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We analyzed Deep Impact High Resolution Instrument (HRI) images acquiredwithin the first seconds after collision of the Deep Impact impactor with thenucleus of comet 9P/Tempel 1. These images reveal an optically thick ejectaplume that casts a shadow on the surface of the nucleus. Using the 3D radiativetransfer code HYPERION we simulated light scattering by the ejecta plume,taking into account multiple scattering of light from the ejecta, thesurrounding nuclear surface and the actual observational geometry (including anupdated plume orientation geometry that accounts for the latest 9P/Tempel 1shape model). Our primary dust model parameters were the number density ofparticles, their size distribution and composition. We defined the compositionthrough the density of an individual particle and the ratio of its materialconstituents, which we considered to be refractories, ice and voids. Theresults of our modeling indicate a dust/ice mass ratio for the ejecta particlesof at least 1. To further constrain the parameters of the model, we checked forconsistency between the ejecta mass resulting from our modeling with the ejectamass estimated by the crater formation modeling. Constraining the particle sizedistribution by results of other studies of the Deep Impact ejecta, we find thenumber density of ejecta particles equal to ~10^4 particles/cm^3 at the base ofthe plume.

Properties of comet 9P/Tempel 1 dust immediately following excavation by Deep Impact