Water isotopes and hydronium in the evolving coma of 67P

Cometary isotopes preserve key information about the early Solar System and its presolar heritage. Rosetta’s long-term in situ observations of comet 67P/Churyumov–Gerasimenko offered the first opportunity to examine isotopic variability within a single comet. Past studies on the D/H ratio came to different results, and hence, we have reassessed the entire dataset carefully. We reassessed the isotopic composition of water in 67P’s coma using a unified Bayesian framework; tested for isotopic heterogeneities across mission phases, latitudes, and cometocentric distances; and evaluated whether hydronium ions (H O ) are present and variable in the Double Focusing Mass Spectrometer's (DFMS) neutral-mode spectra, particularly during dust-driven activity. 3 + We re-analysed the complete ROSINA/DFMS neutral-mode dataset using Bayesian nested sampling. The model accounts for mass-scale calibration, peak overlaps, and physically motivated priors. Water isotopologues were fitted jointly with potential hydronium contributions at 19. Correlations with gas abundances, cometocentric distance, and dust events were used to identify drivers of variability. m/z More than 7600 D/H measurements yielded a mission-average value of , about three times Earth’s value of 1.56 ⋅ 10^ (4.67 -4 , largely invariant with activity. Pre-perihelion latitude differences remain within uncertainties. Oxygen isotopes show a ∼40% enrichment in O and a ∼25% enrichment in O relative to Earth, higher than the previously reported ∼11% (taken from a subset of the data), without systematic spatial or temporal trends. Hydronium is consistently required to reproduce DFMS neutral-mode spectra at 19 and correlates with NH and other volatiles, with strong dust-event enhancements. Laboratory experiments indicate that H O forms mainly inside the DFMS ion source via proton transfer reactions involving H O and NH . 17 18 m/z 3 3 2 3 The isotopic composition of comet 67P reflects stable primordial signatures with little intra-comet variability. Hydronium traces ion–molecule chemistry and precursor availability near the spacecraft (S/C), with variability driven by water abundance, ammonia-mediated proton transfer, and dust-enhanced volatile release.