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Aims. The Rosetta mission provided us with detailed data of the surface of the nucleus of comet 67P/Churyumov-Gerasimenko. In order to better understand the physical processes associated with the comet activity and the surface evolution of its nucleus, we performed a detailed comparative morphometrical analysis of two depressions located in the Ash region. Methods. To detect morphological temporal changes, we compared pre- and post-perihelion high-resolution (pixel scale of 0.07–1.75 m) OSIRIS images of the two depressions. We quantified the changes using the dynamic heights and the gravitational slopes calculated from the digital terrain model of the studied area. In particular, we measured seven geometric parameters associated with the two depressions (length, three width values, height, area, and volume) using the ArcGIS software before and after perihelion. Results. Our comparative morphometrical analysis allowed us to detect and quantify the temporal changes that occurred in two depressions of the Ash region during the last perihelion passage. We find that the two depressions grew by several meters. The area of the smallest depression (structure I) increased by 90 ± 20%, with two preferential growths: one close to the cliff associated with the apparition of new boulders at its foot, and a second one on the opposite side of the cliff. The largest depression (structure II) grew in all directions, increasing in area by 20 ± 5%, and no new deposits have been detected. We interpreted these two depression changes as being driven by the sublimation of ices, which explains their global growth and which can also trigger landslides. The deposits associated with depression II reveal a stair-like topography, indicating that they have accumulated during several successive landslides from different perihelion passages. Overall, these observations bring additional evidence of complex active processes and reshaping events occurring on short timescales (months to years), such as depression growth and landslides, and on longer timescales (decades to millenniums), such as cliff retreat.

In an earlier campaign to characterize the mass of the transiting temperate super-Earth K2-18b with HARPS, a second, non-transiting planet was posited to exist in the system at $\sim 9$ days. Further radial velocity follow-up with the CARMENES spectrograph visible channel revealed a much weaker signal at 9 days which also appeared to vary chromatically and temporally leading to the conclusion that the origin of the 9 day signal was more likely to be related to stellar activity than to being planetary. Here we conduct a detailed re-analysis of all available RV time-series, including a set of 31 previously unpublished HARPS measurements, to investigate the effects of time-sampling and of simultaneous modelling of planetary + activity signals on the existence and origin of the curious 9 day signal. We conclude that the 9 day signal is real and was initially seen to be suppressed in the CARMENES data due to a small number of anomalous measurements, although the exact cause of these anomalies remains unknown. Investigation of the signal's evolution in time, with wavelength, and detailed model comparison reveals that the 9 day signal is most likely planetary in nature. By this analysis, we reconcile the conflicting HARPS and CARMENES results and measure precise and self-consistent planet masses of $m_{p,b} = 8.63 \pm 1.35$ and $m_{p,c}\sin{i_c}=5.62 \pm 0.84$ M$_{\oplus}$. This work, along with the previously published RV papers on the K2-18 planetary system, highlight the importance of understanding one's time-sampling and of simultaneous planet + stochastic activity modelling, particularly when searching for sub-Neptune-sized planets with radial velocities., 19 pages, 10 figures (including 9 interactive figures when viewed in Adobe Acrobat), accepted for publication in Astronomy & Astrophysics