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).

While demanding more detailed correlation studies similar to the work done by Rubin et al. (2023), this might hint at a shared origin of DMS and methanol not only in the comet but maybe even in the formational history.

Additional pieces of the puzzle may be obtained from comparing the chemical complexity associated with cometary bulk ices (Rubin et al. 2019; Schuhmann et al. 2019a,b) to that associated to dust (Altwegg et al. 2020, 2022; H¨anni et al. 2022, 2023).
4.

CONCLUSIONS On Earth, DMS is a degradation product of DMSP, a sulfur-bearing compound produced by eukaryotic marine phytoplankton and a few higher plants (Andreae & Raemdonck 1983; Pilcher 2003).

It has been proposed (Seager et al. 2013) and employed (Madhusudhan et al. 2021, 2023) as a unique biosignature in the search for biological activity on exoplanets.

A thorough reanalysis of the high-resolution mass spectra collected by ROSINA/DFMS at comet 67P around the time of its outbound equinox, when Calmonte et al. (2016) have previously reported a C2H6S signal, provides strong evidence for the presence of DMS in cometary matter.

This is the first evidence of a non-anthropogenic abiotic formation pathway.

However, whether or not the existence of cometary, abiotic DMS is relevant to exoplanetary atmospheres and hence has implications for the use of DMS as a biosignature, remains to be investigated in future work.

Fayolle et al. (2017) have hypothesized for the case of chloromethane (CH3Cl), which is considered a biosignature attributed to oceanic bacteria on Earth (Seager et al. 2016) and has been detected in comet 67P, that exocomets impacting young exoplanetary surfaces may have delivered considerable amounts of material, including 6 ∗105 kg/yr of CH3Cl under peak influx conditions (Brasser et al. 2016).

It is a topic of debate, though, if such a scenario is generally realistic or applicable to DMS and potentially other cometary species.

Independent of DMS being employed as biosignature, we hope the detection of this molecule in pristine cometary matter encourages studies of abiotic formational pathways, for instance for dark molecular cloud conditions or protostellar environments.

Given its respectable abundance relative to methanol derived from this work, DMS may be a promising candidate for ISM molecular searches (spectroscopic reference data are available from Jabri et al. (2016) and Ilyushin et al. (2020)).

Laboratory work by Mahjoub et al. (2017) with astrochemical ice analogs containing hydrogen sulfide and methanol showed a signal on m/z=62 upon electron irradiation followed by temperature programmed desorption.

However, the presented identification of DMS by these authors misses acknowledging the ambiguities created by unit mass resolution EI-MS and simultaneous sublimation of various sulfur-bearing molecules.

Our results strongly motivate further investigation of the ISM sulfur-ice-chemistry as started by Mahjoub et al. (2017), instead focusing on the combination of small reduced sulfur and carbon carriers such as hydrogen sulfide and methane.

ACKNOWLEDGMENTS We gratefully acknowledge the work of the many engineers, technicians and scientists involved in the Rosetta mission and in the planning, construction, and operation of the ROSINA instrument.

Without their contributions, ROSINA would not have produced such outstanding data and scientific results.

Rosetta is an ESA mission with contributions from its member states and NASA.

Work at the University of Bern was funded by the Canton of Bern and the Swiss National Science Foundation (200020 207312).

M.C. acknowledges support from NASA grants 80NSSC18K1280 and 80NSSC20K0651.

J.D.K. acknowledges support from the Belgian Science Policy Office through PRODEX/ROSINA PEA 4000090020 and 4000107705.