Geomorphology and Spectrophotometry of Philae’s Landing Site on Comet 67P Churyumov-Gerasimenko

Abstract

Context. On 12 November 2014 the European mission Rosetta succeeded in deploying a lander, named Philae, on the surface of one of the smallest, low-gravity and more primitive bodies of the Solar System, the comet 67P/Churyumov-Gerasimenko (67P). Aims. The aim of this paper is to provide a comprehensive geomorphological and spectrophotometric analysis of the region where Philae landed in order to give an essential framework for the interpretation of its measurements. Methods. OSIRIS images, coupled with the 3D shape model based on stereo-photogrammetric analysis have been used to interpret the geomorphology of the site. Photometric modelling via Akimov’s parameter-less function has been used to correct the reflectance for the effects of the viewing and illumination conditions. Spectral analysis on co-registered colour cubes have been used to retrieve spectrophotometric properties. Results. The landing site shows a low average reflectance of 0.96% in the orange filter (λ = 649.2 nm) at about 63° phase angle and a global featureless spectrum with an average red spectral slope of 15.2%/100 nm between 480.7 nm (blue filter) and 882.1 nm (NearIR filter). The spatial analysis shows a well-established correlation between the geomorphological units and the photometric characteristics of the surface. In particular smooth deposits have highest reflectance and less steep spectrum if compared to the outcropping material across the area. Conclusions. We interpret the results as indications that Agilkia spectrophotometry is compatible with a composition of organic materials showing small local variegation. Texture and grain size effects alone can not explain the spectral variegation and space weathering is rather unlikely for short-period comets. We therefore conclude that the photometric variegation is due to a compositional differentiation. This might be tentatively interpreted as the effect of the efficient dust-transport processes acting on 67P (Thomas et al. submitted). High activity regions might be the original sources for smooth fine-grained materials which then covered Agilkia as a consequence of airfall of residual material. Further ROSETTA observations performed as the comet gets closer to the Sun would help interpreting the processes working at shaping the landing site and the overall nucleus.