Properties of the dust cloud caused by the Deep Impact experiment

We present an analysis of the observations of the Deep Impact event performed by the OSIRIS narrow angle camera aboard the Rosetta spacecraft over two weeks, in an effort to characterize the cometary dust grains ejected from the nucleus of Comet 9P/Tempel 1. We adopt a Monte Carlo approach to generate calibrated synthetic images, and a linear combination of them is fitted to the calibrated images so as to determine the physical parameters of the dust cloud. Our model considers spherical olivine particles with a density of 3780 kg m−3. It incorporates constraints on the direction of the cone of emission coming from additional images obtained at Pic du Midi observatory, and constraints on the dust terminal velocities coming from the physics of the impact. We find that the slope of the differential dust size distribution of grains with radii <20 μm ( β > 0.008 ) is 3.1 ± 0.3 , a value typical of cometary dust tails. This shows that there is no evidence in our data for an enhancement in sub-micron particles in the ejecta compared to the typical dust distribution of active comets. We estimate the mass of particles with radii <1.4 μm ( β > 0.14 ) to be 1.5 ± 0.2 × 10 5  kg . These particles represent more than 80% of the cross-section of the observed dust cloud. The mass carried by larger particles depends whether the gas significantly increases the kinetic energy of the grains in the inner coma; it lies in the range 1 – 14 × 10 6  kg for particles with radii <100 μm ( β > 0.002 ). We obtain the distribution of terminal velocities reached by the dust after the dust–gas interaction which is very well constrained between 10 and 600 m s−1. It is characterized by Gaussian with a maximum at about 190 m s−1 and a width at half maximum of 150 m s−1.