Helene (moon)

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Helene
High-resolution view of leading hemisphere, showing gullies and apparent dust (regolith) flows (Cassini, June 2011)
Discovery[1]
Discovered byP. Laques
J. Lecacheux
Discovery sitePic du Midi Observatory
Discovery dateMarch 1, 1980
Designations
Designation
Saturn XII
Pronunciation/ˈhɛləniː/[2]
Named after
Helen of Troy (Ἑλένη Helenē)
  • Dione B
  • S/1980 S 6
AdjectivesHelenean /hɛləˈniːən/[3]
Orbital characteristics
377600 km[4]
Eccentricity0.007[4]
2.736916 d[4]
Inclination0.199° (to Saturn's equator)
Satellite ofSaturn
GroupL4 Dione trojan
Physical characteristics
Dimensions45.2 × 39.2 × 26.6 km
(± 0.4 × 0.6 × 0.4 km)[5]
Mean diameter
36.2±0.4 km[5]
Volume24840 km3[a]
Mass(7.1±0.2)×1015 kg[6]
Mean density
0.2926±0.0217 g/cm3[6]
0.0009 m/s2 at longest axis
to 0.0027 m/s2 at poles
0.0065 km/s at longest axis
to 0.0084 km/s at poles
assumed synchronous
zero
Albedo1.67±0.20 (geometric)[7]

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[edit]

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[edit]

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°.[11] These basins are likely the decayed remains of old impact craters.[12]

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.[11]

Simulation models show that the time series of surface activity on Helene is chaotic.[citation needed]

Surface material[edit]

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.[12]

Selected observations[edit]

Mostly raw greyscale images with near infrared or ultraviolet channels.

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

Notes[edit]

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

References[edit]

Citations[edit]

  1. ^ a b Lecacheux1980.
  2. ^ John Walker (1839) A Critical Pronouncing Dictionary and Expositor of the English Language;
    also per "Helena". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
  3. ^ Clarified as Helenéan in Earle (1841) Marathon: and other poems, p. 76.
  4. ^ a b c "Planetary Satellite Mean Orbital Parameters". Jet Propulsion Laboratory. Retrieved 5 June 2023.
  5. ^ a b c Thomas & Helfenstein 2020, p. 2.
  6. ^ a b Jacobson 2022, p. 6.
  7. ^ Verbiscer et al. 2007.
  8. ^ a b IAUC 3496.
  9. ^ IAUC 4609.
  10. ^ Transactions of the International Astronomical Union, Vol. XVIIIA, 1982 (mentioned in IAUC 3872: Satellites of Jupiter and Saturn, September 30, 1983
  11. ^ a b Hirata et al. 2014.
  12. ^ a b Umurhan et al. 2015.

Sources[edit]

Sources

External links[edit]

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