ρd, csρs, and cgρg are the products of heat capacity and mass density of refractory dust, water ice, and water vapour, respectively. Zg is the mass flux of water vapour diffused through the porous top layer of the nucleus. q is the heat flux by conduction, lg is the latent heat of water vapour, and ζg is the flux of water vapour sublimation or deposition. Thermophysical states of the boulder and its surrounding area are investigated on both diurnal and orbital time scale by converging over one rotation or one orbit of the nucleus, respectively.

[Figure: Figure 6. Activities related to the migrating boulder in Khonsu region in early and after 2015 October. The cometary activities parts in the images are substituted with the contrast-stretched counterparts for better visualization. (a) the essential and only image for clues of missing boulder in the short time frame when the boulder made its moving, a is the brightness-enhanced image for identifying the dust emission. The activity event is indicated with dash lines. a0 is the original image and a1 is the synthetic version of local observation. (b) the sequence captured right next two comet rotations after the disappearance of the boulder, showing a prolonged activity after migration event as well as the indication of the location of boulder. Only four images from the sequence are present in this mosaic. (c) an apparent jet next to the boulder and (d) a well-investigated outburst recorded in 2015 December 13 (Hasselmann et al. (2019); El-Maarry et al. (2017), Supp. Mat. S5). Most dust activities appeared in night condition, making dust activities in this restricted area really special. Ejected dust particles dispersed from the badly illuminated area and thus presented an increase in brightness in images.]

[Figure: Figure 7. Timeline combining the boulder migration event and local dust activities with their spatial and temporal correlations. The events are marked on comet-spacecraft distance variation. The blue background displays the zoom-in of timestamps from September 20 to October 10, in which local surface changes and activities peaked after comet’s perihelion.]

Primary model parameters are listed in Table 2. Detailed numerical treatment and model parameters could be found in Hu et al. (2017b) and Hu & Shi (2021). We use a cutout from the SHAP7 shape model of 500 facets containing the boulder at its original location (red-shaded facets in Figure 1) for the thermal modelling. The meter-scale resolution of the shape model allows the investigation of differential thermal development over the boulder and its surroundings.

[Table: Table 2. Parameters of the thermophysical model
Parameter Symbol Value
Diurnal Orbital
Dust heat capacity cd 1200 J K−1 kg−1
Water-ice heat capacity cs 90 + 7.49T J K−1 kg−1
Water vapour heat capacity cs 1400 J K−1 kg−1
Compact dust density ρd 2000 kg m−3
Compact water-ice density ρs 1000 kg m−3
Dust volume-filling fraction f0
0.2 Initial ice volume fraction f0
0.1 Water latent heat lg 2.6 × 106 J kg−1
Step in depth ∆x 1 mm Step in time ∆t tP/40000 tP/2000 Nucleus spin period tP
44655 s]

13 Thermal modelling results show significant dichotomy between the northern and southern sides of the boulder, which is similar to the global dichotomy between the northern and southern hemispheres of 67P due to its large obliquity of ∼40◦. The contrast is essentially induced by received insolation energy. Figure 8a shows the map projection of cumulative insolation calculated for the boulder and its surroundings during the five months from May 2014, when 67P was at a perihelion distance of ∼4 au, until the occurrence of the migration on at the beginning of October 2015. The average energy received by southern side of the boulder is as much as double that of the northern side. Intriguingly, the southern side of the boulder was subject to constant illumination before the migration event, with some of the facets experiencing constant illumination over more than 150 rotations (Figure 8b).

[Figure: Figure 8. Insolation history of the boulder and its surrounding areas before its migration. (a) cumulative insolation between 2014-05-17 and 2015-10-03; The locations of representative facets S and N are indicated with red and blue crosses, respectively; (b) number of rotations with constant illumination, or ‘polar days’, for each facet.]