(a) A comparison of reflected-light spectra for a realistic, cloudy Earth and the abiotic atmospheric state after 4.5 Gyr. The abiotic atmosphere lacks O₂ (and therefore O₃) and CH₄, contributing to differences in absorption. (b) A comparison of the input molecular abundance profiles used to generate each reflected-light spectrum. We address minor differences between the CO₂, H₂O, and N₂ profiles in the accompanying text. Though the true profiles extend to a TOA pressure of 1 × 10⁻⁵ bars, we only show the surface to the upper stratosphere to better emphasize the differences in the water-vapor profiles given the logarithmic scale.