Surface Composition of the Icy Satellites of Saturn from an Ultraviolet Perspective

The structure and composition of the surfaces of icy satellites are affected by primarily exogenic processes, such as E ring grain bombardment and plasma and electron interactions. We propose to investigate the effects of these processes on the surfaces of the inner large icy Saturnian satellites (Mimas, Tethys, Enceladus, Dione and Rhea) through photometric and compositional analyses in the UV region (110-190nm), using mainly data from the Cassini Ultraviolet Imaging Spectrograph (UVIS) instrument. This wavelength region is particularly sensitive to radiation products that are the results of weathering processes. The investigation will provide critical insight into the evolution of the moons’ regoliths and a better understanding of the interactions between the satellites, the E-ring and Saturn’s magnetosphere. In addition, results will be placed in a broader context by including previously-published data from the Hubble Space Telescope (HST). The updated photometric models applied to these older datasets will be complementary to the interpretation of the UVIS data. The key objective is to explore various candidate species for composition and water ice contamination, to produce a broad composition map for each satellite, and assess their potential for endogenic activities.

We propose to start our investigation by analyzing the photometric properties of the five inner large icy Saturnian satellites using the UVIS data. This first modeling step will provide input parameters for the spectral modeling task. Previously published phase curves of Mimas, Tethys and Dione at 180 nm will be updated by adding the entirety of the Cassini data, from the beginning to the end of mission and the first 180 nm phase curves for Enceladus and Rhea will be added. The Hapke photometric model will be used to retrieve the albedo as well as the scattering and texture (roughness and porosity) properties of each satellites. The composition analysis of the icy surfaces will be our second and main step of the project. We will investigate candidate species such as ozone (O3), hydrogen peroxide (H2O2), ammonia (NH3), iron-rich nanograins, and Pluto tholins, which all have absorption features at the FUV wavelengths. However, a recent report of hydrazine monohydrate as principal contaminant of the water ice at Rhea will have us prioritizing this contaminant among the others, for all satellites. Our model will also include an investigation of the water ice grain size, as the UV spectra of the icy satellites exhibit a water ice absorption edge centered near 165 nm, the position of which is sensitive to grain size. The photometric and composition analysis will be first performed on disk-integrated spectra and then on disk-
resolved spectra on specific locations of interest. At each step of the project, data from HST will be compared with the Cassini-UVIS observations, in order to perform comparative planetology across the Saturnian icy satellites and across UV and visible wavelengths.