Editing Helene (moon)

{{short description|Trojan moon of Saturn}}
{{Infobox planet
| name = Helene
| image = Leading hemisphere of Helene - 20110618.jpg
| image_scale = 
| caption = High-resolution view of leading hemisphere, showing gullies and apparent dust ([[regolith]]) flows (''[[Cassini–Huygens|Cassini]]'', June 2011)
| discovery_ref = {{sfn|Lecacheux1980}}
| discoverer = [[Pierre Laques|P. Laques]]<br />[[Jean Lecacheux|J. Lecacheux]]
| discovered = March 1, 1980
| discovery_site = [[Pic du Midi Observatory]]
| mpc_name = Saturn XII
| alt_names = {{plainlist |
* Dione B
* S/1980 S 6
  }}
| pronounced = {{IPAc-en|ˈ|h|ɛ|l|ə|n|iː}}<ref>John Walker (1839) ''A Critical Pronouncing Dictionary and Expositor of the English Language'';<br>also per {{OED|Helena}}</ref>
| named_after = [[Helen of Troy]] (Ἑλένη ''Helenē'')
| adjective = Helenean {{IPAc-en|h|ɛ|l|ə|ˈ|n|iː|ə|n}}<ref>Clarified as ''Helenéan'' in Earle (1841) ''Marathon: and other poems'', p. 76.</ref>
| semimajor = {{val|377600|u=km}}<ref name="jpl-satelem"/>
| eccentricity = {{val|0.007}}<ref name="jpl-satelem"/>
| period = {{val|2.736916|u=d}}<ref name="jpl-satelem"/>
| inclination = 0.199° (to Saturn's equator)
| satellite_of = [[Saturn]]
| group = [[Trojan moon|L<sub>4</sub> Dione trojan]]
| dimensions = {{val|45.2|x|39.2|x|26.6}}&nbsp;km<br/>(±&nbsp;{{val|0.4|x|0.6|x|0.4}}&nbsp;km){{sfn|Thomas|Helfenstein|2020|p=2}}
| mean_diameter = {{val|36.2|0.4|u=km}}{{sfn|Thomas|Helfenstein|2020|p=2}}
| volume = {{val|24840|u=km3}}{{efn|Calculated from Helene's volume-equivalent sphere radius of {{val|18.1|0.2|u=km}} given by Thomas et al. (2020){{sfn|Thomas|Helfenstein|2020|p=2}}}}
| mass = {{val|7.1|0.2|e=15|u=kg}}{{sfn|Jacobson|2022|p=6}}
| density = {{val|0.2926|0.0217|u=g/cm3}}{{sfn|Jacobson|2022|p=6}}
| surface_grav = {{Gr|7.1e-6|22.6|4}}&nbsp;m/s<sup>2</sup> at longest axis<br/>to {{Gr|7.1e-6|13.3|4}}&nbsp;m/s<sup>2</sup> at poles
| escape_velocity = {{V2|7.1e-6|22.6|4}}&nbsp;km/s at longest axis<br/>to {{V2|7.1e-6|13.3|4}}&nbsp;km/s at poles<!-- V2 template uses mass in terms of e+21 kg, radius in terms of km, and number of decimal places -->
| rotation = assumed [[synchronous rotation|synchronous]]
| albedo = {{val|1.67|0.20}} {{nowrap|([[geometric albedo|geometric]]){{sfn|Verbiscer|French|Showalter|Helfenstein|2007}}}}
| axial_tilt = zero
}}

'''Helene''' {{IPAc-en|ˈ|h|ɛ|l|ə|n|iː}} is a [[natural satellite|moon]] of [[Saturn]]. It was discovered by [[Pierre Laques]] and [[Jean Lecacheux]] in 1980 from ground-based observations at [[Pic du Midi Observatory]],{{sfn|Lecacheux1980}} and was designated '''{{nowrap|S/1980 S 6}}'''.{{sfn|IAUC 3496}} In 1988 it was officially named after [[Helen of Troy]], who was the granddaughter of [[Cronus]] (Saturn) in [[Greek mythology]].{{sfn|IAUC 4609}} Helene is also designated '''{{nowrap|Saturn XII}}''' (12), which it was given in 1982, and '''Dione B''',<ref>Transactions of the International Astronomical Union, Vol. XVIIIA, 1982 (mentioned in [[#{{sfnRef|IAUC 3872}}|IAUC 3872: ''Satellites of Jupiter and Saturn'']], September 30, 1983</ref> because it is [[co-orbital]] with [[Dione (moon)|Dione]] and located in its leading [[Lagrangian point]] ({{L4}}). It is one of four known [[trojan moon]]s.
[[File:Animation of Polydeuces's orbit relative to Saturn and Dione.gif|thumb|left|Animation of Helene's orbit relative to Saturn and Dione<br>{{legend2|Magenta|Polydeuces }}{{·}}{{legend2|Cyan|Helene}}{{·}}{{legend2|Lime|Dione}}{{·}}{{legend2|DarkOrange|Saturn}}]]

== Exploration ==

Helene was initially observed from Earth in 1980,{{sfn|IAUC 3496}} and ''[[Voyager program|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&nbsp;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.

== Geology ==

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°.{{sfn|Hirata et al. 2014}} These basins are likely the decayed remains of old impact craters.{{sfn|Umurhan|Howard|Moore|Schenk|2015}}

Thin, elongated km-scale raised grooves trace the slopes of many of Helene's basins, likely representing [[Mass wasting|mass flow]] features and indicating that the moon is undergoing active geologic processes such as [[Mass wasting|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.{{sfn|Hirata et al. 2014}}

Simulation models show that the time series of surface activity on Helene is chaotic.{{citation needed|date=February 2024}}

== Surface material ==

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

Stress-strain laboratory testing of [[Impact gardening|impact-gardened]] lunar regolith samples shows that at low packing densities, they behave like [[Non-Newtonian fluid|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.{{sfn|Umurhan|Howard|Moore|Schenk|2015}}

== Selected observations ==

Mostly raw greyscale images with near infrared or ultraviolet channels.

<gallery heights=150 widths=150>
File:PIA12758 Helene crop.jpg|Flow-like features on Helene's leading hemisphere (''Cassini'', January 2011)
File:Helene over Saturn.jpg|Image of Helene against the backdrop of Saturn's clouds (''Cassini'', March 3, 2010)
File:N00152209 Helene.jpg|Helene's Saturn-facing side, lit by [[Planetshine|saturnshine]] (''Cassini'', March 2010)
File:N00152248 Helene.jpg|Close-up of Helene with Saturn in the background (''Cassini'', March 2010)
File:Helene rev 127 raw 1.jpg|''[[Cassini–Huygens|Cassini]]'' image from March 3, 2010
File:PIA 10544 Helene.jpg|''Cassini'' orbiter image from November 2008
File:Cassini Helene N00086698 CL.jpg|''Cassini'' image taken July 2007
File:Helene - Voyager 2.jpg|''[[Voyager 2]]'' image (August 1981)
</gallery>

== Notes ==
{{notelist}}

== References ==
=== Citations ===
{{reflist|20em|refs=

<ref name="jpl-satelem">{{cite web
  |title      = Planetary Satellite Mean Orbital Parameters
  |url        = https://ssd.jpl.nasa.gov/sats/elem/sep.html
  |publisher  = [[Jet Propulsion Laboratory]]
  |access-date = 5 June 2023}}</ref>

}}

=== Sources ===

'''Sources'''
* {{cite journal
  | last = Lecacheux
  | first = Jean.
  | title = A new satellite of Saturn: Dione B
  | date = July 1980
  | journal = Icarus
  | volume =  43| issue = 1
  | pages = 111–115
 | doi = 10.1016/0019-1035(80)90093-7
  |bibcode = 1980Icar...43..111L }}
* {{cite journal
  | last = Marsden
  | first = Brian G.
  | title = Satellites of Saturn
  | date = July 31, 1980
  | format = discovery
  | journal = IAU Circular
  | volume = 3496
  | url = http://www.cbat.eps.harvard.edu/iauc/03400/03496.html
  | access-date = 2011-12-23
  | ref = {{sfnRef|IAUC 3496}}
  }}
* {{cite journal
  | last = Marsden
  | first = Brian G.
  | title = Satellites of Jupiter and Saturn
  | date = September 30, 1983
  | journal = IAU Circular
  | volume = 3872
  | url = http://www.cbat.eps.harvard.edu/iauc/03800/03872.html
  | access-date = 2011-12-23
  | ref = {{sfnRef|IAUC 3872}}
  }}
* {{cite journal
  | last = Marsden
  | first = Brian G.
  | title = Satellites of Saturn and Uranus
  | date = June 8, 1988
  | format = naming
  | journal = IAU Circular
  | volume = 4609
  | url = http://www.cbat.eps.harvard.edu/iauc/04600/04609.html
  | access-date = 2011-12-23
  | ref = {{sfnRef|IAUC 4609}}
  }}
* {{cite journal| doi=10.1016/j.icarus.2010.01.025| last1=Thomas| first1=P. C.| date=July 2010| title=Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission| journal=Icarus| volume=208| issue=1| pages=395–401| url=http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf| bibcode=2010Icar..208..395T| access-date=2015-09-04| archive-date=2018-12-23| archive-url=https://web.archive.org/web/20181223003125/http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf| url-status=dead}}
* {{cite journal
  | last = Hirata
  | first = N.
  | title = Particle deposition on the saturnian satellites from ephemeral cryovolcanism on Enceladus
  | date = May 27, 2014
  | journal = Geophysical Research Letters
  | volume = 41
  | issue = 12
 | pages = 4135–4141
 | doi = 10.1002/2014GL060470
 | arxiv = 2205.11265
 | bibcode = 2014GeoRL..41.4135H
 | url = https://doi.org/10.1002/2014GL060470
  | access-date = 2024-03-04
  | ref = {{sfnRef|Hirata et al. 2014}}
  }}
* {{cite journal
  |first1 = P. C. |last1 = Thomas
  |first2 = P. |last2 = Helfenstein
  |title      = The small inner satellites of Saturn: Shapes, structures and some implications
  |journal    = Icarus
  |date       = July 2020
  |volume     = 344
  |id         = 113355
  |pages      = 20
  |doi        = 10.1016/j.icarus.2019.06.016
  |bibcode    = 2020Icar..34413355T|s2cid = 197474587 }}
* {{cite journal
  |first1 = Robert A. |last1 = Jacobson
  |title      = The Orbits of the Main Saturnian Satellites, the Saturnian System Gravity Field, and the Orientation of Saturn's Pole
  |journal    = The Astronomical Journal
  |date       = November 2022
  |volume     = 164
  |issue      = 5
  |id         = 199
  |pages      = 19
  |doi-access = free
  |doi        = 10.3847/1538-3881/ac90c9
  |bibcode    = 2022AJ....164..199J}}
* {{cite conference |last1=Umurhan |first1=O. M. |last2=Howard |first2=A. D. |last3=Moore |first3=J. M. |last4=Schenk |first4=P. |last5=White |first5=O. L. |date=2015 |title=Reconstructing Helene's Surface History – Plastics and Snow |conference=46th Lunar and Planetary Science Conference |url=https://www.hou.usra.edu/meetings/lpsc2015/pdf/2400.pdf |access-date=2021-02-12}}
* {{cite journal| doi=10.1126/science.1134681| last1=Verbiscer| first1=A.| last2=French| first2=R.| last3=Showalter| first3=M.| last4=Helfenstein| first4=P.| title=Enceladus: Cosmic Graffiti Artist Caught in the Act| journal=Science| volume=315| issue=5813| page=815| date=9 February 2007| pmid=17289992| bibcode=2007Sci...315..815V| s2cid=21932253}} (supporting online material, table S1)
== External links ==

{{Commons category|Helene (moon)|Helene}}
{{Spoken Wikipedia|Helene (moon).ogg|date=2010-02-06}}
* [https://web.archive.org/web/20151120054640/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sat_Helene Helene Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]; see instead [https://web.archive.org/web/20140421054529/http://saturn.jpl.nasa.gov/science/moons/helene/ Cassini Solstice Mission: Helene]
* [http://www.planetary.org/explore/topics/saturn/helene.html The Planetary Society: Helene]
* [http://www.planetary.org/blog/article/00002385/ Helene has two faces] {{Webarchive|url=https://web.archive.org/web/20100616022007/http://planetary.org/blog/article/00002385/ |date=2010-06-16 }}—[[The Planetary Society]] : Helene Mini Atlas—Mar. 11, 2010
* [http://www.planetary.org/blog/article/00003072/ Cassini catches Helene] {{Webarchive|url=https://web.archive.org/web/20120401234936/http://www.planetary.org/blog/article/00003072/ |date=2012-04-01 }}—The Planetary Society : Video & Views—Jun. 20, 2011

{{Moons of Saturn|state=uncollapsed}}
{{Saturn}}

{{DEFAULTSORT:Helene (Moon)}}
[[Category:Moons of Saturn]]
[[Category:Trojan moons]]
[[Category:Astronomical objects discovered in 1980|19800301]]
[[Category:Moons with a prograde orbit]]
[[Category:Helen of Troy]]