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The Opposition Effect of 67P/Churyumov-Gerasimenko on post-perihelion Rosetta images

Pedro Henrique Hasselmann 1 Maria Antonella Barucci 1 Sonia Fornasier 1 Clément Feller 1 Prasanna Deshapriya 1 Marcello Fulchignoni 1 Bernhard Jost 2 Holger Sierks 3 Cesare Barbieri 4 Philippe Lamy 5 Rafael Rodrigo 6 Detlef Koschny 7 Hans Rickman 8, 9 Michael F. A'Hearn 10 Jean-Loup Bertaux 11 Ivano Bertini 4 Gabriele Cremonese 12 Vania da Deppo 13 Björn Davidsson 14 Stefano Debei 15 Mariolino de Cecco 16 Jakob Deller 3 Marco Fulle 17 Robert W. Gaskell 18 Olivier Groussin 5 Pedro J. Gutiérrez 19 Carsten Güttler 3 Marc Hofmann 3 Stubbe F. Hviid 20 Wing-Huen Ip 21, 22 Laurent Jorda 23 Horst U. Keller 24, 20 Jörg Knollenberg 20 Gabor Kovacs 3, 25 Rainer Kramm 3 Ekkehard Kührt 20 Michael Küppers 26 Luisa-Maria Lara 19 Monica Lazzarin 4 Josè J. Lopez-Moreno 19 Francesco Marzari 4 Stefano Mottola 20 Giampiero Naletto 27, 14, 28 Nilda Oklay 20 Antoine Pommerol 2 Nicolas Thomas 2 Cécilia Tubiana 3 Jean-Baptiste Vincent 20 
Abstract : High-resolution OSIRIS/Rosetta images of 67P/Churyumov-Gerasimenko acquired on the night run of April 9th to 10th 2016 shows, at large scale, an Opposition Effect (OE) spot swooping 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 linear-exponential and Hapke (2012) model. Those features encompass different types of spectral behaviour: a circular mesa, one venous structure and an assemble of bright spots, going from red to blue colours. Both Hapke and linear-exponential parameters indicate a stepwise sharpening of the OE from bright spots to circular mesa. Yet, a very broad non-linear phase curve is verified and no sign of sharp OE associated to coherent-backscattering mechanism observed. We estimate that 67P surface is dominated by opaque, desiccated and larger-than-wavelength irregular grains. Veins and bright spots display photometric properties consistent to surfaces becoming slightly brighter as it is 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 (Fornasier et al., 2015) 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 Imhotep-Ash boundary area (Feller et al., 2016) advocates for mild darkening (< 7%) of the surface at local scale.
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Pedro Henrique Hasselmann, Maria Antonella Barucci, Sonia Fornasier, Clément Feller, Prasanna Deshapriya, et al.. The Opposition Effect of 67P/Churyumov-Gerasimenko on post-perihelion Rosetta images . Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP): Policy P - Oxford Open Option A, 2017, 469 (Suppl_2), pp.S550-S567. ⟨10.1093/mnras/stx1834⟩. ⟨insu-01599000⟩



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