the north face experienced relatively short daytime and received much less energy, resulting in lower temperatures at all depths (Figure 9b). The surface temperature did not exceed 200 K over the entire rotation, while staying below 170 K for 70% of the time. Note that the surface temperature of facet N peaks at an earlier time than facet S, roughly around UTC 12:00. On orbital time scale, the dichotomy is even more evident. Figure 9 c&d show variation of temperatures at depths from one centimeter to one meter at the two facets over one orbital period of 14 6.45 years.

Facet S starts to be illuminated from roughly 200 days before perihelion at the end of January 2015. Amount of insolation on the facet increased till around perihelion, then gradually decreased. At the time of the migration event, temperature at one decimeter depth of facet S reached its orbital maximum of 225 K. Conversely, temperature variation in the top one decimeter of facet N shows two peaks, one at ∼80 days before perihelion at the end of May 2015, the other at ∼150 days after perihelion at the beginning of January 2016. At the time of boulder migration, facet N was experiencing minimum temperatures between the two peaks. The temperature was as low as 110 K at one decimeter deep, reaching its minimum during the perihelion period. As a consequence, sublimation of volatile ices also contrasts between the two facets. Figure 10 shows the intensity of water ice sublimation on the day of boulder migration. The mean flux rate of water vapour at facet S was ∼3.2 × 10−5 kg m−2s−1, three orders of magnitude higher than that at facet N, meaning that gas emission was occurring mainly, if not only on the southern side of the boulder. With uneven gas emission over the boulder, “rocket force” could be at work, as observed on decimeter-sized boulders in the near-nucleus coma (Agarwal et al. 2016). According to the equation (2) in Agarwal et al. (2016), the above flux rate could yield an acceleration of ∼2 × 10−5 m s−2, one order of magnitude smaller than the local gravitational acceleration of ∼2 × 10−4 m s−2. It is interesting to notice that the bulk of sublimation at facet S comes from a larger depth of about five centimeters (Figure 10a). This is due to the persistent illumination on the southern side of the boulder that desiccated the top few centimeters of subsurface. As the heat wave penetrates to deeper interior of the boulder, it could reach reservoirs of more volatile ices, such as carbon dioxide, causing more intensive sublimation activity. Previous works demonstrated the triggering of super-volatile sublimation could be the mechanism driving outbursts on 67P (Skorov et al. 2016; M¨uller et al. 2024).
5. DISCUSSION The wealth of imaging data acquired by Rosetta enabled us to constrain the time of this boulder migration event to within ∼14 hours. However, given the large distance between the spacecraft and the nucleus at the time, direct observation of the boulder’s motion is lacking, leaving this extraordinary cometary geological event to be enigmatic. Nevertheless, our new analysis and modelling results shed light on the possible scenarios for the boulder’s mobilisation, as discussed below:
5.1. Scenario A: Ejection by outburst As proposed in previous works, direct outburst stays the most straightforward scenario responsible for the displacement of the boulder (El-Maarry et al. 2017), in which outgassing at the boulder’s original location transported it to its new location via gas drag force. To lift a 30-m boulder, taking particle size and boulder shape into consideration, El-Maarry et al. (2017) estimated that the minimum gas production rate required is 25 kg·s−1 when the driving agent was H2O. This amounts to around 5.5% of the global production rate of 67P at the time of the migration event, which is ∼450 kg·s−1 (Hansen et al. 2016). An outburst at such scale would likely to release large quantity of dust particles, that could transiently increase the brightness of the coma significantly. For comparison, the gas production rate of the outburst on 2016 February 19 is estimated to be 10 kg·s−1 (Gr¨un et al. 2016), and it caused an increase in the brightness of 67P’s coma by two orders of magnitude (Figure 4 in Gr¨un et al. (2016)). Since the brightness surge usually lasts for less than half an hour, and given the sparse observations at the time, we are not able to conclude at different depths of (a)