TY - JOUR A1 - Grott, Matthias A1 - Knollenberg, J. A1 - Hamm, M. A1 - Ogawa, K. A1 - Jaumann, R. A1 - Otto, Katharina Alexandra A1 - Delbo, M. A1 - Michel, Patrick A1 - Biele, J. A1 - Neumann, Wladimir A1 - Knapmeyer, Martin A1 - Kührt, E. A1 - Senshu, H. A1 - Okada, T. A1 - Helbert, Jorn A1 - Maturilli, A. A1 - Müller, N. A1 - Hagermann, A. A1 - Sakatani, Naoya A1 - Tanaka, S. A1 - Arai, T. A1 - Mottola, Stefano A1 - Tachibana, Shogo A1 - Pelivan, Ivanka A1 - Drube, Line A1 - Vincent, J-B A1 - Yano, Hajime A1 - Pilorget, C. A1 - Matz, K. D. A1 - Schmitz, N. A1 - Koncz, A. A1 - Schröder, Stefan E. A1 - Trauthan, F. A1 - Schlotterer, Markus A1 - Krause, C. A1 - Ho, T-M A1 - Moussi-Soffys, A. T1 - Low thermal conductivity boulder with high porosity identified on C-type asteroid (162173) Ryugu JF - Nature astronomy N2 - C-type asteroids are among the most pristine objects in the Solar System, but little is known about their interior structure and surface properties. Telescopic thermal infrared observations have so far been interpreted in terms of a regolith-covered surface with low thermal conductivity and particle sizes in the centimetre range. This includes observations of C-type asteroid (162173) Ryugu1,2,3. However, on arrival of the Hayabusa2 spacecraft at Ryugu, a regolith cover of sand- to pebble-sized particles was found to be absent4,5 (R.J. et al., manuscript in preparation). Rather, the surface is largely covered by cobbles and boulders, seemingly incompatible with the remote-sensing infrared observations. Here we report on in situ thermal infrared observations of a boulder on the C-type asteroid Ryugu. We found that the boulder’s thermal inertia was much lower than anticipated based on laboratory measurements of meteorites, and that a surface covered by such low-conductivity boulders would be consistent with remote-sensing observations. Our results furthermore indicate high boulder porosities as well as a low tensile strength in the few hundred kilopascal range. The predicted low tensile strength confirms the suspected observational bias6 in our meteorite collections, as such asteroidal material would be too frail to survive atmospheric entry7 Y1 - 2019 U6 - https://doi.org/10.1038/s41550-019-0832-x SN - 2397-3366 VL - 3 IS - 11 SP - 971 EP - 976 PB - Nature Publishing Group CY - London ER - TY - JOUR A1 - Grott, Matthias A1 - Knollenberg, J. A1 - Hamm, M. A1 - Ogawa, K. A1 - Jaumann, R. A1 - Otto, Katharina Alexandra A1 - Delbo, M. A1 - Michel, P. A1 - Biele, J. A1 - Neumann, W. A1 - Knapmeyer, M. A1 - Kuehrt, E. A1 - Senshu, H. A1 - Okada, T. A1 - Helbert, J. A1 - Maturilli, A. A1 - Müller, N. A1 - Hagermann, A. A1 - Sakatani, N. A1 - Tanaka, S. A1 - Arai, T. A1 - Mottola, S. A1 - Tachibana, S. A1 - Pelivan, Ivanka A1 - Drube, L. A1 - Vincent, J-B A1 - Yano, H. A1 - Pilorget, C. A1 - Matz, K. D. A1 - Schmitz, N. A1 - Koncz, A. A1 - Schröder, S. E. A1 - Trauthan, F. A1 - Schlotterer, M. A1 - Krause, C. A1 - Ho, T-M A1 - Moussi-Soffys, A. T1 - Low thermal conductivity boulder with high porosity identified on C-type asteroid (162173) Ryugu JF - Nature astronomy N2 - C-type asteroids are among the most pristine objects in the Solar System, but little is known about their interior structure and surface properties. Telescopic thermal infrared observations have so far been interpreted in terms of a regolith-covered surface with low thermal conductivity and particle sizes in the centimetre range. This includes observations of C-type asteroid (162173) Ryugu1,2,3. However, on arrival of the Hayabusa2 spacecraft at Ryugu, a regolith cover of sand- to pebble-sized particles was found to be absent4,5 (R.J. et al., manuscript in preparation). Rather, the surface is largely covered by cobbles and boulders, seemingly incompatible with the remote-sensing infrared observations. Here we report on in situ thermal infrared observations of a boulder on the C-type asteroid Ryugu. We found that the boulder’s thermal inertia was much lower than anticipated based on laboratory measurements of meteorites, and that a surface covered by such low-conductivity boulders would be consistent with remote-sensing observations. Our results furthermore indicate high boulder porosities as well as a low tensile strength in the few hundred kilopascal range. The predicted low tensile strength confirms the suspected observational bias6 in our meteorite collections, as such asteroidal material would be too frail to survive atmospheric entry7. Y1 - 2020 U6 - https://doi.org/10.1038/s41550-019-0832-x SN - 2397-3366 VL - 3 IS - 11 SP - 971 EP - 976 PB - Nature Publishing Group CY - London ER - TY - JOUR A1 - Kappel, David A1 - Sachse, Manuel A1 - Haack, David A1 - Otto, Katharina A. T1 - Discrete element modeling of boulder and cliff morphologies on comet 67P/Churyumov-Gerasimenko JF - Astronomy and astrophysics : an international weekly journal N2 - 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. KW - comets: general KW - comets: individual: 67P KW - Churyumov-Gerasimenko KW - methods: numerical Y1 - 2020 U6 - https://doi.org/10.1051/0004-6361/201937152 SN - 0004-6361 SN - 1432-0746 VL - 641 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Otto, Katharina Alexandra A1 - Jaumann, R. A1 - Krohn, K. A1 - Spahn, Frank A1 - Raymond, C. A. A1 - Russell, C. T. T1 - The Coriolis effect on mass wasting during the Rheasilvia impact on asteroid Vesta JF - Geophysical research letters N2 - We investigate the influence of the Coriolis force on mass motion related to the Rheasilvia impact on asteroid Vesta. Observations by the NASA Dawn mission revealed a pattern of curved radial ridges, which are related to Coriolis-deflected mass-wasting during the initial modification stage of the crater. Utilizing the projected curvature of the mass-wasting trajectories, we developed a method that enabled investigation of the initial mass wasting of the Rheasilvia impact by observational means. We demonstrate that the Coriolis force can strongly affect the crater formation processes on rapidly rotating objects, and we derive the material's velocities (28.9 ± 22.5 m/s), viscosities (1.5–9.0 × 106 Pa s) and coefficients of friction (0.02–0.81) during the impact modification stage. The duration of the impact modification stage could be estimated to (1.1 ± 0.5) h. By analyzing the velocity distribution with respect to the topography, we deduce that the Rheasilvia impactor hit a heterogeneous target and that the initial crater walls were significantly steeper during the modification stage. Y1 - 2016 U6 - https://doi.org/10.1002/2016GL071539 SN - 0094-8276 SN - 1944-8007 VL - 43 SP - 12340 EP - 12347 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Sachse, Manuel A1 - Kappel, David A1 - Tirsch, Daniela A1 - Otto, Katharina A. T1 - Discrete element modeling of aeolian-like morphologies on comet 67P/Churyumov-Gerasimenko JF - Astronomy and astrophysics : an international weekly journal N2 - Context. Even after the Rosetta mission, some of the mechanical parameters of comet 67P/Churyumov-Gerasimenko's surface material are still not well constrained. They are needed to improve our understanding of cometary activity or for planning sample return procedures. Aims. We discuss the physical process dominating the formation of aeolian-like surface features in the form of moats and wind taillike bedforms around obstacles and investigate the mechanical and geometrical parameters involved. Methods. By applying the discrete element method (DEM) in a low-gravity environment, we numerically simulated the dynamics of the surface layer particles and the particle stream involved in the formation of aeolian-like morphological features. The material is composed of polydisperse spherical particles that consist of a mixture of dust and water ice, with interparticle forces given by the Hertz contact model, cohesion, friction, and rolling friction. We determined a working set of parameters that enables simulations to be reasonably realistic and investigated morphological changes when modifying these parameters. Results. The aeolian-like surface features are reasonably well reproduced using model materials with a tensile strength on the order of 0.1-1 Pa. Stronger materials and obstacles with round shapes impede the formation of a moat and a wind tail. The integrated dust flux required for the formation of moats and wind tails is on the order of 100 kg m(-2), which, based on the timescale of morphological changes inferred from Rosetta images, translates to a near-surface particle density on the order of 10(-6)-10(-4) kg m(-3). Conclusions. DEM modeling of the aeolian-like surface features reveals complex formation mechanisms that involve both deposition of ejected material and surface erosion. More numerical work and additional in situ measurements or sample return missions are needed to better investigate mechanical parameters of cometary surface material and to understand the mechanics of cometary activity. KW - comets: general KW - comets: individual: 67P/Churyumov-Gerasimenko KW - methods: numerical Y1 - 2022 U6 - https://doi.org/10.1051/0004-6361/202141296 SN - 0004-6361 SN - 1432-0746 VL - 662 PB - EDP Sciences CY - Les Ulis ER -