ABSTRACT Technological progress related to astronomical observatories such as the recently launched James Webb Space Telescope (JWST) allows searching for signs of life beyond our Solar System, namely in the form of unambiguous biosignature gases in exoplanetary atmospheres. The tentative assignment of a 1-2.4σ spectral feature observed with JWST in the atmosphere of exoplanet K2-18b to the biosignature gas dimethyl sulfide (DMS; sum formula C2H6S) raised hopes that, although controversial, a second genesis had been found. Terrestrial atmospheric DMS is exclusively stemming from marine biological activity and no natural abiotic source has been identified – neither on Earth nor in space. Therefore, DMS is considered a robust biosignature. Since comets possess a pristine inventory of complex organic molecules of abiotic origin, we have searched high-resolution mass spectra collected at comet 67P/Churyumov-Gerasimenko, target of the European Space Agency’s Rosetta mission, for the signatures of DMS. Previous work reported the presence of a C2H6S signal when the comet was near its equinox but distinction of DMS from its structural isomer ethanethiol remained elusive. Here we reassess these and evaluate additional data. Based on differences in the electron ionization induced fragmentation pattern of the two isomers, we show that DMS is significantly better compatible with the observations. Deviations between expected and observed signal intensities for DMS are < 1σ, while for ethanethiol they are 2-4σ. The local abundance of DMS relative to methanol deduced from these data is (0.13 ± 0.04)%. Our results provide the first evidence for the existence of an abiotic synthetic pathway to DMS in pristine cometary matter and hence motivate more detailed studies of the sulfur chemistry in such matter and its analogs. Future studies need to investigate whether or not the present inference of cometary DMS could provide an abiotic source of DMS in a planetary atmosphere.
Keywords: comets:general – comets: individual: 67P/Churyumov-Gerasimenko – instrumentation: detectors – methods: data analysis 1. INTRODUCTION Based on data collected during an Earth fly-by by the National Aeronautics and Space Administration (NASA) spacecraft Galileo, Sagan et al. (1993) proposed that unique molecular indicators of carbon-based life on Earth could be used to search for and detect extraterrestrial life. Today, those indicators are often referred to as biosignatures or sometimes as biomarkers, the latter term being borrowed from the field of medicine. The use of biosignature gases is especially interesting for the search for life in environments like exoplanets that are not accessible by space missions for in-situ studies. Currently, potential life on exoplanets is only detectable with remote sensing spectroscopic techniques using ground- or space-based observatories. A recent example is NASA’s James Webb Space Telescope, capable of
Corresponding author: Nora H¨anni nora.haenni@unibe.ch 2 H¨anni et al.

investigating exoplanetary atmospheres in the near-to-mid-infrared spectral range in unprecedented detail.

Taking advantage of the constantly increasing resolving power, wavelength coverage, and sensitivity of exoplanet observatories, Seager et al. (2016) compiled a list of terrestrial atmospheric molecules that are produced by biological activity and hence could be employed as biosignatures.

Crucial properties of such biosignature gases are, besides volatility, abundance, and stability under exoplanet-like conditions, unique spectral features and the absence of false positives.

Beyond the candidates already suggested by Sagan et al. (1993), molecular oxygen (O2; produced by photosynthesis) and methane (CH4; produced by biology, for example through methanogenesis), the list in Seager et al. (2016) also mentions the simple sulfur-bearing molecule dimethyl sulfide (DMS; C2H6S).

Methane can be produced by both biotic and abiotic chemical processes, including methanogenic bacteria (Taubner et al. 2018) and oceanic hydrothermal vents (Horita & Berndt 1999).

This may be relevant, for instance, for Saturn’s icy moon Enceladus (Bouquet et al. 2015; Waite et al. 2017).

So far, for DMS, no false positives are known.

However, Raulin & Toupance (1975) have shown experimentally that electrical discharge in a gaseous mixture of hydrogen sulfide (H2S) and methane can lead to the abiotic formation of DMS.