Helene and Dione’s relative surface properties: implications for Helene’s surface evolution

Helene and Dione are two co-orbital moons located within Saturn’s E ring. While Dione is a 562 km-wide spherical moon known for its cratered and relatively dark surface, Helene is smaller (about 17 km wide) with an irregular shape and displays much brighter and smoother terrains on its surface. We present here results from a multi-wavelength (UV to IR) investigation of the Dione-Helene duo, to assess the properties of both surfaces in terms of origin, formation, and evolution. Through the retrieval of the photometric properties and grain sizes on the two moons, we infer the effects of space weathering and other exogenic processes acting on the surfaces.

On January 31, 2011, Helene was observed by all remote-sensing instruments onboard the Cassini spacecraft: UVIS (Ultraviolet Imaging Spectrograph, ISS (Imaging Science Subsystem), VIMS (Visual and Infrared Mapping Spectrometer) and CIRS (Composite Infrared Spectrometer). We report on the photometric characteristics of Helene at wavelengths from 110 nm to 5.2 μm. At all wavelengths, Helene shows signs of a fresher surface less affected by space weathering. Using UVIS data we retrieve a grain size of 4.8 μm. We present the first phase curve of Helene at 610 nm and place our results in the ultraviolet and near-IR in a wider spectral context toward a better understanding of Helene’s surface evolution. Hedman et al. (2020) suggested that either a recent large impact on Helene or an asymmetric flux of E-ring particles could explain the satellite high surface brightness. Results from this study favor the impactor hypothesis to explain Helene’s photometric behavior.