ogenic 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.

DMS falls into the class of low-abundance but unique biosignatures that originate from secondary reactions, i.e., reactions that are not related to energy production or biomass build-up (Seager et al. 2016).

On Earth, the most relevant and best-studied source of DMS is the degradation of dimethylsulfoniopropionate (DMSP; C5H10O2S), a sulfur-bearing compound produced by eukaryotic marine phytoplankton and a few higher plants (Andreae & Raemdonck 1983; Pilcher 2003).

DMS, among a handful of small molecules, has been discussed as potentially observable indicator of aquatic life on habitable exoplanets with a hydrogen-dominated atmosphere (Domagal-Goldman et al. 2011; Seager et al. 2013; Catling et al. 2018; Schwieterman et al. 2018) and, more recently, on a specific class of ocean-covered exoplanets with a hydrogen-rich atmosphere, referred to as Hycean worlds (Madhusudhan et al. 2021; Rigby & Madhusudhan 2024).

Shortly after, Madhusudhan et al. (2023) reported the tentative detection of DMS in the atmosphere of exoplanet K2-18b, a candidate Hycean world, with the James Webb Space Telescope using transit spectroscopy.

Despite the DMS detection’s confidence was only 2.4σ/≈1σ/none (depending on how the relative detector offset was accounted for), this work received world-wide attention as it featured DMS as a potential indicator of marine biological activity on this planet, adding though that more conclusive observational constraints on the DMS atmospheric abundance are needed.

Better constrained abundances of atmospheric species could also underpin the classification of K2-18b as a Hycean world, which is being debated (Wogan et al. 2024; Shorttle et al. 2024; Glein 2024).

Generally, the search for extraterrestrial life should rely on multiple lines of evidence, cf., e.g., Catling et al. (2018) or Schwieterman et al. (2018).

We take a different path in this debate by asking if DMS is present in abiotic cometary matter.

Comets are known to be rich in refractory and volatile organic matter (Sandford 2008; Bardyn et al. 2017; Altwegg et al. 2019) that is thought to be well-preserved since the earliest times of our Solar System (Bockel´ee-Morvan et al. 2000; Drozdovskaya et al. 2019; L´opez-Gallifa et al. 2024), and hence, are a relevant point of reference when it comes to abiotic chemical complexity and diversity.

For this work, we revisited the organosulfur inventory of comet 67P (Calmonte et al. 2016; Mahjoub et al. 2023) and searched for evidence of the presence of DMS, and hence, for an abiotic source of this molecule.

Comet 67P was the target of the European Space Agency’s (ESA’s) Rosetta mission.

It was the first comet to be studied from up-close for an extended time period of two years as it passed through perihelion.

The Rosetta Orbiter Instrument for Ion and Neutral Analysis (ROSINA; Balsiger et al. 2007) with its high-resolution Double Focusing Mass Spectrometer (DFMS) was collecting unique data that unveiled this comet’s chemical inventory in unprecedented detail (Rubin et al. 2019; Altwegg et al. 2019).

Many surprising chemical species were detected, from molecular oxygen (Bieler et al. 2015) to ammonium salts (H¨anni et al. 2019; Altwegg et al. 2020; Poch et al. 2020; Altwegg et al. 2022), and including a plethora of complex organic molecules (Schuhmann et al. 2019a,b; H¨anni et al. 2022, 2023).