Comparison between Dione' and Enceladus' terrain units

Dione has a diameter of 1122 km and a density of ρ = 1.475 g/cm3. Most of Dione' surface is covered by the heavily cratered terrains, located mainly in the trailing hemisphere and crossed by high-albedo wispy streaks that are likely tectonic features. Enceladus has a mean diameter of 504 km and its surface appears to be completely made up of pure water ice. Form data acquired by Cassini spacecraft it was observed a present-day geologic activity coming from the South polar region. Plumes of micron-sized particles of water ice erupting from this region represent the major source of the E-ring. The VIMS spectrometer is able to acquire hyperspectral cubes in the overall spectral range from 0.3 to 5.1 μm. We select VIMS cubes of Dione and Enceladus in the IR range between 0.8 and 5.1 μm and we normalize all spectra at λ=2.232 µm in different illumination conditions effects. We apply a clustering technique to the spectra of each cube based on the supervised method Spectral Angle Mapper (SAM) to emphasize the presence of spectral units. The endmembers used by the SAM for the classification of each terrain type, were selected applying the unsupervised clustering technique k-means to the cubes with the highest spatial resolution. In particular, k-means technique identified nine endmembers for Dione. To summarize the result of the SAM classification, we projected classified cube's pixels on a Dione's cylindrical map. For both satellites, the infrared spectrum is dominated by the prominent signatures of H2O ice /OH bands at 1.5, 2.0 and 3.0 μm. We conclude that a classification method applied to VIMS hyperspectral data is crucial to understand geochemical processes taking place on the surface of the icy satellites. From our analysis we find that several spectral units on the two satellites are characterized by different values of the spectral indices, such as the water ice bands' depths, which are indicators of the water ice grain size and abundance. Particles of water ice coming from the E-ring, deposits on Dione's leading hemisphere, making this side of brighter than the trailing hemisphere. Therefore, a comparison between Dione and Enceladus spectra is crucial to quantify the effectiveness of this mechanism.