spectra have been taken from NIST (Steins 2018) and were scaled to C2H6S on m/z=62 for both DMS and ethanethiol and to CH4S on m/z=48 for methanethiol as those species can be unambiguously assigned to the respective molecular ions. The hatched contribution to C2H4S on m/z=60 can be assigned to thiirane and/or thioacetaldehyde (parent), fragments of which we do not indicate as no reference spectrum is available for thioacetaldehyde. Details are given in the main text.]

Based on Figure 3 as well as Figure 4 and Table 1 in the appendix, DMS consistently reproduces the observed intensities together with methanethiol much better than ethanethiol. However, fragmenting heavier organosulfur species could contribute to the signals between m/z=45 and 61, especially to m/z=61 (C2H5S) and 46 (CH2S), and explain the intensity that is left unexplained in Figure 3 (bottom). We ruled out this possibility based on the following line of reasoning:

Evidence for Abiotic Dimethyl Sulfide 7 1.

The signal of fully saturated C2H6S on m/z=62 can only indicate DMS or ethanethiol or a combination of both, but it cannot be a charge retaining fragment of a heavier organosulfur as such fragments would be unsaturated.

Approximately 40% of the observed intensity on C2H5S on m/z=61 cannot be explained if all the C2H6S signal on m/z=62 is associated to ethanethiol.

If there are no heavier organosulfur parent species whose fragments could explain this residual intensity, then DMS is the only explanation.
2.

Therefore, we searched the ROSINA/DFMS data for observed signals of parent species that theoretically yield C2H5S on m/z=61.

Candidate molecules must contain at least two C atoms, five H atoms, and one S atom.

S-bearing species with molecular masses larger than 61 Da observed on 15 March 2016 are S2 and SO2, both registered on m/z=64, as well as CS2 on m/z=76, and their minor isotopologues.

Those species are abundant and well-studied coma species (Le Roy et al. 2015; Calmonte et al. 2016).

However, they cannot produce the C2H5S fragment signal on m/z=61 and therefore are not relevant.

Up to m/z=100, corresponding to the upper end of the mass range scanned on that day, no species which could contribute to the C2H5S fragment were observed.

Apart from a few pure hydrocarbon species (CmHn), only krypton isotopes have been detected.
3.

The picture is different when the comet was close to its perihelion, beginning of August 2015.

H¨anni et al. (2022) presented and discussed a data set collected on 3 August 2015, when the mass range was scanned up to m/z=140 and the comet’s coma was dusty.

They reported the presence of a series of CmHnS-bearing species which shows that DFMS is indeed able to identify these species – if they were present.

The same authors derived upper limits in cases where distinction of the pure hydrocarbon and the S-bearing species with the same nominal mass was not possible as the exact masses are inseparable even at the mass resolution of DFMS.

Intensities at the positions of the following C2H5S-bearing species have been observed: C3H5S, C3H6S, C3H8S, C4H5S, C4H6S, C6H8S, C3H5OS, C3H6OS, C3H7OS, C4H8OS, and C2H6S2.

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.