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Helene (moon)

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Template:Short description {{#invoke:infobox|infoboxTemplate | class = vcard | titleclass = fn org | title = Helene | image = {{#invoke:InfoboxImage|InfoboxImage|image=Leading hemisphere of Helene - 20110618.jpg|upright={{#if:||1.1}}|alt=}} | caption = High-resolution view of leading hemisphere, showing gullies and apparent dust (regolith) flows (Cassini, June 2011) | headerstyle = {{#if:|background-color:|background-color:#E0CCFF}} | labelstyle = max-width:{{#if:||11em}}; | autoheaders = y

| header1 = DiscoveryTemplate:Sfn

| label2 = Discovered by | data2 = P. Laques
J. Lecacheux | label3 = Discovery site | data3 = Pic du Midi Observatory | label4 = Discovery date | data4 = March 1, 1980 | label5 = Template:Longitem | data5 =

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

| label11 = Template:Longitem | data11 = Saturn XII | label12 = Pronunciation | data12 = Template:IPAc-en<ref>John Walker (1839) A Critical Pronouncing Dictionary and Expositor of the English Language;
also per Template:OED</ref> | label13 = Template:Longitem | data13 = Helen of Troy (Ἑλένη Helenē) | label14 = Template:Longitem | data14 = Template:Plainlist | label15 = Template:Longitem | data15 = | label16 = Adjectives | data16 = Helenean Template:IPAc-en<ref>Clarified as Helenéan in Earle (1841) Marathon: and other poems, p. 76.</ref> | label17 = Symbol | data17 =

| header20 = Orbital characteristics{{#ifeq:|yes| (barycentric)}}

| data21 = | data22 = {{#if: |Epoch {{{epoch}}}}} | data23 = {{#if: | Uncertainty parameter {{{uncertainty}}}}} | label24 = Observation arc | data24 = | label25 = Earliest precovery date | data25 = | label26 = {{#switch:{{{apsis}}} |apsis|gee|barion|center|centre|(apsis)=Apo{{{apsis}}} |Ap{{#if:|{{{apsis}}}|helion}}}} | data26 = | label27 = Peri{{#if:|{{{apsis}}}|helion}} | data27 = | label28 = Peri{{#if:|{{{apsis}}}|apsis}} | data28 = | label29 = {{#switch:{{{apsis}}} |helion|astron=Ap{{{apsis}}} |Apo{{#if:|{{{apsis}}}|apsis}}}} | data29 = | label30 = Periastron | data30 = | label31 = Apoastron | data31 = | label32 = Template:Longitem | data32 = Template:Val<ref name="jpl-satelem"/> | label33 = Template:Longitem | data33 = | label34 = Eccentricity | data34 = Template:Val<ref name="jpl-satelem"/> | label35 = Template:Longitem | data35 = Template:Val<ref name="jpl-satelem"/> | label36 = Template:Longitem | data36 = | label37 = Template:Longitem | data37 = | label38 = Template:Longitem | data38 = | label39 = Template:Longitem | data39 = | label40 = Inclination | data40 = 0.199° (to Saturn's equator) | label41 = Template:Longitem | data41 = | label42 = Template:Longitem | data42 = | label43 = Template:Longitem | data43 = | label44 = Template:Longitem | data44 = | label45 = Template:Longitem | data45 = | label46 = Template:Nowrap | data46 = | label47 = Satellite of | data47 = Saturn | label48 = Group | data48 = L4 Dione trojan | label49 = {{#switch: |yes|true=Satellites |Known satellites}} | data49 = | label50 = Star | data50 = | label51 = Earth MOID | data51 = | label52 = Mercury MOID | data52 = | label53 = Venus MOID | data53 = | label54 = Mars MOID | data54 = | label55 = Jupiter MOID | data55 = | label56 = Saturn MOID | data56 = | label57 = Uranus MOID | data57 = | label58 = Neptune MOID | data58 = | label59 = TJupiter | data59 =

| header60 = Proper orbital elements

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({{#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)Template:Sfn | label72 = Template:Longitem | data72 = Template:ValTemplate:Sfn | 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:ValTemplate:Efn | label80 = Mass | data80 = Template:ValTemplate:Sfn | label81 = Template:Longitem | data81 = Template:ValTemplate:Sfn | label82 = Template:Longitem | data82 = Template:Gr m/s2 at longest axis
to Template:Gr m/s2 at poles | 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 = assumed 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 = Template:Val Template:Nowrap | label94 = Temperature | data94 =

| data100 = {{#if:|

{{#if:|}}{{#if:|}}{{#if:|}}{{#if:|}}
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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Helene Template:IPAc-en is a moon of Saturn. It was discovered by Pierre Laques and Jean Lecacheux in 1980 from ground-based observations at Pic du Midi Observatory,Template:Sfn and was designated Template:Nowrap.Template:Sfn In 1988 it was officially named after Helen of Troy, who was the granddaughter of Cronus (Saturn) in Greek mythology.Template:Sfn Helene is also designated Template:Nowrap (12), which it was given in 1982, and Dione B,<ref>Transactions of the International Astronomical Union, Vol. XVIIIA, 1982 (mentioned in [[#Template:SfnRef|IAUC 3872: Satellites of Jupiter and Saturn]], September 30, 1983</ref> because it is co-orbital with Dione and located in its leading Lagrangian point (Template:L4). It is one of four known trojan moons.

File:Animation of Polydeuces's orbit relative to Saturn and Dione.gif
Animation of Helene's orbit relative to Saturn and Dione
Template:Legend2Template:·Template:Legend2Template:·Template:Legend2Template:·Template:Legend2

ExplorationEdit

Helene was initially observed from Earth in 1980,Template:Sfn and Voyager flybys of Saturn in the early 1980s allowed much closer views. The Cassini–Huygens mission, which went into orbit around Saturn in 2004, provided still better views, and allowed more in-depth analysis of Helene, including views of the surface under different lighting conditions. Some of the closest images of Helene to date are from the Cassini spacecraft's 1800 km flyby on March 3, 2010, and another very successful imaging sequence occurred in June 2011. There were many other approaches over the course of the Cassini mission.

GeologyEdit

Images of Helene taken by the Cassini spacecraft, with resolutions of up to 24 meters per pixel, show a landscape characterized by broad 2–10km scale depressions with interior slopes no greater than 12°.Template:Sfn These basins are likely the decayed remains of old impact craters.Template:Sfn

Thin, elongated km-scale raised grooves trace the slopes of many of Helene's basins, likely representing mass flow features and indicating that the moon is undergoing active geologic processes such as mass-wasting and erosion. Digital elevation models suggest that the grooves have a positive relief of between 50 and 100 meters. Helene has more than 70 craters, while it shows a bimodal appearance—the heavily cratered trailing hemisphere exhibits a crater density ten times greater than the smooth-looking leading hemisphere.Template:Sfn

Simulation models show that the time series of surface activity on Helene is chaotic.Template:Citation needed

Surface materialEdit

Helene's surface material is of relatively high reflectance, suggesting grain sizes between 1 and 100 micrometers. Small craters appear somewhat buried, suggesting recent accretional processes of some sort.

Stress-strain laboratory testing of impact-gardened lunar regolith samples shows that at low packing densities, they behave like Non-Newtonian “Bingham” materials, i.e., having the plastic quality of candle-wax and glaciers. This observation suggests that Helene's snow-like surface material may behave as a non-Newtonian mass flow and could be primarily responsible for the visible flow patterns seen on its low-gravity surface.Template:Sfn

Selected observationsEdit

Mostly raw greyscale images with near infrared or ultraviolet channels.

  • PIA12758 Helene crop.jpg

    Flow-like features on Helene's leading hemisphere (Cassini, January 2011)

  • Helene over Saturn.jpg

    Image of Helene against the backdrop of Saturn's clouds (Cassini, March 3, 2010)

  • N00152209 Helene.jpg

    Helene's Saturn-facing side, lit by saturnshine (Cassini, March 2010)

  • N00152248 Helene.jpg

    Close-up of Helene with Saturn in the background (Cassini, March 2010)

  • Helene rev 127 raw 1.jpg

    Cassini image from March 3, 2010

  • PIA 10544 Helene.jpg

    Cassini orbiter image from November 2008

  • Cassini Helene N00086698 CL.jpg

    Cassini image taken July 2007

  • Helene - Voyager 2.jpg

    Voyager 2 image (August 1981)

NotesEdit

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ReferencesEdit

CitationsEdit

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SourcesEdit

Sources

External linksEdit

Template:Sister project Template:Spoken Wikipedia

Template:Moons of Saturn Template:Saturn

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