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

Abstract

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.

The Philae lander reveals low-strength primitive ice inside cometary boulders

“…, matching values previously observed in freshly exposed water ice from outbursts 4 and water ice in shadow 5,6 . At the end of the crevice, Philae made a 0.25-metre-deep impression in the boulder ice, providing in situ measurements confirming that primitive ice has a very low compressive strength (less than 12 pascals, softer than freshly fallen light snow) and allowing a key estimation to be made of the porosity (75 ± 7 per cent) of the boulders' icy interiors.…”

Spatial Distribution of Ultraviolet Emission from Cometary Activity at 67P/Churyumov-Gerasimenko

“…This overhang area previously had an outburst identified in Vincent et al (2016) (outburst 28 in Table 1) that saw a relative increase in brightness of 10% on 2015 September 10, just under two months before the activity increase discussed in this paper. However, five outbursts were identified with similar timestamps on that date in Sobek and Anukhet as well, making it difficult to attribute any observed ultraviolet emission in Alice observations on that day to a specific region (Fornasier et al 2019). There are two outburst sites located farther south in the Geb region that produce outburst vectors that do not have a south pole parallel component and will be eliminated as a possibility.…”

New constraints on the chemical composition and outgassing of 67P/Churyumov-Gerasimenko

“…However, locally, morphological changes and erosion have been identified in particular with dust redistribution, cliffs collapses and boulders movements (Groussin et al 2015, Hu et al 2017, El-Maary et al, 2017. The mass loss can reach in places up to 14 m as estimated by Fornasier et al, (2019) in the Anhur region located in the southern hemisphere.…”