S2.

We carefully checked if those species and their potential fragments, which are mostly m/z=57, 64, 74, 76, 84, and 94 according to Steins (2018), are present also on 15 March 2016.

However, the observed intensities are compatible with relevant and securely identified pure hydrocarbon species such as benzene.

No sign of C2H5S-bearing species could be found.
4.

The exact mass scale and isotopic relative abundances also excludes contributions of C2H6S isotopologues.

They are well below the uncertainty of the measurements.

Having ruled out contributions of heavier organosulfur species, we find DMS compatible (deviations between expected and observed intensities < 1σ) and ethanethiol much less compatible (deviations between expected and observed intensities 2-4σ) with ROSINA/DFMS data from 15 and 17 March 2016.

Neither DMS nor possible precursors such as DMSP or other C-S-bearing molecules have been detected in the spacecraft background (Schl¨appi et al. 2010) and also reactions at the hot filament of the DFMS’ ionization chamber are highly unlikely.

Industrially, DMS is produced from the reaction of methanol and hydrogen disulfide over a hot aluminum oxide catalyst (Roy 2000).

Both these molecules are common in comets, including comet 67P, where their abundance usually is on the order of 1% relative to water (Rubin et al. 2019).

Nevertheless, the ionization chamber’s open design, which does not allow pressure build-up, as well as the shielding of the hot filament impede chemical reactions.

The ion source itself remained below 25◦C during the whole mission.

We therefore argue that the observed DMS must be of cometary origin.

Various lines of evidence, collected in H¨anni et al. (2022), indicate a formation of volatile cometary organic molecules in the early Solar System history rather than on the comet itself.

In order to estimate the abundance of DMS in comet 67P, we sum over all signals associated to DMS in the data set collected on 15 March 2016 between 03:30 and 04:14 UTC and give the result relative to methanol (CH3OH).

Methanol, a representative of complex organic molecules such as DMS itself, is not only common in comets but also in the interstellar medium (ISM), see, e.g., Bockel´ee-Morvan et al. (2000) or Drozdovskaya et al. (2019).

From the analyzed spectra (Figures 2 and 3), we derive a relative abundance DMS/methanol = (0.13±0.04)% and methanethiol/methanol = (0.9 ± 0.27)%.

The relatively large, estimated error accounts for the circumstance that that neither the ionization cross-sections nor the detector sensitivities of those molecules are known.

Detailed information on the estimation of the error margins of abundances relative to methanol are in H¨anni et al. (2023).

Notably, abundances of specific molecules in comet 67P may expose pronounced time-variability.

A time series for selected species has been published by L¨auter et al. (2020).

This time-variability means that the relative abundance values reported for DMS in this work represent a snapshot.

In Appendix A.1, we delve into the time variability by comparing the data shown in Figures 2 and 3 to other data sets collected around the time of 67P’s outbound 8 H¨anni et al.

equinox in March 2016, in October 2014 (Calmonte et al. 2016), while the spacecraft was orbiting the comet at a constant cometocentric distance of 10 km for roughly two weeks, and during its perihelion passage in early August 2015 (H¨anni et al. 2022).

Due to contamination of the data by heavy organosulfur species, full deconvolution is only possible for the March 2016 data.

Although the signals are varying slightly in intensity, as can be seen in Figure 4, the deduced C2H5S/C2H6S abundance ratios collected in Table 1, remain stable within error margins.

Three out of four ratios are compatible with DMS as parent molecule within 1σ, while all four are off from the expectation value for ethanethiol by more than 3σ.

Furthermore, the C2H6S/CH4O abundance ratios were tabulated for the said data sets and demonstrate stability under similar conditions (March 2016 and October 2014).