Local study of photometric variations at comet 67P/Churyumov-Gerasimenko: a point of view from VIRTIS/Rosetta

From August 2014 to September 2016, the Rosetta spacecraft has been orbiting comet 67P and has obtained information on the origin and evolution of comet. VIRTIS, the Visible, Infrared and Thermal Imaging Spectrometer collected reflectance spectra of the nucleus surface with two different channels: VIRTIS-M and VIRTIS-H. The first one is an imaging spectrometer ranging from 0.25 to 5.1 μm and it covers a large field of view. The second one is a point spectrometer ranging from 1.9 to 5.0 μm with higher spectral resolution. Both channels observed comet 67P under many different illumination conditions and observation geometries (e.g., between 20° to 110° phase angle). VIRTIS spectra revealed a very low single scattering albedo of 0.06 @ 0.65 μm and red slopes in the visible and in the near infrared (Capaccioni et al., 2015; Ciarniello et al., 2015). Thanks to the long duration of the Rosetta mission it is possible to perform a photometric study through time and to monitor potential changes of surface spectral properties (e.g. spectral slope, single scattering albedo) associated with varying activity. In this context, we defined specific periods of observation and compared them between each other to identify surface changes. Using a Lommel-Seeliger model as a first step, we studied the spatial distribution of the single scattering albedo (SSA) and spectral slope at the local or regional scale. In the period between the end of 2014 and the beginning of 2015, we observe SSA variations which are neither spatially homogeneous nor uniform (i.e., increase and decrease). These changes, observed both in the visible and in the NIR, may be related to different processes such as dust deposit/removal or changes in the ice content in the sub-surface. We’ve also used a Hapke model to evaluate the effect of the photometric function on the results. We will also study other periods of observations, in particular around perihelion in August 2015 when the activity was reaching a maximum. VIRTIS-M infrared cryocooler has failed in April 2015 and it is no longer possible to investigate the photometric variations in this wavelength range on large areas. However investigations are still possible with the visible channel