Evolution of pits at the surface of 67P/Churyumov-Gerasimenko

Abstract

Context. The observation of pits at the surface of comets offers the opportunity to take a glimpse into the properties and the mechanisms that shape a nucleus through cometary activity. If the origin of these pits is still a matter of debate, multiple studies have recently suggested that known phase transitions (such as volatile sublimation or amorphous water ice crystallization) alone could not have carved these morphological features on the surface of 67P/Churyumov-Gerasimenko (hereafter 67P). Aims. We want to understand how the progressive modification of 67P's surface due to cometary activity might have affected the characteristics of pits and alcoves. In particular, we aim to understand whether signatures of the formation mechanism of these surface morphological features can still be identified. Methods. To quantify the amount of erosion sustained at the surface of 67P since it arrived on its currently observed orbit, we selected 380 facets of a medium-resolution shape model of the nucleus, sampling 30 pits and alcoves across the surface. We computed the surface energy balance with a high temporal resolution, including shadowing and self-heating contributions. We then applied a thermal evolution model to assess the amount of erosion sustained after ten orbital revolutions under current illumination conditions. Results. We find that the maximum erosion sustained after ten orbital revolutions is on the order of 80 m, for facets located in the southern hemisphere. We thus confirm that progressive erosion cannot form pits and alcoves, as local erosion is much lower than their observed depth and diameter. We find that plateaus tend to erode more than bottoms, especially for the deepest depressions, and that some differential erosion can affect their morphology. As a general rule, our results suggest that sharp morphological features tend to be erased by progressive erosion. Conclusions. This study supports the assumption that deep circular pits, such as Seth_01, are the least processed morphological features at the surface of 67P, or the best preserved since their formation.

Pits on Jupiter-family Comets and the Age of Cometary Surfaces

“…We select a minimum of 10 pits of each nucleus, located across all latitudes of each nucleus. Indeed, we show that seasonal effects dominate the global erosion trends on 67P (Benseguane et al 2022), so covering the entire nucleus is important in this study too. We thus apply selection criteria similar to 67P.…”

“…In this work, we want to understand how cometary activity may modify these surface structures and whether signatures of their formation process can be inferred from their expected evolution through sustained activity. We thus apply the same method as used to study the evolution of pits at the surface of 67P (Benseguane et al 2022) to quantify the amount of erosion sustained by pits at the surface of 9P, 81P, and 103P under their current illumination conditions. We note that 19P/Borrelly cannot be included in this study, as there is no shape model of its nucleus with a sufficient spatial resolution to be used.…”

“…The surfaces of comet nuclei display a diversity of morphological features (terraces, fractures, or boulders, for example; Massironi et al 2015;El-Maarry et al 2019) that have a complex interplay with cometary activity and the cycle of material across the nucleus (e.g., see Pajola et al 2022, for a review). Surface depressions offer the opportunity to connect the subsurface properties of comet nuclei with the thermophysical processes actively shaping them (e.g., Vincent et al 2015b;Benseguane et al 2022;Davidsson et al 2022). Shallow depressions (a few meters deep) generally observed on smooth terrains might be seasonal in nature, shaped by sublimation-driven activity orbit after orbit (Groussin et al 2015;Vincent et al 2016;El-Maarry et al 2017;Birch et al 2019;Bouquety et al 2021;Davidsson et al 2022).…”

“…Shallow depressions (a few meters deep) generally observed on smooth terrains might be seasonal in nature, shaped by sublimation-driven activity orbit after orbit (Groussin et al 2015;Vincent et al 2016;El-Maarry et al 2017;Birch et al 2019;Bouquety et al 2021;Davidsson et al 2022). However, larger depressions, typically tens to several hundreds of meters deep, are also observed (Vincent et al 2015a;El-Maarry et al 2019) and cannot be linked with such seasonal activity (Benseguane et al 2022). These structures, also known as pits, have been observed on all of the Jupiter-family comets (JFCs) imaged by spacecraft (Pajola et al 2022).…”

“…At the surface of 67P/Churyumov-Gerasimenko (hereafter 67P), they are mostly present on the northern hemisphere (Leon-Dasi et al 2021). We recently studied their evolution through erosion as a result of thermally driven water-ice sublimation under current illumination conditions (Benseguane et al 2022). We showed that through sustained cometary activity, erosion tends to erase such sharp morphological features.…”