Simulated transit transmission spectra for the O₂ desiccated (upper panel) and O₂ outgassing (lower panel) atmospheres. The y axes are the relative transit depths and show the modeled atmospheric signal. We show only the 10 bar atmospheres, as the 100 bar atmospheres are qualitatively similar. The desiccated atmospheres are dominated by CO₂ and O₂–O₂. The presence of weaker O₃ bands and CO are indicative of the desiccated environment. The outgassing atmospheres have additional features from H₂O and SO₂. The hotter atmospheres maintain substantial stratospheric H₂O, and the outer planets, too cold to maintain H₂O, build up SO₂, which would otherwise condense with H₂O to form sulfuric acid.