Helene is a
moon
The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
of
Saturn
Saturn is the sixth planet from the Sun and the second largest in the Solar System, after Jupiter. It is a gas giant, with an average radius of about 9 times that of Earth. It has an eighth the average density of Earth, but is over 95 tim ...
. It was discovered by
Pierre Laques and
Jean Lecacheux in 1980 from ground-based observations at
Pic du Midi Observatory, and was designated . In 1988 it was officially named after
Helen of Troy
Helen (), also known as Helen of Troy, or Helen of Sparta, and in Latin as Helena, was a figure in Greek mythology said to have been the most beautiful woman in the world. She was believed to have been the daughter of Zeus and Leda (mythology), ...
, who was the granddaughter of
Cronus
In ancient Greek religion and Greek mythology, mythology, Cronus, Cronos, or Kronos ( or ; ) was the leader and youngest of the Titans, the children of Gaia (Earth) and Uranus (mythology), Uranus (Sky). He overthrew his father and ruled dur ...
(Saturn) in
Greek mythology
Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of ancient Greek folklore, today absorbed alongside Roman mythology into the broader designation of classical mythology. These stories conc ...
. 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 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 (mechanics), equilibrium for small-mass objects under the gravity, gravitational influence of two massive orbit, orbiting b ...
(). It is one of four known
trojan moons.
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, 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, i ...
'' 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 24 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.
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.
Simulation models show that the time series of surface activity on Helene is chaotic.
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
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.
Selected observations
Mostly raw greyscale images with near infrared or ultraviolet channels.
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 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'' 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
''Voyager 2'' is a space probe launched by NASA on August 20, 1977, as a part of the Voyager program. It was launched on a trajectory towards the gas giants (Jupiter and Saturn) and enabled further encounters with the ice giants (Uranus and ...
'' image (August 1981)
Notes
References
Citations
Sources
Sources
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* (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, a ...
: Helene Mini Atlas—Mar. 11, 2010
Cassini catches Helene—The Planetary Society : Video & Views—Jun. 20, 2011
{{DEFAULTSORT:Helene (Moon)
Moons of Saturn
Trojan moons
19800301
Moons with a prograde orbit
Helen of Troy