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.

N.F.W.L. acknowledges support from the Swiss National Science Foundation (SNSF) Ambizione grant 193453.

S.F.W. acknowledges support of the SNSF Eccellenza Professorial Fellowship PCEFP2 181150.

We are grateful to Daniel Kitzmann for enlightening discussions on the JWST observations of K2-18b.

Evidence for Abiotic Dimethyl Sulfide 9
APPENDIX
A.

APPENDIX INFORMATION A.1.

Time Variability of the Targeted Signals In order to investigate the time variability of the targeted S-bearing species, we have compared several m/z=62 and m/z=61 ROSINA/DFMS mass spectra mostly from mid-March 2016, see Figure 4 and Table 1.

In March 2016, comet 67P passed its outbound equinox, whith the spacecraft at a heliocentric distance of 2.58 au and at a cometocentric distance of approximately 13.5 km.

Due to the rotation of the comet and the orbit of the spacecraft, the observable longitudes and latitudes were constantly changing on a short time scale, although DFMS has a large field of view of 20◦.

In addition to the short-term variability, observed as a result of said changes in observational geometries or in cometary activity, there are other factors having an impact in the long-term.

As the comet moved around the Sun and passed its dust and gas outflow activity maximum shortly after reaching perihelion in August 2015, the Rosetta orbiter had to constantly readjust its relative position to the comet’s surface for safety reasons.

Sometimes, the distance also was changed for scientific reasons.

For instance, shortly after the mid-March 2016 data was collected, the spacecraft retracted from the comet in favor of plasma studies to much greater cometocentric distances.

As the local densities vary with 1/r2, where r is the cometocentric distance, relatively weak signals, < 1% relative to water, are difficult to observe.

For this reason, DMS was mainly observed when the spacecraft was close to the comet’s surface at the beginning of the mission or around the analyzed equinox period.

Its characteristic signal, i.e., C2H6S on m/z=62, was just above detection limit when the comet was very active around perihelion, even though the spacecraft was at a relatively large cometocentric distance.

Table 1 summarizes the spacecraft’s relative position together with the C2H5S/C2H6S and the C2H6S/CH4O abundance ratios for the four spectra shown in Figure 4 plus two additional ones.

For the latter two, from 10 October 2014 (Calmonte et al. 2017) and 3 August 2015 (H¨anni et al. 2022), no full analysis can be done as, in both cases, heavier CnHmS species and potentially even C5H are contaminating the C2H5S signal.

The derived C2H5S/C2H6S ratios thus have a large uncertainties.

From the listed abundance ratios we can conclude two things: First, three out of four C2H5S/C2H6S abundance ratios are compatible with DMS within 1σ, while for ethanethiol all are off by more than 3σ.

Note that for DMS, the expected C2H5S/C2H6S ratio is 33%, while for ethanethiol it is 15% (Steins 2018).

Second, the C2H6S/CH4O abundance ratios, where CH4O corresponds to the methanol molecular ion, are surprisingly constant too, except for the one in August 2015 when there was a lot of sublimation also from dust and the cometary activity was highly variable.

In October 2014 and March 2016, however, the comet was at a comparable heliocentric distance of around 3 au and the activity was more stable and rather comparable.

The respective C2H6S/CH4O abundance ratios are identical within 1σ error margins.