The opposition effect of 67P/Churyumov-Gerasimenko on post-perihelion Rosetta images
Jorda, L.; Lamy, P. L.; Bertaux, J. -L.; Sierks, H.; Fulle, M.; Kührt, E.; Güttler, C.; Keller, H. U.; Mottola, S.; Barbieri, C.; Rodrigo, R.; Koschny, D.; Rickman, H.; Barucci, M. A.; Bertini, I.; Cremonese, G.; Da Deppo, V.; Davidsson, B.; Debei, S.; De Cecco, M.; Deller, J.; Fornasier, S.; Groussin, O.; Hofmann, M.; Hviid, S. F.; Ip, W. -H.; Knollenberg, J.; Kovacs, G.; Küppers, M.; Lazzarin, M.; Marzari, F.; Naletto, G.; Oklay, N.; Thomas, N.; Tubiana, C.; Vincent, J. -B.; Pommerol, A.; Fulchignoni, M.; Feller, C.; Lopez-Moreno, J. J.; Gaskell, R. W.; Kramm, R.; Hasselmann, P. H.; Gutierrez, P. J.; Deshapriya, J. D. P.; A'Hearn, M.; Lara, M. L.; Jost, B.
France, Switzerland, Germany, Italy, Spain, Netherlands, Sweden, Poland, United States, Taiwan, China, Hungary
Abstract
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° to 17°. 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.