H3O+ and D/H ratios in dust-related outgassing at 67P: Evidence for salt-rich grains?

Cometary dust is a key tracer of early Solar System material, preserving primitive components that offer insight into comet formation and evolution. ESA’s Rosetta mission to comet 67P/Churyumov-Gerasimenko (67P) provided an unprecedented opportunity to study the dust and gas environment of a comet's inner coma in situ, over a two-year period surrounding its perihelion in August 2015. Due to the spacecraft’s low relative velocity with respect to the nucleus, dust particles could be collected with minimal alteration (Longobardo et al., 2022).

Dedicated dust instruments aboard Rosetta, including GIADA (Grain Impact Analyzer and Dust Accumulator) and COSIMA (Cometary Secondary Ion Mass Analyzer), characterized dust grain velocities, sizes, and densities (Agarwal et al., 2017), while MIDAS (Micro-Imaging Dust Analysis System), the first Atomic Force Microscope flown in space, revealed detailed micro- and nanoscale structures (Bentley et al., 2016; Mannel et al., 2019). In addition, the ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) suite – though primarily a gas analyzer – also contributed to dust studies. Its COPS (Comet Pressure Sensor) detected transient dust events (Pestoni et al., 2023) and the DFMS (Double Focusing Mass Spectrometer) enabled chemical analysis of volatiles released during phases with a lot of dust in the coma (Hänni et al., 2022).

We report on the detection of H₃O⁺ (hydronium ion) at m/z = 19 in DFMS’s neutral mode and its correlation with gas species associated with dust events, pointing to a link between H₃O⁺ formation and salt-bearing dust grains. Salts, particularly ammonium salts, have also been proposed as key contributors to the nitrogen depletion observed in cometary comae (Altwegg et al., 2020, 2022; Poch et al., 2020). Additionally, we present a comparative analysis of the D/H ratio in H₂O in the ambient gas coma versus that observed during dust events, when water was released from grains. These results are further compared to D/H ratios from the literature for the gas and dust phase, for instance in the refractory organics measured by the COSIMA instrument (Paquette et al., 2021).

We discuss the implications of the presence of H₃O⁺ released from dust grains, its potential formation pathways, and isotopic ratios for understanding salt chemistry and the coupling between volatile and solid phases in cometary comae.

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