Helene was initially observed from Earth in 1980,[8] andVoyager flybys of Saturn in the early 1980s allowed much closer views. TheCassini–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 theCassini 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 theCassini mission.
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 representingmass flow features and indicating that the moon is undergoing active geologic processes such asmass-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]
Helene's surface material is of relatively high reflectance, suggesting grain sizes between 1 and 100 micrometers. Small craters appear somewhat buried, suggesting recentaccretional processes of some sort.
Stress-strain laboratory testing ofimpact-gardened lunar regolith samples shows that at low packing densities, they behave likeNon-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]
