Analysis of Images of Comet 67P/Churyumov-Gerasimenko Obtained by Osiris/Rosetta: Dust Particles Investigation

The aim of this thesis is to study and characterize the dust in the coma of the comet 67P/Churyumov-Gerasimenko in order to increase our knowledge about the nature and behavior of comets. These objects are the most pristine of our solar system and their investigation allows to understand the physical and chemical conditions that took place in the primordial environment where they formed. In turn, dust is a fundamental constituent of comets and provides valuable information on their composition, structure and evolution. The importance of the dust lies in the possibility to use it as a proxy to derive properties of the interior of the cometary nuclei. Indeed, the activity produced when the comet approaches the Sun releases both surface material and pristine matter from the inside of the nucleus that cannot be reached otherwise. The ESA Rosetta mission to the comet 67P/Churyumov-Gerasimenko revealed a new insight of the cometary dust providing precious information about its composition, dimension, size distribution and structure. For the rst time it was possible to acquire images of single dust aggregates and to give a comprehensive description of their properties, thanks to the information obtained by all the instrumentation on board the spacecraft. The overall processes that act on the comets are not yet fully understood and there are still open questions that need to be claried. It is therefore essential to continue to investigate this complex scenario to unveil the real nature of these primordial objects. The rst work of this thesis concerns the photometric analysis of images of dust grains, taken by the OSIRIS Narrow Angle Camera (NAC). The large amount of data forced us to develop an automatic pipeline to identify and to analyze the particle tracks present in the images. We measured the spectral slope of about 2000 grains in 555 images taken in 18 days of observation from July 2015 to January 2016. Then we evaluate their variability with respect to the distance from the nucleus and from the Sun. To better characterize the grains composition we perform also a spectrophotometric analysis of a subgroup of 339 grains. To check their nature, we compared their three point spectra with the ones of several nucleus terrains identifying dierent types of grains spectra. The second work of this thesis treats the scattering properties of cometary dust analogs, in collaboration with the Instituto de Astrofsica de Andalusa, Granada at the Cosmic Dust Laboratory. Laboratory experiments are of fundamental importance to interpret the data since they allow to study complex phenomena in a monitored environment, favoring the evaluation of the eects due to specic physical and chemical properties of the dust. We measure the phase function and the linear polarization curve of 7 samples, consisting on 4 meteorites, 2 silicates and 1 organic compound with the aim to compare our results with the observational data of 67P and other comets. Our last work, which is still in progress, consists in the analysis of dust motion with the aim to provide observational constraints to the dynamical models of cometary dust in the inner coma as well as to the shape of the single grains. We focus our investigation on the 15 groups of images taken from July 2015 to January 2016 and we measure the rotational periods, the axis ratio and the direction of motion on a dataset of more than 500 grains.