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.

Given the likely existence of highly-volatile ices at that depth, the southern side of the boulder could experience explosive outgassing events. Meanwhile, the northern side was undergoing its ‘winter’, with temperatures staying below 120 K at all depths. This dichotomy in activity would lead to a contrast in outgassing flux on the two sides, exerting a net propulsion towards the north. In fact, the boulder performed much as a miniature nucleus. Globally, 67P’s southern hemisphere experienced active summer during perihelion passage, the intense solar heating led to the morphological and topographic diversity between the two hemisphere and triggered peak dust activity, introducing mass transport across nucleus from south to north. In this scenario, the source of the mini-outburst on October 3 (Figure 6a) could be the boulder itself. It would serve as an unexhausted source for the mini outburst on October 04 as well (Figure 6b1 ∼b4) . Or, the dust activity on October 4 might be sustained by newly exposed icy areas on the boulder due to its displacement.
5.4. Comparison with the other Khonsu “jumping boulder” Hasselmann et al. (2019) found in the northern high bank of Khonsu another noticeable “jumping boulder” with even larger dimension of ∼50 m. They argued that it was likely initially ejected by an outburst somewhere outside Khonsu region, travelled over the nucleus before falling back onto the surface. It possibly performed a gentle impact on the nearby cliff before touchdown while maintaining its intact structure. Similar to the boulder discussed in this work, the northern jumping boulder’s inferred movement is temporally aligned with a series of activities that could have been linked to its movement or interaction with the cliff. However, the relationship between the boulder migration event discussed in this work and surrounding dust activities is less direct. The only concrete observation during the migration process is linked to a night-time mini-outburst, the source of which remains unclear. It is possible that the activity is from the boulder itself if the motion was triggered by its own activity, or the dust activity could be from its new location triggered by the boulder’s impact. The dust emanation observed in the following days at the beginning of October could be activities from the freshly exposed icy surfaces of the migrated boulder or its surroundings. Given the similar observational evidence, it is possible that the two Khonsu “jumping boulders” could share a similar movement triggering mechanism. Both boulders could be involved in a scenario of direct ejection by outburst, with a high gas production rate comparable to that reported in Agarwal et al. (2017), or they could have been triggered and propelled by their own activity. However, as we do not know the original position of the other boulder in the high bank, it is difficult to compare quantitatively the two events.
6. CONCLUSION AND OUTLOOK
- 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.