Photometry of dust grains of comet 67P and connection with nucleus regions G.Cremonese, E.Simioni, R.Ragazzoni, I.Bertini, F.La Forgia, M.Pajola, S.Fornasier, N.Oklay, M.Lazzarin, and the Osiris team We have analyzed the images of the NAC obtained on 10 September 2014 The data set is composed of 6 pairs of images obtained with two different filters, F24 and F28 We have identified 70 dust tracks all over the 6 pairs There are no grains identified in more than one pair The S/N of the tracks most likely was very close to 3 We have analyzed each track measuring FWHM, length, integrated flux, and color We are comparing the dust color grains with the color of the nucleus regions in order to trace the origin of the single grain

Error in parameter estimates Automatic detection of dust grain tracks Laplace filtered image Boolean mask Dilatation and erosion process Lower limit on eccentricity of closed isocurves Error in parameter estimates FWHM Length Flux 2.1 % 5.0% 7.2% FWHM and length definition

Histogram of the 70 dust grains measured

Projection of all the dust grains on the image plane at 29 km from S/C Projection of all the dust grains on the image plane at 29 km from S/C. The circle size is related to the track length. Red circles correspond to positive reddening, and negative for blue ones.

Nuclear regions considered for the color comparison Nuclear regions considered for the color comparison. The corresponding images have been selected according to their phase angle that is very close to 90° as for the dust grains.

Slope maps of the selected images

Comparison between dust grain reddening (red histogram) and nucleus reddening (black)

Discussion The comparison between the two histograms shows a main peak where most of the grains have a color very similar to the nucleus, i.e. they have been released directly from the surface or sub-surface There is a secondary peak showing a low number of grains having a negative reddening. We have images only in two filters and it is difficult to explain these values, but we may raise one hypothesis: The grains have a different composition with respect to the observed surface. There could be a strong absorption in the spectral range covered by the F28 filter, for instance at 700 nm as we observe in some asteroids and it is due to features associated with hydrated minerals, as for instance Fe2+Fe3+ (Vilas and Gaffey, 1989). There are few grains having a reddening higher than the nucleus regions, as we can see on the right side of the histogram. We may have grains characterized by an absorption in the F24, as for instance in the range 390-470 nm as observed on the Earth, in some measurements performed in the Antarctic region on glacier ice and snow (Warren et al., 2006, Appl.Opt., 45, 5320). Even in this spectral range we may have a feature associated with hydrated minerals at 430 nm, Fe3+ (Vilas et al., 1993).