Pan (moon)

Template:Short description Template:For multi Template:Use dmy dates {{#invoke:infobox|infoboxTemplate | class = vcard | titleclass = fn org | title = Pan | image = {{#invoke:InfoboxImage|InfoboxImage|image=Pan color PIA21449.png|upright={{#if:||1.1}}|alt=}} | caption = Color photo of Pan by Cassini in March 2017Template:Efn | headerstyle = {{#if:|background-color:|background-color:#E0CCFF}} | labelstyle = max-width:{{#if:||11em}}; | autoheaders = y

| header1 = Discovery

| label2 = Discovered by | data2 = M. R. Showalter | label3 = Discovery site | data3 = | label4 = Discovery date | data4 = July 16, 1990 | label5 = Template:Longitem | data5 =

| header10 = {{#if:|Designations|Designations}}

| label11 = Template:Longitem | data11 = Saturn XVIII | label12 = Pronunciation | data12 = Template:IPAc-en | label13 = Template:Longitem | data13 = Πάν Pān | label14 = Template:Longitem | data14 = S/1981 S 13 | label15 = Template:Longitem | data15 = | label16 = Adjectives | data16 = Pandean Template:IPAc-en<ref>Template:OED</ref> | label17 = Symbol | data17 =

| header20 = Orbital characteristics{{#ifeq:|yes| (barycentric)}}<ref name="Jacobson2008">Template:Cite journal</ref>

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| header60 = Proper orbital elements

| label61 = Template:Longitem | data61 = {{#if: |{{{p_semimajor}}} AU}} | label62 = Template:Longitem | data62 = | label63 = Template:Longitem | data63 = | label64 = Template:Longitem | data64 = {{#if: |{{{p_mean_motion}}} degTemplate:\yr}} | label65 = Template:Longitem | data65 = {{#if:|{{#expr:360/1 round 5}} yr
({{#expr:365.25*360/1 round 3}} d) }} | label66 = Template:Longitem | data66 = {{#if:|{{{perihelion_rate}}} arcsecTemplate:\yr }} | label67 = Template:Longitem | data67 = {{#if:|{{{node_rate}}} arcsecTemplate:\yr}}

| header70 = Template:Anchor{{#if:| Physical characteristics|Physical characteristics}}

| label71 = Dimensions | data71 = Template:Val km
(± Template:Val km)<ref name="Thomas2020"/>Template:Rp | label72 = Template:Longitem | data72 = Template:Val<ref name="Thomas2020"/>Template:Rp | label73 = Template:Longitem | data73 = | label74 = Template:Longitem | data74 = | label75 = Template:Longitem | data75 = | label76 = Flattening | data76 = | label77 = Circumference | data77 = | label78 = Template:Longitem | data78 = | label79 = Volume | data79 = Template:Val<ref name="Thomas2020"/>Template:Rp | label80 = Mass | data80 = Template:Val<ref name="Thomas2020"/>Template:Rp | label81 = Template:Longitem | data81 = Template:Val<ref name="Thomas2020"/>Template:Rp | label82 = Template:Longitem | data82 = Template:Val<ref name="Thomas2020"/>Template:Rp | label83 = Template:Longitem | data83 = | label84 = Template:Longitem | data84 = Template:V2 km/s at longest axis
to Template:V2 km/s at poles | label85 = Template:Longitem | data85 = synchronous | label86 = Template:Longitem | data86 = | label87 = Template:Longitem | data87 = | label88 = Template:Longitem | data88 = zero | label89 = Template:Longitem | data89 = | label90 = Template:Longitem | data90 = | label91 = Template:Longitem | data91 = | label92 = Template:Longitem | data92 = | label93 = {{#if: |Template:Longitem |Albedo}} | data93 = 0.5 | label94 = Temperature | data94 = ≈ 78 K

| data100 = {{#if:|

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Surface temp.	min	mean	max
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| label101 = Surface absorbed dose rate | data101 = | label102 = Surface equivalent dose rate | data102 = | label103 = Template:Longitem | data103 = | label104 = Template:Longitem | data104 = | label105 = Template:Longitem | data105 = | label106 = Template:Longitem | data106 = | label107 = Template:Longitem | data107 =

| header110 = Atmosphere

| label111 = Template:Longitem | data111 = | label112 = Template:Longitem | data112 = | label113 = Composition by volume | data113 =

| below = {{#if:||Template:Reflist }}

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Pan is the innermost named moon of Saturn.<ref name="Moons of Saturn">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> It is approximately 35 kilometres across and 23 km wide and orbits within the Encke Gap in Saturn's A Ring. Pan is a ring shepherd and is responsible for keeping the Encke Gap free of ring particles. It is sometimes described as having the appearance of a walnut, or ravioli.<ref name="Ravioli">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Pan was discovered by Mark R. Showalter in 1990 from analysis of old Voyager 2 probe photos and received the provisional designation Template:Nowrap because the discovery images dated back to 1981.<ref name="iauc5052">IAUC 5052: Saturn July 16, 1990 (discovery)</ref>

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.<ref>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</ref> 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 5–10Template:E Saturn masses, and inferred that there was only a single moon within the Encke gap.<ref name="Showalter1986">Template:Cite journal</ref> The actual semi-major axis differs by 19 km, and the actual mass is 8.6Template:E 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.<ref name="Showalter1990">Template:Cite journal</ref><ref name="Showalter1991">Template:Cite journal</ref>

Name

The moon was named on 16 September 1991<ref>IAUC 5347: Satellites of Saturn and Neptune 1991 September 16 (naming the moon)</ref> 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 Template:Nowrap.<ref name="Gazetteer">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

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.<ref name="SaturnRingStats"/>

Geography

File:Pan (moon) edge-on.jpg

Pan, photographed by Cassini on March 7, 2017. The thin equatorial ridge is clearly visible.

Cassini scientists have described Pan as "walnut-shaped"<ref>"PIA08320: Cruising with Pan", Planetary Photojournal.</ref> 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.<ref>Template:Cite news</ref><ref name="Space empanada">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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).<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Pandean ringlet

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

Gallery

• PIA06534 Encke Division.jpg

  Closeup of the Encke Gap, showing the central ringlet that is coincident with Pan's orbit.

• Pans highway 3-05-07.jpg

  Cassini image showing Pan orbiting in the Encke Gap.

• Pan side view.jpg

  Equatorial view of Pan from Cassini, with the rings of Saturn surrounding the moon.

• 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.

• PIA21436 - Pan, 3 versions.jpg

  Stretched (4x) images, processed in various ways.

See also

• List of natural satellites

External links

• Pan Profile by NASA's Solar System Exploration
• The Planetary Society: Pan