Astronomy:Helene (moon)

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Short description: Trojan moon of Saturn
Helene
Leading hemisphere of Helene - 20110618.jpg
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/ˈhɛlən/[2]
Named afterHelen of Troy (Ἑλένη Helenē)
  • Dione B
  • S/1980 S 6
AdjectivesHelenean /hɛləˈnən/[3]
Orbital characteristics
377600 km[4]
Eccentricity0.007[4]
Orbital period2.736916 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 diameter36.2±0.4 km[5]
Volume24840 km3[lower-alpha 1]
Mass(7.1±0.2)×1015 kg[6]
Mean density0.2926±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
Rotation periodassumed synchronous
Axial tiltzero
Albedo1.67±0.20 (geometric) [7]


Helene /ˈhɛlən/ 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.

Notes

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

References

Citations

  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.
  4. 4.0 4.1 4.2 "Planetary Satellite Mean Orbital Parameters". Jet Propulsion Laboratory. https://ssd.jpl.nasa.gov/sats/elem/sep.html. 
  5. 5.0 5.1 5.2 Thomas & Helfenstein 2020, p. 2.
  6. 6.0 6.1 Jacobson 2022, p. 6.
  7. Verbiscer et al. 2007.
  8. 8.0 8.1 IAUC 3496.
  9. IAUC 4609.
  10. Transactions of the International Astronomical Union, Vol. XVIIIA, 1982 (mentioned in IAUC 3872}}|IAUC 3872: Satellites of Jupiter and Saturn, September 30, 1983
  11. 11.0 11.1 Umurhan et al. 2015.

Sources

Sources

External links