Albedo variegation on Comet 67P/Churyumov–Gerasimenko

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 $\mathcal {W}$ of the local single–scattering albedo. We find a wide range $0.02 \lesssim \mathcal {W}\lesssim 0.09$ around an average of $\mathcal {W}=0.055$. 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.

CO2-driven surface changes in the Hapi region on Comet 67P/Churyumov-Gerasimenko

“…Further analysis is needed to demonstrate whether the CO 2 -driven ejection of a ≥ 5 m thick layer indeed is possible, i. e., if sufficient CO 2 pressure can be reached at such depths to overcome the tensile strength of the overlying icedust mixture. Whereas Bouquety et al (2022) see subsidence as the major pit-forming mechanism, we here see the collapse as a smaller prelude, that does not lead to detectable morphological changes (put perhaps leads to darkening, as previously mentioned; Davidsson et al 2022a). In the currently proposed scenario, morphological changes come after compaction and darkening, and are due to ejection of material, not subsidence.…”

“…The compaction we propose to have taken place in late October or early November has not been observed in the form of a measurable subsidence. However, Davidsson et al (2022a) demonstrate that the single-scattering albedo at Hapi D was reduced between 2014 August 30 and December 10. They suggest that this darkening (presumably caused by a decrease of porosity and an increased coherent effect, as small brighter grains started acting as larger and darker optically effective particles) is a manifestation of the compaction that actually was observable by OSIRIS.…”