Exposed bright features on the comet 67P/Churyumov-Gerasimenko: distribution and evolution

Abstract

Context. Since its arrival at the comet 67P/Churyumov-Gerasimenko in August 2014, the Rosetta spacecraft followed the comet as it went past the perihelion and beyond until September 2016. During this time there were many scientific instruments operating on board Rosetta to study the comet and its evolution in unprecedented detail. In this context, our study focusses on the distribution and evolution of exposed bright features that have been observed by OSIRIS, which is the scientific imaging instrument aboard Rosetta. <BR /> Aims: We envisage investigating various morphologies of exposed bright features and the mechanisms that triggered their appearance. <BR /> Methods: We co-registered multi-filter observations of OSIRIS images that are available in reflectance. The Lommel-Seeliger disk function was used to correct for the illumination conditions and the resulting colour cubes were used to perform spectrophotometric analyses on regions of interest. <BR /> Results: We present a catalogue of 57 exposed bright features observed on the nucleus of the comet, all of which are attributed to the presence of H<SUB>2</SUB>O ice on the comet. Furthermore, we categorise these patches under four different morphologies and present geometric albedos for each category. <BR /> Conclusions: Although the nucleus of 67P/Churyumov-Gerasimenko appears to be dark in general, there are localised H<SUB>2</SUB>O ice sources on the comet. Cometary activity escalates towards the perihelion passage and reveals such volatile ices. We propose that isolated H<SUB>2</SUB>O ice patches found in smooth terrains in regions, such as Imhotep, Bes, and Hapi, result from frost as an aftermath of the cessation of the diurnal water cycle on the comet as it recedes from perihelion. Upon the comet's return to perihelion, such patches are revealed when sublimation-driven erosion removes the thin dust layers that got deposited earlier. More powerful activity sources such as cometary outbursts are capable of revealing much fresher, less contaminated H<SUB>2</SUB>O ice that is preserved with consolidated cometary material, as observed on exposed patches resting on boulders. This is corroborated by our albedo calculations that attribute higher albedos for bright features with formations related to outbursts.