Color Variegation and Inner Stratified Structure of comet 67P/Churyumov-Gerasimenko

The international space mission Rosetta was approved in 1993 by the European Space Agency (ESA), within the program Horizon 2000 for the exploration of the minor bodies of the Solar System, and launched on 2 March 2004. The spacecraft was constituted by the Rosetta orbiter and the Philae lander, the latter projected to anchor at the cometary nucleus of the comet, collect images and perform superficial analyses. The mission concluded on September 2016 with the controlled landing of the probe on the comet. The instruments onboard the spacecraft were chosen to perform a wide range of analyses, among which image analysis, spectroscopy and spectrophotometry. In particular, the images used in this work were acquired with the imaging system OSIRIS (Optical, Spectroscopic and Infrared Remote Imaging System) and precisely with the Narrow Angle Camera (NAC), projected to acquire high resolution images of the surface in the NUV-VIS-NIR wavelength (Keller et al., 2007). The importance of the Rosetta mission lies in the fact that it is the first mission that set as main objective the close-range study of the nucleus of a comet, approaching specifically the comet 67P/Churyumov-Gerasimenko of the Jupiter-family comets. The spacecraft performed a series of controlled orbits around the comet, allowing its observation, and escorted it during its perihelion passage and, above all, was the first mission to land a lander on the surface of a cometary body. The images acquired by OSIRIS allowed to describe a cometary body with a bi-lobed shape and with vast depressions, pits, scarps, terraces, cuestas, mesas, fractures, mass-wasting deposits, landslides, continuous stratifications and fine material deposits (Sierks et al., 2015; Thomas et al., 2015). Studies on the stratification allowed to detect on both lobes of the cometary nucleus a onion-like stratification, which should have formed previously the merging of the two lobes through a low velocity collision (Massironi et al., 2015; Davidsson et al., 2016). With the aim to identify distinct envelopes of the onion-like stratification and to verify their spectrophotometric variations, two sequences of multispectral OSIRIS NAC images acquired after the perihelion passage and framing the minor lobe of comet 67P where selected. These images were overlapped to build two multispectral cubes to which were then applied photometric corrections to delete the effects due to the different illumination conditions. To the cubes was successively applied a two-classes Maximum Likelihood supervised classification, on which the training areas were selected on the identified consolidated outcrops and relative deposits on the basis of the superficial reflectance and structural elevation defined by the ellipsoid-based model of the internal structure of the nucleus (Penasa et al., acce pted). The classification highlighted the presence of an inner and an outer envelope (each constituted by numerous stata) with different spectral properties, in particular with the inner shell brighter than the other one. These difference could be due to different textural properties or to compositional variations between the two envelopes.