Surface evolution of the Anhur region on comet 67P/Churyumov-Gerasimenko from high-resolution OSIRIS images

Context. The southern hemisphere of comet 67P/Churyumov-Gerasimenko (67P) became observable by the Rosetta mission in March
2015, a few months before cometary southern vernal equinox. The Anhur region in the southern part of the comet’s larger lobe was
found to be highly eroded, enriched in volatiles, and highly active.
Aims. We analyze high-resolution images of the Anhur region pre- and post-perihelion acquired by the OSIRIS imaging system on
board the Rosetta mission. The Narrow Angle Camera is particularly useful for studying the evolution in Anhur in terms of morphological
changes and color variations.
Methods. Radiance factor images processed by the OSIRIS pipeline were coregistered, reprojected onto the 3D shape model of the
comet, and corrected for the illumination conditions.
Results. We find a number of morphological changes in the Anhur region that are related to formation of new scarps; removal of
dust coatings; localized resurfacing in some areas, including boulders displacements; and vanishing structures, which implies localized
mass loss that we estimate to be higher than 50 million kg. The strongest changes took place in and nearby the Anhur canyon-like structure,
where significant dust cover was removed, an entire structure vanished, and many boulders were rearranged. All such changes
are potentially associated with one of the most intense outbursts registered by Rosetta during its observations, which occurred one day
before perihelion passage. Moreover, in the niche at the foot of a new observed scarp, we also see evidence of water ice exposure that
persisted for at least six months. The abundance of water ice, evaluated from a linear mixing model, is relatively high (>20%). Our
results confirm that the Anhur region is volatile-rich and probably is the area on 67P with the most pristine exposures near perihelion.