Reflectance spectra of Titan tholin between 7000 and 10 cm-1 Interpretation of Cassini/CIRS observation of Saturn's satellite Phoebe

Context. Laboratory experiments provide a great support to astronomical studies in that they are the most suited technique to reproduce, model and interpret the observational data. Aims. We investigate the spectroscopic properties of particulate and flat slab of tholin samples in a wide MIR and FIR spectral range at cryogenic temperature to contribute to the interpretation of the observation of Saturn satellite Phoebe obtained with the Cassini CIRS instrument. Methods. Reflectance spectra of Titan tholin were obtained in the 7000-10 cm(-1) spectral region by a Fourier transform spectrometer operating in vacuum. Several optical setups were used to cover this wide spectral range. Specular and diffuse reflectance spectra were obtained. A cryostat was interfaced to the spectrometer for reflectance measurements at low temperatures. It was cooled by a continuum flux of cryogenic fluid from about 300 to 72 K. Results. It is shown that powder and slab tholin have a different reflectance that depends on geometrical factors and wavelength ranges. The emissivity of Phoebe is reproduced by flat slab tholin covered by a thin layer of water ice. Conclusions. A considerable amount of compact smooth millimeter-size carbonaceous compounds are present on the Phoebe satellite covered with water ice and tholin regolith. This confirms a surface highly processed by small object impacts and a peculiar nature of Phoebe with respect to other Saturn satellites. It could be a primitive Kuiper belt object captured by Saturn that contains an high amount of HCN-like polymers active in prebiotic chemistry.