Geography

Cassini scientists have described Pan as "walnut-shaped" 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. 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).

Pandean ringlet

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

Gallery

File:PIA06534 Encke Division.jpg|Closeup of the Encke Gap, showing the central ringlet that is coincident with Pan's orbit. File:Pans highway 3-05-07.jpg|Cassini image showing Pan orbiting in the Encke Gap. File:Pan side view.jpg|Equatorial view of Pan from Cassini, with the rings of Saturn surrounding the moon. File:Pan Rev09.2x.jpg|Pan in the center of the image, occupying the Encke Gap in Saturn's rings. Its walnut-like shape is clearly visible. File:PIA21436 - Pan, 3 versions.jpg|Stretched (4x) images, processed in various ways.

Notes

References

References

1. {{OED. Pandean
2. Jacobson, R. A.. (2008). "Revised orbits of Saturn's small inner satellites". [[Astronomical Journal]].
3. "Saturn - Moons". NASA.
4. "Saturn's Tiny Moon Pan Looks Like a Ravioli". CNET.
5. [http://www.cbat.eps.harvard.edu/iauc/05000/05052.html IAUC 5052: ''Saturn''] July 16, 1990 (discovery)
6. Cuzzi, J. N.; and Scargle, J. D.; [http://adsabs.harvard.edu//full/seri/ApJ../0292//0000276.000.html ''Wavy Edges Suggest Moonlet in Encke's Gap''], Astrophysical Journal, Vol. 292 (May 1, 1985), pp. 276–290
7. Showalter, M. R.. (1986). "Satellite "wakes" and the orbit of the Encke Gap moonlet". Icarus.
8. Showalter, M. R.. (1990). "Visual Detection of 1981 S 13, the Encke Gap Moonlet". Bulletin of the American Astronomical Society.
9. Showalter, M. R.. (1991). "Visual detection of 1981 S 13, Saturn's eighteenth satellite, and its role in the Encke gap". Nature.
10. [http://www.cbat.eps.harvard.edu/iauc/05300/05347.html IAUC 5347: ''Satellites of Saturn and Neptune''] 1991 September 16 (naming the moon)
11. (July 21, 2006). "Planet and Satellite Names and Discoverers". USGS Astrogeology.
12. [http://photojournal.jpl.nasa.gov/catalog/PIA08320 "PIA08320: Cruising with Pan"], ''[[Planetary Photojournal]]''.
13. (10 March 2017). "Pan, Moon of Saturn, Looks Like a Cosmic Ravioli (or Maybe a Walnut)". The New York Times.
14. Perkins, Sid. (2017-03-09). "Stunning close-up of Saturn's moon, Pan, reveals a space empanada".
15. (22 May 2018). "Here's Why Saturn's Inner Moons Are Shaped Like Ravioli and Potatoes".
16. (2013). "Of horseshoes and heliotropes: Dynamics of dust in the Encke Gap". Icarus.
17. (2005). "Cassini Imaging Science: Initial Results on Saturn's Rings and Small Satellites". Science.
18. "Vital Statistics for Saturn's Rings and Inner Satellites". NASA Planetary Data System.