Seasonal erosion and restoration of the dust cover on comet 67P/Churyumov-Gerasimenko as observed by OSIRIS onboard Rosetta

Abstract

Context. Dust deposits or dust cover are a prevalent morphology in the northern hemi-nucleus of comet 67P/Churyumov-Gerasimenko (67P). The evolution of the dust deposits was captured by the OSIRIS camera system onboard the Rosetta spacecraft having escorted the comet for over two years. The observations shed light on the fundamental role of cometary activity in shaping and transforming the surface morphology. Aims. We aim to present OSIRIS observations of surface changes over the dust deposits before and after perihelion. The distribution of changes and a timeline of their occurrence are provided. We perform a data analysis to quantify the surface changes and investigate their correlation to water activity from the dust deposits. We further discuss how the results of our investigation are related to other findings from the Rosetta mission. Methods. Surface changes were detected via systematic comparison of images, and quantified using shape-from-shading technique. Thermal models were applied to estimate the erosion of water ice in response to the increasing insolation over the areas where surface changes occurred. Modeling results were used for the interpretation of the observed surface changes. Results. Surface changes discussed here were concentrated at mid-latitudes, between about 20• N and 40• N, marking a global transition from the dust-covered to rugged terrains. The changes were distributed in open areas exposed to ample solar illumination and likely subject to enhanced surface erosion before perihelion. The occurrence of changes followed the southward migration of the sub-solar point across the latitudes of their distribution. The erosion at locations of most changes was at least about 0.5 m, but most likely did not exceed several meters. The erosive features before perihelion had given way to a fresh, smooth cover of dust deposits after perihelion, suggesting that the dust deposits had been globally restored by at least about 1 m with ejecta from the intensely illuminated southern hemi-nucleus around perihelion, when the north was inactive during polar night. Conclusions. The erosion and restoration of the northern dust deposits are morphological expressions of seasonality on 67P. Based on observations and thermal modeling results, it is inferred that the dust deposits contained a few percent of water ice in mass on average. Local inhomogeneity in water abundance at spatial scales below tens of meters is likely. We suspect that dust ejected from the deposits may not have escaped the comet in bulk. That is, at least half of the ejected mass was afloat in the inner-coma or/and redeposited over other areas of the nucleus.

An orbital water-ice cycle on comet 67P from colour changes

“…We have evidence [6], in fact, that water ice-rich layers are immediately available under the dust layer, as shown in areas of recent disruption of the surface [23] and as demonstrated by many thermal models applied to cometary nuclei [24,25]. Between 20% [21] to 50% [26] of the dust grains ejected around perihelion from the south pole, the region which receives the maximum insolation at that time, fall-back preferentially on the northern hemisphere. The fall-back flux is dominated by decimetre-sizes dust aggregates in which a small fraction of water ice can be maintained [21].…”

Cometary Nuclei—From Giotto to Rosetta

“…Seasonal variations of the cover by back fall were observed at several places particularly at northern mid latitudes. The thickness of the cover was estimated to 0.5 to 1 m (Hu et al 2017). A more precise measure of 1.4 m could be derived for the variation of the ground level at Hapi using the height of outcrops (Cambianica et al 2020).…”

Migrating Scarps as a Significant Driver for Cometary Surface Evolution

“…Instead, many rapid changes were detected in what were previously thought to be the comparably inactive smooth terrains. These changes occurred in the the months leading up to perihelion and include both morphological changes like depressions (El‐Maarry et al, ; Groussin et al, ) and “honeycombs” (Hu et al, ) and spectroscopic (De Sanctis et al, ; Fornasier et al, ). Together, these changes suggest the nucleus removed an overlaying layer of dust prior to receiving its newest layer at perihelion.…”