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Research for possible biosignature gases on habitable exoplanet atmosphere isaccelerating. We add isoprene, C5H8, to the roster of biosignature gases. Wefound that formation of isoprene geochemical formation is highlythermodynamically disfavored and has no known abiotic false positives. Theisoprene production rate on Earth rivals that of methane (~ 500 Tg yr-1). OnEarth, isoprene is rapidly destroyed by oxygen-containing radicals, but itsproduction is ubiquitous to a diverse array of evolutionarily distantorganisms, from bacteria to plants and animals-few, if any at all, volatilesecondary metabolite has a larger evolutionary reach. While non-photochemicalsinks of isoprene may exist, the destruction of isoprene in an anoxicatmosphere is mainly driven by photochemistry. Motivated by the concept thatisoprene might accumulate in anoxic environments, we model the photochemistryand spectroscopic detection of isoprene in habitable temperature, rockyexoplanet anoxic atmospheres with a variety of atmosphere compositions underdifferent host star UV fluxes. Limited by an assumed 10 ppm instrument noisefloor, habitable atmosphere characterization using JWST is only achievable withtransit signal similar or larger than that for a super-Earth sized exoplanettransiting an M dwarf star with an H2-dominated atmosphere. Unfortunately,isoprene cannot accumulate to detectable abundance without entering a run-awayphase, which occurs at a very high production rate, ~ 100 times Earth'sproduction rate. In this run-away scenario isoprene will accumulate to > 100ppm and its spectral features are detectable with ~ 20 JWST transits. Onecaveat is that some spectral features are hard to be distinguished from that ofmethane. Despite these challenges, isoprene is worth adding to the menu ofpotential biosignature gases.

Assessment of Isoprene as a Possible Biosignature Gas in Exoplanets with Anoxic Atmospheres