Evidence for Abiotic Dimethyl Sulfide in Cometary Matter

TL;DR

This study provides the first evidence of abiotic dimethyl sulfide (DMS) in cometary matter, suggesting complex sulfur chemistry can occur without biological processes, which has implications for understanding organic molecules in space.

Contribution

The paper demonstrates the presence of abiotic DMS in comet 67P, challenging the notion that DMS is exclusively a biosignature and revealing new insights into cometary sulfur chemistry.

Findings

DMS is compatible with observed mass spectra better than ethanethiol.

DMS abundance in comet 67P is approximately 0.13%.

First evidence of abiotic DMS formation in cometary matter.

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 $1-2.4\sigma$ spectral feature observed with JWST in the atmosphere of exoplanet K2-18b to the biosignature gas dimethyl sulfide (DMS; sum formula C$_2$H$_6$S) 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 $<1\sigma$, while for ethanethiol they are $2-4\sigma$. The local abundance of DMS relative to methanol deduced from these data is (0.13$\pm$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. [...]