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Sulfur gases are common components in the volcanic and biological emission onEarth, and are expected to be important input gases for atmospheres onterrestrial exoplanets. We study the atmospheric composition and the spectra ofterrestrial exoplanets with sulfur compounds (i.e., H2S and SO2) emitted fromtheir surfaces. We use a comprehensive one-dimensional photochemistry model andradiative transfer model to investigate the sulfur chemistry in atmospheresranging from reducing to oxidizing. The most important finding is that both H2Sand SO2 are chemically short-lived in virtually all types of atmospheres onterrestrial exoplanets, based on models of H2, N2, and CO2 atmospheres. Thisimplies that direct detection of surface sulfur emission is unlikely, as theirsurface emission rates need to be extremely high (>1000 times Earth's volcanicsulfur emission) for these gases to build up to a detectable level. We alsofind that sulfur compounds emitted from the surface lead to photochemicalformation of elemental sulfur and sulfuric acid in the atmosphere, which wouldcondense to form aerosols if saturated. For terrestrial exoplanets in thehabitable zone of Sun-like stars or M stars, Earth-like sulfur emission ratesresult in optically thick haze composed of elemental sulfur in reducingH2-dominated atmospheres for a wide range of particle diameters (0.1-1 micron),which is assumed as a free parameter in our simulations. In oxidizedatmospheres composed of N2 and CO2, optically thick haze, composed of elementalsulfur aerosols (S8) or sulfuric acid aerosols (H2SO4), will form if thesurface sulfur emission is 2 orders of magnitude more than the volcanic sulfuremission of Earth. Although direct detection of H2S and SO2 by their spectralfeatures is unlikely, their emission might be inferred by observingaerosol-related features in reflected light with future generation spacetelescopes.

Photochemistry in Terrestrial Exoplanet Atmospheres II: H2S and SO2 Photochemistry in Anoxic Atmospheres