The impact of stellar UV radiation on reduced, hydrogen-dominated atmospheres

The stellar UV radiation has a major influence on planetary atmospheres, affecting not only the observable spectra, but also the atmospheric chemistry, dynamics, and evolution. These processes can have further implications for planetary habitability. Broadly speaking, we can divide planetary atmospheres into oxidized and reduced, the latter mostly dominated by hydrogen and its compounds. In this work we will focus on reduced atmospheres, which can be found on a diverse range of planets, from gas giants and mini-Neptunes, all the way to rocky bodies, such as Titan or the pre-biotic Early Earth. Recent studies have shown that large, iron-rich impactors could have generated reduced atmospheres on the Early Earth lasting millions of years and potentially led to the production of large amounts of organic hazes via UV-driven photochemistry. These organic compounds not only play a key role in pre-biotic chemistry on Early Earth, but can also influence the global atmospheric structure as well as the observed spectral signatures of all types of reduced atmospheres. Numerous exoplanets detected to date are believed to have hazes in their atmosphere, which can potentially explain their observed spectra. Further, the strong UV radiation and flares of M-dwarfs must be considered when modeling the atmospheres of the many exoplanets discovered orbiting them. Here we present new atmospheric models that take into account the feedback between UV-driven photochemistry, haze formation, and atmospheric structure. We calculate the effects of UV radiation on the atmospheric composition and the resulting high-resolution spectra for a sample of reduced atmospheres and input stellar spectra, and compare the results with those obtained using standard equilibrium chemistry calculations.