We revisited the relocation of a ∼30m boulder in the Khonsu region of comet 67P. With a ∼140m displacement, this is the most prominent boulder migration event observed on 67P.
- Utilizing a high-resolution shape model, we derived a detailed timeline for the event, narrowing the occurrence of migration to within 14 hours on 2015 October 3, roughly two months after 67P’s perihelion.
- Night-time mini-outbursts are found, both during the migration period, and in the following day, near the new location of the boulder.

- The boulder and its surroundings displayed a substantial morphological changes based on comparisons of pre- and post-perihelion observations.
- Thermophysical modelling shows that before the boulder’s migration, cumulative insolation on southern and northern sides of the boulder had different patterns. Analysis of the thermal history of the boulder identified a significant north-south dichotomy in the variation of temperature and flux rate. The displacement co-occurred with maximum temperature on the southern side 10 centimeters below the surface.
- New findings neither confirm nor invalidate hypotheses for the boulder’s migration in previous works. However, in addition to direct outburst and seismic vibration, we propose a third mechanism for the triggering of the boulder movement, which is by the propulsion due to intense outgassing on its southern side. However, the actual scenario might be a combination of several triggers.
- Displacement of large boulders on cometary surfaces presents a unique opportunity for probing cometary activity as well as a nucleus’ physical properties. Boulder-sized debris ranging from decimeter to meter sizes were discovered being ejected and falling back, (Shi et al. 2024; Agarwal et al. 2016), potentially driven by the sublimation of more volatile ices, such as carbon dioxide. For the displacement of even larger chunks like the one discussed here, mechanisms could be diverse and need to be more thoroughly explored, particularly regarding their interactions with local topography and dust activity. To this end, we plan to reconstruct time-varying topography of local Khonsu region to quantitatively depict changes of the boulder and its surroundings. This may also help in reconstructing a possible migration scenario using numerical methods.
The authors thank an anonymous referee for insightful comments that helped improve the manuscript significantly. This work is supported by the National Natural Science Foundation of China (No. 12233003). MRELM acknowledges funding from the KU internal grant (8474000336-KU-SPSG). XS thanks members of the ISSI International Team “Closing The Gap Between Ground Based And In-Situ Observations Of Cometary Dust Activity: Investigating Comet 67P To Gain A Deeper Understanding Of Other Comets” for inspirational discussions that helped to improve this study. OSIRIS was built by a consortium led by the Max-Planck-Institut f¨ur Sonnensystemforschung, G¨ottingen, Germany, in collaboration with CISAS, University of Padova, Italy, the Laboratoire d’Astrophysique de Marseille, France, the Instituto de Astrof´ısica de Andalucia, CSIC, Granada, Spain, the Scientific Support Office of the European Space Agency, Noordwijk, The Netherlands, the Instituto Nacional de T´ecnica Aeroespacial, Madrid, Spain, the Universidad Polit´echnica de Madrid, Spain, the Department of Physics and Astronomy of Uppsala University, Sweden, and the Institut f¨ur Datentechnik und Kommunikationsnetze der Technischen Universit¨at Braunschweig, Germany. The support of the national funding agencies of Germany (DLR), France (CNES), Italy (ASI), Spain (MEC), Sweden (SNSB), and the ESA Technical Directorate is gratefully acknowledged.
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