Modeling the Spectrophotometric Properties of 67/P Churyumov-Gerasimenko as Seen by VIRTIS-M Onboard Rosetta

Abstract:

The Rosetta spacecraft is investigating comet 67P/Churyumov-Gerasimenko since mid-July 2014. VIRTIS-M, the Visible InfraRed Thermal Imaging Spectrometer, onboard the orbiter (Coradini et al., 2007) has acquired data of the nucleus’ surface in the 0.25-5.1 µm spectral range.

We discuss the modeling of the radiance and reflectance spectra by applying the Hapke radiative transfer model with the aim to characterize the compositional properties of the surface in terms of refractory material, water and carbon dioxide ices, and other minor species abundances. Moreover this method allows us to infer the grain size distribution across the surface.

The best retrieval is achieved by leaving free the parameters of the model in order to minimize the residuals: (signal – model) / noise. To achieve a correct characterization of the noise we have derived an instrumental signal to noise simulator tool, which has been validated against the reference signal measured by VIRTIS-M on two internal calibration sources.

The availability of observations taken at different viewing and illumination geometries have allowed us to characterize the surface regolith’s photometric properties and to include them in our model. Finally, the uncertainties of the derived parameters are estimated through the propagation of the errors.

The modeling of VIRTIS data is supported by comparison with laboratory spectra of analogue materials.

Authors acknowledge the funding from Italian and French Space Agencies.