Modeling the Radiance from coma/jets: Application to Tempel 1 coma and Enceladus jets

We will describe our model for the determination of the radiance from a cometary coma, composed of dust and gases, under different illumination conditions, gas and particle compositions and abundances. The study of dust/ice and gas in the coma is one of the major goals of the VIRTIS (Visible and Infrared Thermal Imaging Spectrometer) instrument, an imaging spectrometer onboard of the Rosetta spacecraft working in the spectral range of 0.25-5 μm. The Rosetta S/C will encounter the comet 67P/CG in mid-2014 at a distance of about 3.25AU and it will follow the comet for more than year during its trajectory around the Sun. The results of our model are the synthetic spectra, in the VIRTIS spectral range, computed at different distances from the sun and with different dust grains models (size distribution, composition and vertical distribution), taken from the literature. The gases composition, spatial distribution and coma properties are provided by the Inner Coma Environment Simulation tools (ICES) available at http://ices.engin.umich.edu/. The synthetic spectra derived are presently used to forecast the signal expected at the instrument and to correctly plan the observations at the comet. It will also be used to interpret the spectra obtained during the observation phases at the comet 67/P. As a validation of our model we have applied it to the infrared observations of the coma of the comet Tempel 1 performed by HRII onboard the Deep Impact spacecraft and to the available infrared data obtained from the Cassini-VIMS instrument during the fly-byes with the Saturn's moon Enceladus. The Cassini instruments results have shown jets that spew from the below the surface and consist of both water vapor, icy particles and several organic compounds.