Activity and composition of periodic comets 67P/Churyumov-Gerasimenko and 103P/Hartley 2 at two different perihelion passages

Context. Comets are among the most primitive Solar System bodies, and they preserve material from its early formation stages. Studying their activity and composition provides key insights into the physical and chemical conditions of the primordial solar nebula. Periodic comets, such as 67P/Churyumov─Gerasimenko and 103P/Hartley 2, offer the opportunity for comparative and long-term monitoring over successive perihelion passages. Aims. Through photometry and spectroscopy, we studied the evolution of the activity and chemical composition of comet 67P during its 2025 and 2021 perihelion passages and of comet 103P during its 2010 and 2023 passages. For each comet, we aim to compare their behavior from one apparition to another. Methods. We used the TRAPPIST telescopes to monitor the comets using broadband and narrowband filters. From the broadband images, we produced light curves and computed color indices for each passage, and we derived the activity slopes. We used a Haser model to compute the production rates of five gaseous species (CN, C2, C3, OH, and NH) and derived the proxy parameter Afρ for dust activity. We also observed both comets in spectroscopy during their most recent apparition using the Himalayan Chandra Telescope and compared the spectroscopic data to our results obtained through photometry. Results. For both comets, our analysis of coma colors does not reveal any significant change from one passage to the other, indicating that the properties of the released dust grains are similar. Our values of the color indices are consistent with the mean values for Jupiter-family comets. We measured a slight increase in the gas and dust activities of comet 67P between 2015 and 2021, probably due to the small change in the comet's orbit that led the perihelion distance to decrease from 1.24 au for the first apparition to 1.21 au for the second one. Regarding 103P, we unambiguously measured a decrease (of at least 50%) in the gas and dust activities between 2010 and 2023, showing a different behavior for this young, active comet. We find a typical chemical composition for both comets and detect no variation of the C2-to-CN production rate ratios and dust-to-gas ratios from one passage to the other, indicating constant compositions, even if the level of activity has changed for 103P.