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.

2016). However, due to the low resolution of ∼19 m px−1 and the uncertainty in spacecraft orientations, it is unclear whether the outburst is from the unilluminated boulder itself or its surroundings. Here the boulder had already disappeared from its original location as inferred from the comparison between actual and synthetic images (Figure 6a0 & a1). Whether it reached its new location is debatable. According to the synthetic image, top of the boulder should be illuminated if it was already at the new location, which is not the case in the actual image. However, the identification of the boulder at its new location is subject to the accuracy of the shape model. If the boulder’s actual dimensions are smaller than that represented in the shape model, its appearance in the synthetic image would not necessarily indicate its the new location. Since the migration path was not illuminated, it is unclear whether the boulder was in motion or had already reached its new location. Roughly two rotations later, on 2015 October 4, a sequence of images with cadence of 10 minutes show the development of another mini-outburst in the same area close to the new location of the boulder (four frames are shown in Figure 6 b1 to b4, the sequence in appendix). Again, the smooth terrain containing the newly located boulder and the source of the mini-outburst was in darkness during the time, and we could only identify from synthetic images that the source of the activity is in immediate proximity to the boulder or on it (Figure 14). Roughly six days after the boulder migration, on 2015 October 9, a jet appeared next to the illuminated boulder (Figure 6c), which was more probably in the peak stage of dust ejection phenomena similar to mini-outburst documented by Knollenberg et al. (2016). Later on December 13, in similar insolation condition a massive night-time outburst originated from the patch on the edge of the cluster of lineaments near the original position of the boulder (Figure 6d, Hasselmann et al. (2019); El-Maarry et al. (2017)). A detailed timeline of events, including the evolution of local environment of migrating boulder as well as the dust activities is summarized in Figure 7 .
4. THERMOPHYSICAL ANALYSIS Given the better-constrained migration time of the boulder, we are able to investigate its thermal condition leading to the event. We performed facet-wise thermophysical modeling with a generic 1-D thermophysical code (Hu et al. 2017a,b; Hu & Shi 2021). The Energy input on a certain facet at any epoch is calculated as the instant insolation, which takes into consideration the orientation of the facet, bolometric bond albedo, as well as topographic shadowing effect (Hu et al. 2017b). The model is constructed such that both energy and volatile mass are conserved, which allows us to estimate the temporal variation of temperatures as well as ice content at different depths of each facet (Hu & Shi 2021). The conservation equations are as follows:
(cdρd + csρs + cgρg)∂T
∂x = −∂q
∂x −lgζg (1)
∂t + cgZg ∂T
∂ρg
∂t + ∂Zg
∂x = ζg (2) where cdρd, 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.