Organic Aerosols in Anoxic and Oxic Atmospheres of Earth-like Exoplanets: VUV-MIR Spectroscopy of CHON Tholins

Organic hazes can drastically impact the characterization and detection of exoplanet atmospheres. We explore the effects of the transition from an anoxic to oxic atmosphere on the molecular structure of organic aerosols, by producing aerosols in N2:CO2:CH4-rich plasmas with increasing CO2/CH4 gas ratios. We performed a spectroscopic study on the resulting aerosol analogs, CHON-rich tholins, from the vacuum-ultraviolet (VUV) to the mid-infrared (MIR), i.e., the 0.13–10 μm range. VUV spectroscopy revealed the presence of π–π* and n–π* transitions in the 200–500 nm range in all samples. These are attributed to electronic transitions in amine groups. As the CO2/CH4 ratio increases, new bands emerge that can be attributed to electronic transitions in hydroxyl and carboxyl functional groups. MIR spectroscopy showed that the molecular structure of oxidized aerosols is dominated by the CO2/CH4 ratio. Band deconvolution revealed the oxygenation of the organic matrix via the formation of oxygenated functional groups, including amide, hydroxyl, and carbonyl groups. For the most oxidized aerosols, absorption is greatest in the 0.13–0.3 μm and 6–10 μm regions. Some of the oxidized samples were further irradiated with VUV photons, resulting in the formation of π–π* bands in the 200–250 nm region and a decrease in the band intensities below 200 nm, attributed to amine and nitrile losses. We discuss how oxidized aerosols could provide a detectable (bio-)signature in exoplanet atmospheres.