Rosetta/OSIRIS observations of 67P nucleus during the April 2016 flyby: high-resolution spectrophotometry

Abstract : From August 2014 to September 2016, the Rosetta spacecraft followed comet 67P/Churyumov–Gerasimenko along its orbit. After the comet passed perihelion, Rosetta performed a flyby manoeuvre over the Imhotep–Khepry transition in April 2016. The OSIRIS/Narrow-Angle-Camera (NAC) acquired 112 observations with mainly three broadband filters (centered at 480, 649, and 743 nm) at a resolution of up to 0.53 m/px and for phase angles between 0.095° and 62°. We have investigated the morphological and spectrophotometrical properties of this area using the OSIRIS/NAC high-resolution observations. We assembled the observations into coregistered color cubes. Using a 3D shape model, we produced the illumination conditions and georeference for each observation. We mapped the observations of the transition to investigate its geomorphology. Observations were photometrically corrected using the Lommel–Seeliger disk law. Spectrophotometric analyses were performed on the coregistered color cubes. These data were used to estimate the local phase reddening. The Imhotep–Khepry transition hosts numerous and varied types of terrains and features. We observe an association between a feature’s nature, its reflectance, and its spectral slopes. Fine material deposits exhibit an average reflectance and spectral slope, while terrains with diamictons, consolidated material, degraded outcrops, or features such as somber boulders present a lower-than-average reflectance and higher-than-average spectral slope. Bright surfaces present here a spectral behavior consistent with terrains enriched in water-ice. We find a phase-reddening slope of 0.064 ± 0.001%/100 nm/° at 2.7 au outbound, similar to the one obtained at 2.3 au inbound during the February 2015 flyby. Identified as the source region of multiple jets and a host of water-ice material, the Imhotep–Khepry transition appeared in April 2016, close to the frost line, to further harbor several potential locations with exposed water-ice material among its numerous different morphological terrain units.