Boulder Mobility on Comets: Insights from Rosetta Observations and Numerical Modelling

Boulder displacement on comets, including the migration of rocks up to tens of meters and relocation of decimeter- to meter-scale debris, are critical processes in reshaping the nucleus as well as redistributing volatile materials. During its over-two-year rendezvous with comet 67P/Churyumov–Gerasimenko (hereafter 67P), European Space Agency’s Rosetta spacecraft revealed a wide range of boulder activities. Decimeter- to meter-scale chunks were detected in the near-nucleus coma, with many ultimately falling back onto the comet's surface [1,2,3]. Boulder migration was also observed directly on the nucleus, like falling and bouncing of chunks from cliffs [4,5]. Different scenarios have been proposed for the destabilization of cometary boulders, such as occurrence of an outburst underneath the boulder, seismic vibrations from nearby active sources [6,7], acceleration by the surrounding asymmetric gas field [3], and 'rocket force' caused by volatile activity within the boulder [8].The most striking boulder displacement event occurred in the Khonsu region of 67P's southern hemisphere, where a ~30 m boulder was found to have moved ~140 m during the perihelion passage [6]. Recently, via a systematic search through imaging data obtained by Rosetta's OSIRIS camera system, we successfully narrowed the time of this event to within 14 hours on October 3, 2015 [8]. Observations also show numerous changes in the boulder's surrounding area, as well as localized night-time dust activities coinciding with the displacement [8]. These new observational constraints enable synthetic analysis on the boulder's triggeringmechanism in the context of its thermal history and dynamical environment.In this work, we present latest findings in investigating the unique event of Khonsu boulder. Leveraging the Discrete Element Method (DEM), we investigate the destabilization of the boulder under various conditions. Drawing from methodologies previously applied to asteroid surface evolution [e.g.,9, 10], we model 67P's nucleus surface as a granular medium, with physical properties constrained by Rosetta's in-situ observation data including those derived from Philae's landing dynamics [11,12,13]. Our simulations incorporate realistic topography, mechanical and thermal properties to assess the susceptibility of boulders of different scales to destabilizing forces. Preliminary results suggest the critical role of the comet's seismic efficiency in governing the mobility of large surface blocks, comparable to the impact-induced seismic shaking dynamics on asteroids [14,15]. These findings provide new insights into the physical conditions required to trigger major boulder displacements on comets, with direct implications for understanding surface evolution and volatile redistribution across small Solar System bodies.   References1. Agarwal, J. et al. Evidence of sub-surface energy storage in comet 67P from the outburst of 2016 July 03. Monthly Notices of the Royal Astronomical Society 469, s606–s625 (2017).2. Pfeifer, M., Agarwal, J. & Schröter, M. On the trail of a comet’s tail: A particle tracking algorithm for comet 67P/Churyumov-Gerasimenko. A&A 659, A171 (2022).3. Shi, X. et al. Diurnal Ejection of Boulder Clusters on Comet 67P Lasting beyond 3 au. ApJL 961, L16 (2024).4. Pajola, M. et al. 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