Biogenic sulfur gases as biosignatures on temperate sub-Neptune waterworlds

Abstract

Theoretical predictions and observational data indicate a class of sub-Neptune exoplanets may have water-rich interiors covered by hydrogen-dominated atmospheres. Provided suitable climate conditions, such planets could host surface liquid oceans. Motivated by recent JWST observations of K2-18 b, we self-consistently model the photochemistry and potential detectability of biogenic sulfur gases in the atmospheres of temperate sub-Neptune waterworlds for the first time. On Earth today, organic sulfur compounds produced by marine biota are rapidly destroyed by photochemical processes before they can accumulate to significant levels. Domagal-Goldman et al. (2011) suggest that detectable biogenic sulfur signatures could emerge in Archean-like atmospheres with higher biological production or low UV flux. In this study, we explore biogenic sulfur across a wide range of biological fluxes and stellar UV environments. Critically, the main photochemical sinks are absent on the nightside of tidally locked planets. To address this, we further perform experiments with a 3D GCM and a 2D photochemical model (VULCAN 2D (Tsai et al. 2024)) to simulate the global distribution of biogenic gases to investigate their terminator concentrations as seen via transmission spectroscopy. Our models indicate that biogenic sulfur gases can rise to potentially detectable levels on hydrogen-rich waterworlds, but only for enhanced global biosulfur flux ($\gtrsim$20 times modern Earth's flux). We find that it is challenging to identify DMS at 3.4 $\mu m$ where it strongly overlaps with CH$_4$, whereas it is more plausible to detect DMS and companion byproducts, ethylene (C$_2$H$_4$) and ethane (C$_2$H$_6$), in the mid-infrared between 9 and 13 $\mu m$.