Albedo variegation on Comet 67P/Churyumov-Gerasimenko

TL;DR

This study analyzes the albedo variegation on Comet 67P's nucleus using photometric modeling of OSIRIS images, revealing correlations with terrain morphology and stratigraphy, and proposing porosity and compaction effects as causes.

Contribution

It introduces a novel approach to map local albedo variations on the comet's surface and links these to morphological and stratigraphic features, highlighting the role of porosity and compaction.

Findings

Smooth terrain is brighter than consolidated terrain.

Albedo variegation correlates with morphology and stratigraphy.

Darkening of smooth terrain is linked to subsidence and compaction.

Abstract

We here study the level of albedo variegation on the nucleus of Comet 67P/Churyumov-Gerasimenko. This is done by fitting the parameters of a standard photometric phase function model to disk-average radiance factor data in images acquired by the Rosetta/OSIRIS Narrow Angle Camera in the orange filter. Local discrepancies between the observed radiance factor and the disk-average solution are interpreted as a proxy W of the local single-scattering albedo. We find a wide range 0.02<W<0.09 around an average of W=0.055. The observed albedo variegation is strongly correlated with nucleus morphology - smooth terrain is brighter, and consolidated terrain is darker, than average. Furthermore, we find that smooth terrain darken prior to morphological changes, and that stratigraphically low terrain (with respect to the centre of each nucleus lobe) is brighter than stratigraphically high terrain. We propose that the observed albedo variegation is due to differences in porosity and the coherent effect: compaction causes small brighter particles to act collectively as larger optically effective particles, that are darker. Accordingly, we consider the dark consolidated terrain materials more compacted than smooth terrain materials, and darkening of the latter is due to subsidence.