small boulder, and rectangles mark a lumpy relief in the possible route of migration. (a) side view observation of Khonsu region with the best spatial resolution before perihelion. (b) front view of boulder’s neighbouring areas. (c) observation of post-perihelion morphological state of local region from side view. (d) front view for inspecting the boulder’s surrounding after perihelion. Red arrows and red hollow cross mark the original location of the boulder, green arrows point to the boulder after its migration and orange dash lines identify the “pancake” feature.]

Numerous changes were detected including mass excavation processes and cases of instability, conforming with conclusions of previous studies that Khonsu is one of the most active zones on the nucleus (Hasselmann et al. 2019; Fornasier et al. 2019). Before its migration, the boulder situated between a shallow scarp and a cluster of outcrops (highlighted in Figure 5b & d), on a hill with rough texture. To quantitatively evaluate the boulder’s original dynamic environment, we chose 370 facets on the slope between the boulder’s original and final positions 11. The average effective slope of these facets is ∼25.5◦with a typical value of ∼23◦(Figure 11b), compared to the global typical value of ∼14◦over the 2 million facets. Close to the original position of boulder, the effective surface slope can reach ∼46◦. Therefore, the dynamic topography around the boulder could facilitate its motion once it was destabilized. After the migration, the boulder ended up in a smooth terrain near the “pancake” feature, surrounded by a cluster of small boulders. This smooth terrain covered ∼135 × 100 m2 before perihelion, in which a small boulder was almost buried (Figure 5b). After perihelion, however, this area shrunk as the top unconsolidated layer was likely exhumed, revealing the complex features on its outskirts along with the exposure of the small boulder (Figure 5b & d, marked by small circles near the “pancake” feature). Unlike the bouncing boulder in the comet’s smooth neck region (Vincent et al. 2019), as the migrating boulder moved northward downslope (Figure 11), its possible path displayed no clear track other than some mild mass removal. Only a lumpy relief located within the surface slope was detected have been eroded (Figure 5a & c, marked by small rectangles). Erosion processes were quite common in the nearby region. To the east of the boulder’s original location, the cluster of outcrops broke into three segments, accompanied with the exposure of several bright patches (Figure 5d). A small boulder on the south of the slope where the boulder originally stood appeared to have reoriented due to certain perturbation (Figure 5b & d, marked by small circles next to the original location of the migrating boulder). The lack of track could have different indications depending on the migration scenario of the boulder. It would be a natural consequence if the boulder was ejected from its original location into the near-nucleus space and subsequently fell back on the nucleus at its new location. In the case that the boulder was triggered and rolled downslope, the absence of obvious trail on the ground could imply a relatively high compressive strength of the surface. However, the observation of ‘no clear track’ is limited by the resolutions of pre-migration images, that are lower than 1.5 m pixel−1. If a clear mark was left, like those left by the Philae lander on its way to the final landing site (O’Rourke et al. 2020), it would serve as perfect experiments to derive the mechanical properties of the cometary material in the local surface.
3.3. Local dust activity Our inspection through the imaging data also reveal that the Khonsu region was a site of frequent night-time dust activity. In the image taken at UTC 19:30 on 2015 October 3 (Figure 4b), a minioutburst is detected emanating from the smooth terrain near the “pancake” feature (Figure 6a), when we stretched the contrast of the image. The mini-outburst showed a disperse morphology with the dust particles spreading out in all directions from a center point. The brightness in the center of dust emanation was at the same order as the general surface, one order higher than the dark environment. We constrained its source location by intersecting the boresight of the camera with the shape model. The inferred emission site is near the boulder’s new location (see projected 10 image in Figure 12), similar to the condition of the “mini-outburst” found in Imhotep region early in the mission (Knollenberg et al.