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M.I. Blecka (Space Research Centre)
Currently conducted studies on modelling the radiance of the nucleus and coma of the comet P/Wirtanen in the 0.25-5.0 microns range are presented. These studies are directly related to the VIRTIS spectrometer planned for the Rosetta mission. One of the most important scientific objectives of the VIRTIS spectrometer is to determine mineralogical composition of grains on the surface of nucleus and present in the coma. These grains can have both silicatic and icy composition and this composition can be detected through their radiance spectra. The shapes of the spectra depend on the mixing ratios of composition materials and the size distribution of grains on the surface and in the coma. The main goal of the model presented here is to evaluate the influence of ice - particulate mixtures on Mie efficiencies, and reflectance of the comet nucleus as well as on the intensity and shapes of radiance spectra near the comet P/Wirtanen. The various approach to the problem of the influence of relative proportion of minerals in the mixture on Mie efficiencies and the single scattering albedo of grains have been take into consideration. Using the Hapke theory the spectral reflectance/emittance have been calculated. The radiation transfer model includes the contribution of major gas species in the coma as well as the one of solar radiation scattered by the dust particles at short wavelengths and thermally emitted from dust at longer wavelengths. As possible dust composition for the grains present on the nucleus surface and released in the coma we have considered astronomical silicates, olivine and H2O ice. Thermodynamical parameters in the comet's environment have been taken from the available physical models of the comet P/Wirtanen. For modelling the radiance spectra we used Mie and Hapke theories and some options of the MODTRAN code.
Acknowledgements: This work has been supported by the Polish State Committee for Scientific Research under grant 2P03C00515.