Pan is the innermost namedmoon of Saturn.[4] It is approximately 35 kilometres across and 23 km wide and orbits within theEncke Gap in Saturn'sA Ring. Pan is aring shepherd and is responsible for keeping the Encke Gap free of ring particles. It is sometimes described as having the appearance of a walnut, orraviolo.[5]
Pan was discovered byMark R. Showalter in 1990 from analysis of oldVoyager 2 probe photos and received the provisional designationS/1981 S 13 because the discovery images dated back to 1981.[6]
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, Showalteret 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 5–10×10−12 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 allVoyager 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 elevenVoyager 2 images.[9][10]
The moon was named on 16 September 1991[11] after the mythologicalGreek god namedPan, who was (among other things) the god of shepherds. This is a reference to Pan's role as ashepherd moon. It is also designatedSaturn XVIII.[12]
Theeccentricity of Pan's orbit causes its distance from Saturn to vary by ~4 km. Itsinclination, which would cause it to move up and down, is not distinguishable from zero with present data. TheEncke Gap, within which Pan orbits, is about 322 km wide.[13]
Cassini scientists have described Pan as "walnut-shaped"[14] owing to theequatorial ridge, similar to that onAtlas, 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 spaceempanada, a form of stuffed bread or pastry, as well as aravioli.[15][16] A new study suggests that the bizarre shape of Pan could also be due to collisions between tinymoonlets, thus causing them to merge and form Pan (known as the pyramidal regime formation scenario).[17]
The Encke Gap contains a ringlet that is coincident with Pan's orbit, indicating that Pan maintains the particles inhorseshoe orbits.[18] A second ringlet is periodically disrupted by Pan, similarly to how theF Ring is disturbed byPrometheus.[19]
^This color photo of Pan was created by combining separate photos taken in infrared, green, and ultraviolet spectral filters of theCassini spacecraft's narrow-angle Imaging Science Subsystem (ISS) camera on 7 March 2017. This view of the moon's northern hemisphere shows itswalnut-like appearance, with a highly inclined equatorial ridge almost eclipsing the moon's southern hemisphere from view.
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