Located within Saturn’s diffuse E ring, the icy satellites Helene and Dione share the same orbit, 6.26 Saturn radii away from the giant planet. Helene is one of Dione’s two Trojan moons and is situated in the leading Lagrangian point (L4) of Dione’s orbit. While Dione is a spherical moon of 562 km diameter known for its cratered and UV dark surface, Helene is irregular in shape, approximately 17 km in diameter, and displays much brighter and smoother terrains on parts of its surface. We present here results of a multi-wavelength (UV to IR) investigation of the Dione-Helene duo, to assess the properties of both surfaces in term of origin, formation, and evolution. Through the retrieval of the photometric properties of the two moons we infer the effects of space weathering and other exogenic processes acting on the surfaces, which can be tied, in return, to the relative surface ages of the two satellites. We aim to provide a complete evaluation of the Dione and Helene surfaces across the UV-IR spectrum.
Until now, most investigations have focused only on one wavelength domain, telling only part of the story. In this report, we used the synergy of four of the Cassini instruments: UVIS (Ultraviolet Imaging Spectrograph), ISS (Imaging Science Subsystem), VIMS (Visual and Infrared Mapping Spectrometer) and CIRS (Composite Infrared Spectrometer). Using composite disk-integrated spectra of both satellites to conduct photometric modeling, we provide estimates for porosity (>50%) and material compaction at Dione and Helene, across multiple layers of the surface, as each wavelength probes a different layer of the surface. Searches for correlations of basic properties (albedo, scattering properties, texture, grain size, composition, porosity, thermal properties) between Dione and Helene indicate a larger amount of fresh water-ice at Helene and differences in grain sizes across terrains. Retrieved Helene daytime surface temperatures, of 83.3 ± 0.9 K and 88.8 ± 0.8K, did not permit constraints on thermal inertia, but provided an upper limit of 0.75 for Helene’s albedo in the longitude range 223-288 deg and a loose constraint on albedo between 0.25 and 0.70 for longitudes 180-238 deg.