Helene 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
The Pic du Midi de Bigorre or simply the Pic du Midi (elevation ) is a mountain in the French Pyrenees. It is the site of the Pic du Midi Observatory.
Pic du Midi Observatory
The Pic du Midi Observatory (french: Observatoire du Pic du Mi ...
, and was designated . In 1988 it was officially named after
Helen of Troy, who was the granddaughter of
Cronus
In Ancient Greek religion and mythology, Cronus, Cronos, or Kronos ( or , from el, Κρόνος, ''Krónos'') was the leader and youngest of the first generation of Titans, the divine descendants of the primordial Gaia (Mother Earth) an ...
(Saturn) in
Greek mythology. Helene is also designated (12), which it was given in 1982, and Dione B,
[Transactions of the International Astronomical Union, Vol. XVIIIA, 1982 (mentioned in IAUC 3872: ''Satellites of Jupiter and Saturn'', September 30, 1983] because it is
co-orbital
In astronomy, a co-orbital configuration is a configuration of two or more astronomical objects (such as asteroids, moons, or planets) orbiting at the same, or very similar, distance from their primary, i.e. they are in a 1:1 mean-motion resonance ...
with
Dione and located in its leading
Lagrangian point
In celestial mechanics, the Lagrange points (; also Lagrangian points or libration points) are points of equilibrium for small-mass objects under the influence of two massive orbiting bodies. Mathematically, this involves the solution of ...
(). It is one of four known
trojan moon
In astronomy, a co-orbital configuration is a configuration of two or more astronomical objects (such as asteroids, moons, or planets) orbiting at the same, or very similar, distance from their primary, i.e. they are in a 1:1 mean-motion resonance ...
s.
Exploration
Helene was initially observed from Earth in 1980, and ''
Voyager'' flybys of Saturn in the early 1980s allowed much closer views. The ''
Cassini–Huygens
''Cassini–Huygens'' ( ), commonly called ''Cassini'', was a space-research mission by NASA, the European Space Agency (ESA), and the Italian Space Agency (ASI) to send a space probe to study the planet Saturn and its system, including its r ...
'' 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 have been 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.
[O. M. Umurhan, A. D. Howard, J. M. Moore, P. Schenk and O. L. White (2015).]
Reconstructing Helene's Surface History - Plastics and Snow
46th Lunar and Planetary Science Conference. Retrieved 2021-02-12.
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.
Selected observations
Mostly raw greyscale images with near infrared or ultraviolet channels.
References
Sources
*
*
*
*
*
* (supporting online material, table S1)
External links
Helene Profileb
NASA's Solar System Exploration see instea
Cassini Solstice Mission: HeleneHelene has two faces��
The Planetary Society
The Planetary Society is an American internationally-active non-governmental nonprofit organization. It is involved in research, public outreach, and political space advocacy for engineering projects related to astronomy, planetary science, an ...
: Helene Mini Atlas—Mar. 11, 2010
Cassini catches Helene��The Planetary Society : Video & Views—Jun. 20, 2011
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Moons of Saturn
Trojan moons
19800301
Moons with a prograde orbit