facet N; (c) Orbital temperature variation at different depths of facet S; (d) the same as (c) but for facet N. Vertical gray line indicates the time of the boulder migration.]

[Figure: Figure 10. Flux rate of water vapour during one rotation of the nucleus starting from 2015-10-03T10:00:00]

facet S and (b) facet N whether such an outburst did occur within the 14 hour window of the migration event. Outbursts 16 are also commonly accompanied by surface changes, often revealing more pristine subsurface with higher ice content.

The best example might be the collapse of Aswan cliff that revealed fresh nucleus material with albedo six times that of the bulk value (Pajola et al. 2017). Surface changes also occur following the transition of water ice from the amorphous to the crystalline state. The outburst on 2016 July 3, with an estimated gas production rate of (18.4 ± 10.6) kg · s−1 affected an area of ∼10 m radius, creating an ice patch of 15×5 m2 (Agarwal et al. 2017). In the case of Khonsu boulder, the ‘after’ observations do not show prominent increase in albedo at the original location of the boulder (Figure 4 c0 & d0). However, the resolution of ∼15 m hinders quantitative analysis on the surface change immediately after the boulder migration, if there was any. While numerous surface changes were found near the boulder, as described in section 3.2, it is not clear if these changes are directly related to the boulder migration event.
5.2. Scenario B: Destabilisation by nearby activities Activities in neighbouring areas could destabilise the boulder and trigger its downslope movement either by eroding the slope it resided on (El-Maarry et al. 2017), or by seismic shaking (Hasselmann et al. 2019). Rosetta has observed many significant surface changes in Khonsu region after comet perihelion, including formation of cracks and pits, cliff collapses, and the boulder migrated into the northern high bank (Hasselmann et al. 2019). As the cohesion of surface materials on comet is considerably weaker than that on Earth, a small activity can induce perturbation that has a relatively large impact in proximity, as the theoretical calculations of P and S wave velocities are comparable to the escape velocity (Vincent et al. 2016b). Indeed, the night-time mini outbursts we found on both the day of the migration (Oct. 03) might perform as the source of destabilisation. It has been noticed that the original location of the boulder was close to the source of a massive outburst that occurred on 2015 December 13. This outburst revealed an ice-rich patch on the surface and had a similar behavior as another outburst that occurred on 2016 July 3 (Hasselmann et al. 2019). Seismic shaking has been observed on several asteroids, and was suggested as the cause for regolith transport (Richardson et al. 2020; Miyamoto et al. 2007). Impact-induced vibrations can play an important role in the evolution of regolith on irregular-shape small bodies, for the the seismic acceleration caused is comparable to surface gravity and responsible for gravel destabilization (Figure 4 of Miyamoto et al. (2007)). Yet the propagation of seismic wave significantly depends on the mechanical properties of the body. Bodies like Ryugu shows extremely low seismic efficiency as was found with the SCI impact by Hayabusa2 (Nishiyama et al. 2021; Michikami et al. 2019). In fact, 67P shows numerous ‘boulders-on-boulders’ features similar to those on Ryugu, that might imply a similarly low seismic efficiency (Figure 15).
5.3. Scenario C: boulder’s own activity Here we propose a third possible scenario responsible for the migration, which involves the boulder’s own activity. Noticeable boulders (≥7 meter) on 67P are presumed to be ice-rich and can be an inventory of volatiles (Pommerol et al. 2015; Pajola et al. 2015), just like the comet nucleus (Rubin et al. 2023). The migrating boulder in Khonsu, with such a distinctive 30-meter scale, may preserve different volatiles at different depths. As the thermophysical analysis shows, the boulder was subjected to heterogeneous insolation conditions that resulted in asymmetric energy accumulation and thermal history between its southern and northern sides. Profiles of orbital temperature and water flux variation show that the mobilisation event coincided with peaking of temperatures at the 17 depth of 10 centimeter on the boulder’s southern side, reaching over 200 K.