Dimethylsulfide on Earth is biological. Marine phytoplankton produce it in quantities large enough to influence cloud formation. When JWST detected possible dimethylsulfide in the atmosphere of K2-18 b — a sub-Neptune in the habitable zone — the implication was obvious: something alive might be making it.
Jordan, Tsai, Rimmer, and Shorttle test the alternative. In a reducing atmosphere rich in hydrogen and methane, abiotic chemistry can produce dimethylsulfide and dimethyldisulfide through pathways that don't require biology. The critical question is whether these pathways produce enough to match the observed abundances.
One abiotic pathway does. But its efficiency depends on reaction rate parameters that have never been measured in the laboratory — the chemistry occurs at temperatures and pressures found on sub-Neptunes but not easily reproduced on Earth. The biosignature claim hangs on unmeasured experimental values.
The complication deepens. The spectral features attributed to dimethylsulfide overlap with those of simple hydrocarbons like ethane, which is an expected photochemical product in hydrogen-rich atmospheres. The same spectral bump could be biology, abiotic organosulfur chemistry, or just ethane. Three explanations, one observation, no way to distinguish without either better spectra or the missing laboratory measurements.
The sulfur chemistry also explains a broader pattern. Hydrogen sulfide photochemistry produces sulfur hazes in these atmospheres, and variations in sulfur abundance across the sub-Neptune population could explain why some appear hazy and others clear — a diversity JWST has already observed.
The biosignature that excited everyone may be a by-product of the atmosphere talking to itself. The biology is possible; the chemistry is also possible; and the measurement that could tell them apart doesn't exist yet.