Astronomy:Helene (moon)

Helene
	High-resolution view of leading hemisphere, showing gullies and apparent dust (regolith) flows (Cassini, June 2011)
Discovery
Discovered by	P. Laques; J. Lecacheux
Discovery site	Pic du Midi Observatory
Discovery date	March 1, 1980
Designations
Designation	Saturn XII
Pronunciation	/ˈhɛləniː/
Named after	Helen of Troy (Ἑλένη Helenē)
Alternative names	Dione B; S/1980 S 6;
Adjectives	Helenean /hɛləˈniːən/
Orbital characteristics
Semi-major axis	377600 km
Eccentricity	0.007
Orbital period	2.736916 d
Inclination	0.199° (to Saturn's equator)
Satellite of	Saturn
Group	L4 Dione trojan
Physical characteristics
Dimensions	45.2 × 39.2 × 26.6 km; (± 0.4 × 0.6 × 0.4 km)
Mean diameter	36.2±0.4 km
Volume	24840 km3
Mass	(7.1±0.2)×1015 kg
Mean density	0.2926±0.0217 g/cm3
surface gravity	0.0009 m/s2 at longest axis; to 0.0027 m/s2 at poles
escape velocity	0.0065 km/s at longest axis; to 0.0084 km/s at poles
Rotation period	assumed synchronous
Axial tilt	zero
Albedo	1.67±0.20 (geometric)

Short description: Trojan moon of Saturn

Helene /ˈhɛləniː/ 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,^[1] and was designated S/1980 S 6.^[8] In 1988 it was officially named after Helen of Troy, who was the granddaughter of Cronus (Saturn) in Greek mythology.^[9] Helene is also designated Saturn XII (12), which it was given in 1982, and Dione B,^[10] because it is co-orbital with Dione and located in its leading Lagrangian point (L₄). It is one of four known trojan moons.

Animation of Helene's orbit relative to Saturn and Dione
Polydeuces · Helene · Dione · Saturn

Exploration

Helene was initially observed from Earth in 1980,^[8] 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.

Geology

Images of Helene taken by the Cassini spacecraft, with resolutions of up to 42 meters per pixel, show a landscape characterized by broad 2–10km scale depressions with interior slopes no greater than 12°. These basins are likely the decayed remains of old impact craters.^[11]

Thin, elongated km-scale raised grooves trace the slopes of many of Helene's basins, and likely represent mass flow features, 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.

Simulation models show that the time series of surface activity on Helene is chaotic.

Surface material

Helene's surface material is of a 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 show 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.^[11]

Selected observations

Mostly raw greyscale images with near infrared or ultraviolet channels.

Flow-like features on Helene's leading hemisphere (Cassini, January 2011)
Image of Helene against the backdrop of Saturn's clouds (Cassini, March 3, 2010)
Helene's Saturn-facing side, lit by saturnshine (Cassini, March 2010)
Close-up of Helene with Saturn in the background (Cassini, March 2010)
Cassini image from March 3, 2010
Cassini orbiter image from November 2008
Cassini image taken July 2007
Voyager 2 image (August 1981)

Notes

↑ Calculated from Helene's volume-equivalent sphere radius of 18.1±0.2 km given by Thomas et al. (2020)^[5]

References

Citations

↑ ^1.0 ^1.1 Lecacheux1980.
↑ John Walker (1839) A Critical Pronouncing Dictionary and Expositor of the English Language;
also per Helena (3rd ed.), Oxford University Press, September 2005, http://oed.com/search?searchType=dictionary&q=Helena (Subscription or UK public library membership required.)
↑ Clarified as Helenéan in Earle (1841) Marathon: and other poems, p. 76.
↑ ^4.0 ^4.1 ^4.2 "Planetary Satellite Mean Orbital Parameters". Jet Propulsion Laboratory. https://ssd.jpl.nasa.gov/sats/elem/sep.html.
↑ ^5.0 ^5.1 ^5.2 Thomas & Helfenstein 2020, p. 2.
↑ ^6.0 ^6.1 Jacobson 2022, p. 6.
↑ Verbiscer et al. 2007.
↑ ^8.0 ^8.1 IAUC 3496.
↑ IAUC 4609.
↑ Transactions of the International Astronomical Union, Vol. XVIIIA, 1982 (mentioned in IAUC 3872}}|IAUC 3872: Satellites of Jupiter and Saturn, September 30, 1983
↑ ^11.0 ^11.1 Umurhan et al. 2015.

Sources

Sources

Lecacheux, Jean. (July 1980). "A new satellite of Saturn: Dione B". Icarus 43 (1): 111–115. doi:10.1016/0019-1035(80)90093-7. Bibcode: 1980Icar...43..111L.
Marsden, Brian G. (July 31, 1980). "Satellites of Saturn" (discovery). IAU Circular 3496. http://www.cbat.eps.harvard.edu/iauc/03400/03496.html. Retrieved 2011-12-23.
Marsden, Brian G. (September 30, 1983). "Satellites of Jupiter and Saturn". IAU Circular 3872. http://www.cbat.eps.harvard.edu/iauc/03800/03872.html. Retrieved 2011-12-23.
Marsden, Brian G. (June 8, 1988). "Satellites of Saturn and Uranus" (naming). IAU Circular 4609. http://www.cbat.eps.harvard.edu/iauc/04600/04609.html. Retrieved 2011-12-23.
Thomas, P. C. (July 2010). "Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission". Icarus 208 (1): 395–401. doi:10.1016/j.icarus.2010.01.025. Bibcode: 2010Icar..208..395T. http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf. Retrieved 2015-09-04.
Thomas, P. C.; Helfenstein, P. (July 2020). "The small inner satellites of Saturn: Shapes, structures and some implications". Icarus 344: 20. doi:10.1016/j.icarus.2019.06.016. 113355. Bibcode: 2020Icar..34413355T.
Jacobson, Robert A. (November 2022). "The Orbits of the Main Saturnian Satellites, the Saturnian System Gravity Field, and the Orientation of Saturn's Pole". The Astronomical Journal 164 (5): 19. doi:10.3847/1538-3881/ac90c9. 199. Bibcode: 2022AJ....164..199J. </ref>
Umurhan, O. M.; Howard, A. D.; Moore, J. M.; Schenk, P.; White, O. L. (2015). "Reconstructing Helene's Surface History – Plastics and Snow". 46th Lunar and Planetary Science Conference. https://www.hou.usra.edu/meetings/lpsc2015/pdf/2400.pdf. Retrieved 2021-02-12. </ref>
Verbiscer, A.; French, R.; Showalter, M.; Helfenstein, P. (9 February 2007). "Enceladus: Cosmic Graffiti Artist Caught in the Act". Science 315 (5813): 815. doi:10.1126/science.1134681. PMID 17289992. Bibcode: 2007Sci...315..815V. (supporting online material, table S1)

External links

Helene Profile by NASA's Solar System Exploration; see instead Cassini Solstice Mission: Helene
The Planetary Society: Helene
Helene has two faces —The Planetary Society : Helene Mini Atlas—Mar. 11, 2010
Cassini catches Helene —The Planetary Society : Video & Views—Jun. 20, 2011

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Original source: https://en.wikipedia.org/wiki/Helene (moon). Read more

[6] Calculated from Helene's volume-equivalent sphere radius of 18.1±0.2 km given by Thomas et al. (2020)^[5]

[FOOTNOTELecacheux1980-1] 1.0 ^1.1 Lecacheux1980.

[2] John Walker (1839) A Critical Pronouncing Dictionary and Expositor of the English Language;
also per Helena (3rd ed.), Oxford University Press, September 2005, http://oed.com/search?searchType=dictionary&q=Helena (Subscription or UK public library membership required.)

[3] Clarified as Helenéan in Earle (1841) Marathon: and other poems, p. 76.

[jpl-satelem-4] 4.0 ^4.1 ^4.2 "Planetary Satellite Mean Orbital Parameters". Jet Propulsion Laboratory. https://ssd.jpl.nasa.gov/sats/elem/sep.html.

[FOOTNOTEThomasHelfenstein20202-5] 5.0 ^5.1 ^5.2 Thomas & Helfenstein 2020, p. 2.

[FOOTNOTEJacobson20226-7] 6.0 ^6.1 Jacobson 2022, p. 6.

[FOOTNOTEVerbiscerFrenchShowalterHelfenstein2007-8] Verbiscer et al. 2007.

[FOOTNOTEIAUC_3496-9] 8.0 ^8.1 IAUC 3496.

[FOOTNOTEIAUC_4609-10] IAUC 4609.

[11] Transactions of the International Astronomical Union, Vol. XVIIIA, 1982 (mentioned in IAUC 3872}}|IAUC 3872: Satellites of Jupiter and Saturn, September 30, 1983

[FOOTNOTEUmurhanHowardMooreSchenk2015-12] 11.0 ^11.1 Umurhan et al. 2015.

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v t e Moons of Saturn
Listed in approximately increasing distance from Saturn.
Ring shepherds	S/2009 S 1 Ring moonlets Pan Daphnis Atlas Prometheus Pandora
Co-orbitals	Epimetheus Janus
G Ring	Aegaeon
Alkyonides	Methone Anthe Pallene
Inner large (with trojans)	Mimas Enceladus Tethys Telesto Calypso Dione Helene Polydeuces
Outer large	Rhea Titan Hyperion Iapetus
Inuit group	Kiviuq Ijiraq Paaliaq S/2004 S 29 S/2004 S 31 Siarnaq Tarqeq
Norse group	Phoebe Skathi S/2004 S 37 S/2007 S 2 Skoll S/2004 S 13(?) Greip Hyrrokkin Jarnsaxa Mundilfari S/2006 S 1 S/2004 S 17(?) Bergelmir Narvi S/2004 S 20 Suttungr Hati S/2004 S 12(?) S/2004 S 27 Farbauti Thrymr S/2004 S 30 Aegir S/2007 S 3(?) Bestla S/2004 S 7(?) S/2004 S 22(?) S/2004 S 23(?) S/2004 S 25(?) S/2004 S 32(?) S/2006 S 3 S/2004 S 38(?) Fenrir S/2004 S 28(?) Surtur Kari S/2004 S 35(?) Ymir S/2004 S 21(?) Loge S/2004 S 36(?) S/2004 S 39(?) S/2004 S 33(?) S/2004 S 34(?) Fornjot S/2004 S 26(?)
Gallic group	Albiorix Bebhionn Erriapus Tarvos
Prograde outer satellites	S/2004 S 24
Rings of Saturn Cassini–Huygens Themis Chiron S/2004 S 6 S/2004 S 4 S/2004 S 3 In fiction

v t e Saturn
Outline of Saturn
Geography	Dragon Storm Great White Spot Hexagon Magnetosphere Rings
Moons	S/2009 S 1 Ring moonlets Pan Daphnis Atlas Prometheus S/2004 S 6 S/2004 S 4 S/2004 S 3 Pandora Epimetheus Janus Aegaeon Mimas Methone Anthe Pallene Enceladus Tethys Telesto Calypso Dione Helene Polydeuces Rhea Titan Hyperion Iapetus Kiviuq Ijiraq Phoebe Paaliaq Skathi S/2004 S 37 Albiorix S/2007 S 2 S/2004 S 29 Bebhionn Erriapus Skoll S/2004 S 31 Siarnaq Tarqeq S/2004 S 13 Greip Hyrrokkin Jarnsaxa Tarvos Mundilfari S/2006 S 1 S/2004 S 17 Bergelmir Narvi S/2004 S 20 Suttungr Hati S/2004 S 12 S/2004 S 27 Farbauti Thrymr S/2004 S 30 Aegir S/2007 S 3 Bestla S/2004 S 7 S/2004 S 22 S/2004 S 23 S/2004 S 25 S/2004 S 32 S/2006 S 3 S/2004 S 38 Fenrir S/2004 S 28 Surtur Kari S/2004 S 35 Ymir S/2004 S 21 S/2004 S 24 Loge S/2004 S 36 S/2004 S 39 S/2004 S 33 S/2004 S 34 Fornjot S/2004 S 26
Astronomy	Delta Octantis Saturn-crossing minor planets
Exploration	Cassini–Huygens (Huygens) timeline retirement Pioneer 11 Voyager program Voyager 1 Voyager 2
Related	Fiction Moons The Day the Earth Smiled In Saturn's Rings (2018 documentary)

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Discovery ^[1]
High-resolution view of leading hemisphere, showing gullies and apparent dust (regolith) flows (Cassini, June 2011)
Discovered by	P. Laques J. Lecacheux
Discovery site	Pic du Midi Observatory
Discovery date	March 1, 1980
Designations
Designation	Saturn XII
Pronunciation	/ˈhɛləniː/^[2]
Named after	Helen of Troy (Ἑλένη Helenē)
Alternative names	Dione B S/1980 S 6
Adjectives	Helenean /hɛləˈniːən/^[3]
Orbital characteristics
Semi-major axis	377600 km^[4]
Eccentricity	0.007^[4]
Orbital period	2.736916 d^[4]
Inclination	0.199° (to Saturn's equator)
Satellite of	Saturn
Group	L₄ Dione trojan
Physical characteristics
Dimensions	45.2 × 39.2 × 26.6 km (± 0.4 × 0.6 × 0.4 km)^[5]
Mean diameter	36.2±0.4 km^[5]
Volume	24840 km³^{[lower-alpha 1]}
Mass	(7.1±0.2)×10¹⁵ kg^[6]
Mean density	0.2926±0.0217 g/cm³^[6]
surface gravity	0.0009 m/s² at longest axis to 0.0027 m/s² at poles
escape velocity	0.0065 km/s at longest axis to 0.0084 km/s at poles
Rotation period	assumed synchronous
Axial tilt	zero
Albedo	1.67±0.20 (geometric) ^[7]

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Astronomy:Helene (moon)

Exploration

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