The size of gas absorption features relative to the noise level for the Earth (top) and reduced-cloud Venus (bottom) cases. The sizes of the features are calculated by removing each gas from the full spectrum one at a time and taking the difference between the spectra. For both planets, features are small at short wavelengths as these wavelengths are dominated by clouds and scattering to a greater extent. All features other than CO₂ appear weak for Venus. Around 30 transits is the minimum required to detect O₃ for the Earth case.