unveiled this comet’s chemical inventory in unprecedented detail (Rubin et al. 2019; Altwegg et al. 2019).

Many surprising chemical species were detected, from molecular oxygen (Bieler et al. 2015) to ammonium salts (H¨anni et al. 2019; Altwegg et al. 2020; Poch et al. 2020; Altwegg et al. 2022), and including a plethora of complex organic molecules (Schuhmann et al. 2019a,b; H¨anni et al. 2022, 2023).

Calmonte et al. (2016) mentioned the presence of a signal at the exact mass of C2H6S in multiple ROSINA/DFMS mass spectra.

However, due to overlap with the nearby C5H2 signal as well as structural isomerism of DMS (CH3–S–CH3) and ethanethiol (CH3–CH2–SH), these authors did not make conclusive statements regarding the presence or absence of DMS in comet 67P.

Here, we revisit data from Calmonte et al. (2016) and, thanks to a more precisely constrained mass scale, we arrive at a spectral deconvolution that allows the distinction of DMS and ethanethiol.

Technical and methodological details are described in Section 2, while Section 3 presents and discusses our results.

Section 4 summarizes our findings, stating how they motivate further research on the sulfur chemistry in the early Solar System, and discusses potential implications for exoplanetary science.

Evidence for Abiotic Dimethyl Sulfide 3 2.

INSTRUMENT AND METHOD ROSINA/DFMS data collection and reduction have been described in detail by Le Roy et al. (2015) and Calmonte et al. (2016).

In the following, we will therefore limit ourselves to the presentation of the most relevant aspects.
2.1.

Data Acquisition with DFMS DFMS, together with a time-of-flight mass spectrometer and a pressure gauge making up the ROSINA instrument suite (Balsiger et al. 2007), was designed to study the volatile species in comet 67P’s coma.

Cometary neutrals entering the DFMS’ ionization chamber were subjected to electron ionization (EI) using a 45 eV electron beam.

Ions were subsequently extracted from the ion source and passed through an electrostatic analyzer and a permanent magnet, where the electric and magnetic fields led to separation of the species according to their mass-per-charge ratio (m/z).

The ions were detected by a stack of two micro channel plates mounted in a Chevron configuration.

The released charge was collected on two rows of position sensitive linear electron detector array anodes.

Scanning through the DFMS’ mass range was done by stepping from integer mass-per-charge to integer mass-per-charge sequentially and took approximately 30 s per mass spectrum, of which 20 s was the integration time.

We note that the micro channel plate detector works in analog mode.

This means that the charge deposited on the anodes during integration, i.e., the detector signal, depends not only on the number of impacting ions, but also on their energy, their structure and size, as well as on the detector gain.

The detector gain is determined by the set gain step (i.e., the voltage used to amplify the detector signal) and the individual pixel gain.

The latter factor relates to the inhomogeneous detector aging that affects the detector on timescales on the order of weeks and months.

The pixel gain is corrected by frequently measured correction factors.

Detailed information on pixel gain corrections have been presented in Le Roy et al. (2015) and De Keyser et al. (2019) or in Calmonte (2015) and Schroeder (2020).

In subsection 2.3, we estimate the errors, including uncertainties introduced by the detector gain.

The main data set analyzed for this work was collected on 15 March 2016 between 03:30 and 04:14 UTC, during comet 67P’s outbound equinox, and covers the mass range m/z=13–100.

That day, the most intense C2H6S signal was observed, based on the DFMS data evaluated so far.

DFMS achieved a mass resolution of m/∆m=3000 at 1% of the peak height at m/z=28 (Balsiger et al. 2007), which is exceptional for space instruments and allows searching for the fingerprints of specific molecules in a complex analyte mixture as we explain in detail the in following two subsections.
2.2.

Electron Ionization-Induced Fragmentation EI mostly yields singly charged cations of the analyte molecules, which we refer to as molecular ions (M), or fragments thereof.