The dust coma environment of 67P/Churyumov-Gerasimenko as observed by VIRTIS

ESA's Rosetta spacecraft had the unique opportunity to be in the vicinity of comet 67P/Churyumov-Gerasimenko for 2.5 years, observing the regions of the coma within a few km of the surface and to study the dynamical and physical properties of the emitted dust. In this work, we will present a summary of the dust coma investigations during the pre-perihelion and the perihelion passage, performed by the Visible InfraRed Thermal Imaging Spectrometer (VIRTIS) onboard Rosetta, in the spectral range between 0.2 to 5 _m (Coradini et al. 2007). During the pre-perihelion period, the main dust results can be summarized as follows: 1-The general spatial distribution of the dust is strongly correlated with the presence of H2O emission and is significantly different from the distribution of CO2 emission; 2-The sunward radial profiles of the dust show that the dust particles, lifted off the nucleus, are accelerated by the gas in the first 4-5 km and then transit into the free flowing collision-less zone with a 1/_ column density profile, where _ is the distance from the center of the comet; 4- The colour of the dust in the coma has a fairly steep red slope in the VIS of about 9 to 12 ± 1 % per 100 nm and a much shallower slope in the IR with values of 1.7 ± 0.2 % per 100 nm. The colours do not show any spatial variation, nor any variation with distance from the nucleus. This indicates that the dust expelled into the coma from the nucleus is quite uniform, although it could also be that any changes in its composition (Rinaldi et al. 2016). 5- Combining the VIRTIS data with those by GIADA, the dust detector onboard Rosetta, we evaluate the dust size distribution power-law index that is greater than -3.1. This index matches the one determined using GIADA (Della Corte et al. 2016) March 2015 data indicating that the inner coma radiance is dominated by particles larger than 10 _m (Rinaldi et al. 2017).During the perihelion period, we focused on the analysis of the outburst events. The outbursts are characterized by a sudden and short increase of the dust emission, from localized areas with variable degree of collimation, followed by a gradual decrease of activity with a life time ranging between 6 to 24 minutes. The quick onset is correlated with a change of the visible dust colour from redder with 15-18 ± 3 %/100 nm, to bluer with values around 7-10 ± 0.3%/100 nm (Rinaldi et al. 2018, in preparation). The combination of VIS and IR dust properties reveals the presence of very small (less than 100 nm) and bright grains in the ejecta that could be formed by silicate material, implying the thermal degradation of the carbonaceous material, or icy grains (Bockelee-Morvan et al 2017). The dust coma from the outburst was found to expand at speeds between 22.2 ±2.2 to 30.2 ± 1.4 m/s for the "big" outbursts and 38.4 ± 2.1 to 64.9 ±10.6 m/s for "mini" ones. The total ejected mass during an outburst event is estimated to lie between 100 to 6000 kg for a duration of 6 - 24 minutes, assuming a size distribution indexes between -2.5 to -3 (Rinaldi et al. 2018, in preparation).References:Rinaldi G., et al., 2016, MNRAS, 462, S547. Rinaldi G., et al., 2017, MNRAS 469S, 598R. Bockelee-Morvan D., et al., 2017, MNRAS, 469, S443. Rinaldi G., et al., 2018, in preparation. Coradini A., et al., 2007, Space Sci. Rev., 128, 529. Della Corte V., et al., 2016, Acta Astronautica, 126, 205