Surface Morphology of Comets and Associated Evolutionary Processes: A Review of Rosetta’s Observations of 67P/Churyumov–Gerasimenko

Lethuillier

Kreuzig

et al. 2021

A&A

Aims. The morphology of cometary surfaces can provide important information to constrain the composition and evolution of comets. In this work, we investigate the sublimation behavior of comet analog materials and how the sample composition affects the evolution of morphological features in laboratory experiments. In our experiments, we used dust ice mixtures as analog material to form observed cometary morphologies. Methods. We used ice-dust mixtures in different mixing ratios as cometary analog material. In order to obtain realistic results, we scaled the expected cohesive and gravitational forces on comets to laboratory conditions. The samples were placed in a vacuum sublimation chamber and permanently cooled down to temperatures below 150 K to simulate the space environment. In the experiment, the samples were insolated with a light source from two different directions and alterations on the surface were recorded with a camera. Results. We find that the morphology of sublimation residues of ice-dust mixtures is strongly dependent on the ice-dust ratio as well as the insolation direction. High amounts of ice cause constant surface alteration and lead to exotic morphologies. Low amounts cause fewer and more episodic surface changes during its sublimation. Collapse events resulting in irregular and very rough surfaces occur during horizontal insolation.

Section: Vol% and 33 Vol% Water Ice Contentmentioning

confidence: 95%

Section: Particle Scalingmentioning

confidence: 99%

Sublimation of ice-dust mixtures in cooled vacuum environments to reproduce cometary morphologies

Lethuillier

Kreuzig

et al. 2021

A&A

“…In the 1990s sublimation experiments with dust-ice mixtures were performed as part of the KOSI (KOmetenSImulation) project to simulate and understand these cometary surface processes (e.g., Grün et al 1989Grün et al , 1993Lämmerzahl et al 1995). However, these experiments are not suitable and were not designed to understand complex morphologies such as cliffs, cracks, or mass movements that have only later been observed on 67P/Churyumov-Gerasimenko (Thomas et al 2015;Pajola et al 2016;El-Maarry et al 2019). The composition and grain size of the material, interparticle forces, and porosity are of fundamental importance in processes that form these morphologies.…”

Section: Introductionmentioning

confidence: 99%

Tensile strength of dust-ice mixtures and their relevance as cometary analog material

Otto

Gundlach

et al. 2020

A&A

Aims. The tensile strength of granular matter is of great importance to our understanding of the evolution of comets and to our attempts to reproduce processes on cometary surfaces in laboratory experiments. In this work, we investigate the tensile strength of three different materials and their mixtures, which can be used as cometary analog materials in the laboratory. Methods. We used two types of siliceous dusts and granular water ice whose polydisperse particles were either angular or spherical. Our samples were cooled to below 150 K to better simulate the conditions of a cometary surface and to avoid thermal alteration of the material. We used the Brazilian disk test method to exert stress on the cooled samples and determine the tensile strength at the moment the samples broke. Results. We find that the tensile strength of two component mixtures is strongly dominated by the component with the higher tensile strength. The materials made of mostly angular dust particles have a lower filling fraction, but a higher tensile strength compared to materials made of spherical particles. Furthermore, the tensile strength of the cooled components is substantially lower than the tensile strength of the same components at room temperature. This implies that the surface energy of the investigated materials at low temperatures is significantly lower than previously assumed.

“…Remote sensing data of cometary nuclei have revealed a wealth of morphologic features that cover the surfaces of comets (Sunshine et al 2016;El-Maarry et al 2019). These include features unique to comets, such as pits (Vincent et al 2015) and quasi-circular depressions (Brownlee et al 2004), but they also encompass features known from other planetary bodies, such as mass-wasting deposits (Thomas et al 2015a), boulder fields (Pajola et al 2015), and sedimentary deposits (Thomas et al 2015b).…”

Section: Introductionmentioning

confidence: 99%

Discrete element modeling of boulder and cliff morphologies on comet 67P/Churyumov-Gerasimenko

Kappel

Sachse

et al. 2020

A&A

Context. Even after the Rosetta mission, some of the mechanical parameters of comet 67P/Churyumov-Gerasimenko’s surface material are not yet well constrained. These parameters are needed to improve our understanding of cometary activity or for planning sample return missions. Aims. We study some of the physical processes involved in the formation of selected surface features and investigate the mechanical and geometrical parameters involved. Methods. Applying the discrete element method (DEM) in a low-gravity environment, we numerically simulated the surface layer particle dynamics involved in the formation of selected morphological features. The material considered is a mixture of polydisperse ice and dust spheres with inter-particle forces given by the Hertz contact model, translational friction, rolling friction, cohesion from unsintered contacts, and optionally due to bonds from ice sintering. We determined a working set of parameters that enables the simulations to be reasonably realistic and investigated morphological changes due to modifications thereof. Results. The selected morphological features are reasonably well reproduced using model materials with a tensile strength on the order of 1–10 Pa. Increasing the diameters of the spherical particles decreases the material strength, and increasing the friction leads to a more brittle but somewhat stronger material. High friction is required to make the material sufficiently brittle to match observations, which points to the presence of very rough, even angular particles. Reasonable seismic activity does not suffice to trigger the collapses of cliffs without material heterogeneities or structural defects. Conclusions. DEM modeling can be a powerful tool to investigate mechanical parameters of cometary surface material. However, many uncertainties arise from our limited understanding of particle shapes, spatial configurations, and size distributions, all on multiple length scales. Further numerical work, in situ measurements, and sample return missions are needed to better understand the mechanics of cometary material and cometary activity.