ROSINA ion zoo at Comet 67P

Abstract

Context. The Rosetta spacecraft escorted Comet 67P/Churyumov-Gerasimenko for 2 yr along its journey through the Solar System between 3.8 and 1.24 au. Thanks to the high resolution mass spectrometer on board Rosetta, the detailed ion composition within a coma has been accurately assessed in situ for the very first time. Aims. Previous cometary missions, such as Giotto, did not have the instrumental capabilities to identify the exact nature of the plasma in a coma because the mass resolution of the spectrometers onboard was too low to separate ion species with similar masses. In contrast, the Double Focusing Mass Spectrometer (DFMS), part of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis on board Rosetta (ROSINA), with its high mass resolution mode, outperformed all of them, revealing the diversity of cometary ions. Methods. We calibrated and analysed the set of spectra acquired by DFMS in ion mode from October 2014 to April 2016. In particular, we focused on the range from 13–39 u q−1. The high mass resolution of DFMS allows for accurate identifications of ions with quasi-similar masses, separating 13C+ from CH+, for instance. Results. We confirm the presence in situ of predicted cations at comets, such as CHm+ (m = 1−4), HnO+ (n = 1−3), O+, Na+, and several ionised and protonated molecules. Prior to Rosetta, only a fraction of them had been confirmed from Earth-based observations. In addition, we report for the first time the unambiguous presence of a molecular dication in the gas envelope of a Solar System body, namely CO2++.

The Evolution of the Electron Number Density in the Coma of Comet 67P at the Location of Rosetta from 2015 November through 2016 March

“…+ is expected to form pre-dominantly via a series of two ion-neutral reactions (Beth et al, 2016). On the other hand, at lower activity and closer to the nucleus, the variable and often low H 3 O + /H 2 O + number density ratios measured by ROSINA/DFMS (Fuselier et al, 2015) does not really line up with the ions being collisionally coupled to the neutrals.…”

“…To clarify this; chemical models (e.g., Vigren & Galand, 2013;Fuselier et al, 2015;Heritier et al, 2017;Vigren 2018;Beth et al, 2019) that run with the assumption that the ions are cold and moves radially outward with the neutral gas, predicts H 3 O + to most often largely dominate over H 2 O + at the location of Rosetta and this has no strong support in the ROSINA/DFMS ion measurements at low to moderate activity. It should be mentioned that further investigations are needed to come to an understanding of the observed highly variable H 3 O + /H 2 O + ratio (in particular the highly variable H 2 O + , Beth et al, 2016) to confirm whether or not it actually is due to a lack of collisional coupling.…”

Knowledge Gaps in the Cometary Spectra of Oxygen-Bearing Molecular Cations

“…It is unclear whether this O 2 was present at the comet's formation, is trapped in cometary ices or clathrates (Luspay-Kuti et al 2018), or is produced by a chemical reaction with materials on the surface or in the coma (Fortenberry et al 2019). Albeit its presence in cometary atmospheres was first suggested 70 years ago (Swings & Page 1950), the molecular oxygen cation actually has not been detected remotely to date in either cometary or interstellar media (Glinski et al 2004), and was only spuriously detected in situ by Rosetta's ROSINA instrument (Beth et al 2020). Three faint rotational lines of X 3 Σ − g O 2 were finally detected in 2011 towards Orion (Goldsmith et al 2011), but the cation remains elusive for detection in the ISM as well as in comets.…”

“…This paper presents a critical analysis for the state of spectroscopic data currently available (Table 1), as well as existing data gaps, for diatomic and triatomic oxygen-bearing cations that are abundant in comets: H 2 O + , CO + 2 , CO + , OH + , and O + 2 . These five cations are produced by photodissociation or electron-impact of the most common cometary neutral species: H 2 O, CO 2 , and CO (Bockelée-Morvan & Biver 2017; Beth et al 2020). Our discussion is limited to transitions above 200 nm, roughly the lower limit for the quantum efficiency of most CCD detectors requiring different hardware for observation.…”

Abiotic molecular oxygen production -- ionic pathway from sulphur dioxide

“…Also, preliminary work by Larimian et al [7] suggests as possible reaction path CO 2+ 2 → O + 2 + C + . Since CO 2+ 2 dications were detected in the outer atmospheres of those Solar System bodies [8,9], the high O + 2 concentration in the ionospheres of both Venus and Mars may have some contribution from CO 2+ 2 dissociation. However, these findings cannot explain the high abundance of O 2 and related ions in other planetary atmospheres where CO 2 is not a dominant component.…”