Helene (moon)

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 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 image (August 1981)

Notes

References

Citations

Sources

Sources

• {{cite journal
• {{cite journal | access-date = 2011-12-23
• {{cite journal | access-date = 2011-12-23
• {{cite journal | access-date = 2011-12-23
• {{cite journal | access-date = 2024-03-04
• {{cite journal
• {{cite journal |doi-access = free
• (supporting online material, table S1)

References

1. John Walker (1839) ''A Critical Pronouncing Dictionary and Expositor of the English Language'';
   also per {{OED. Helena
2. Clarified as ''Helenéan'' in Earle (1841) ''Marathon: and other poems'', p. 76.
3. Transactions of the International Astronomical Union, Vol. XVIIIA, 1982 (mentioned in [[#{{sfnRef. IAUC 3872. IAUC 3872: ''Satellites of Jupiter and Saturn'']], September 30, 1983