P. H. Hasselmann, M. A. Barucci, S. Fornasier, C. Feller, J. D. P. Deshapriya, M. Fulchignoni, B. Jost, H. Sierks, C. Barbieri, P. L. Lamy, R. Rodrigo, D. Koschny, H. Rickman, M. A’Hearn, J.-L. Bertaux, I. Bertini, G. Cremonese, V. Da Deppo, B. Davidsson, S. Debei, M. De Cecco, J. Deller, M. Fulle, R. W. Gaskell, O. Groussin, P. J. Gutierrez, C. Güttler, M. Hofmann, S. F. Hviid, W.-H. Ip, L. Jorda, H. U. Keller, J. Knollenberg, G. Kovacs, R. Kramm, E. Kührt, M. Küppers, M. L. Lara, M. Lazzarin, J. J. Lopez-Moreno, F. Marzari, S. Mottola, G. Naletto, N. Oklay, A. Pommerol, N. Thomas, C. Tubiana, J.-B. Vincent. 2017. The opposition effect of 67P/Churyumov-Gerasimenko on post-perihelion Rosetta images. Monthly Notices of the Royal Astronomical Society, 469, DOI: 10.1093/mnras/stx1834, International Conference on Cometary Science – Comets – A New Vision after Rosetta and Philae
High-resolution OSIRIS/Rosetta images of 67P/Churyumov-Gerasimenko acquired on the night run of 2016 April 9-10 show, at large scale, an opposition effect (OE) spot sweeping across Imhotep as the phase angle ranges from 0 degrees to 17 degrees. In this work, we fitted the phase curve of the whole surface imaged as well as three particular features using both the linear-exponential and Hapke models. These features encompass different types of spectral behaviour: a circular mesa, one venous structure and an assemblage of bright spots, going from red to blue colours. Both the Hapke and linear-exponential parameters indicate a stepwise sharpening of the OE from bright spots to circular mesa. Yet a very broad nonlinear phase curve is verified and no sign of sharp OE associated with a coherent-backscattering mechanism is observed. We estimate that the 67P surface is dominated by opaque, desiccated and larger-than-wavelength irregular grains. Veins and bright spots display photometric properties consistent with surfaces becoming slightly brighter as they are enriched by high-albedo ice grains. We also report the estimation of normal albedo for all cometary regions observed throughout the image sequence. Comparison to pre-perihelion results indicates that far better insolation of northern brighter regions, i.e. Hapi, Hathor and Seth, is sufficient to explain mismatches on the photometric parameters. However, metre-scale photometric analysis of the Imhotep-Ash boundary area advocates for mild darkening (<7 per cent) of the surface at local scale.
