Notes. In the third column we provide the values used to generate the Venus-twin spectrum introduced in Sect. 2.1. These values provide the ground truth for all retrievals. The fourth column lists the prior distributions used in the retrievals. With 𝒰(x, y), we denote a boxcar prior with lower threshold x and upper threshold y; 𝒢(µ, σ) indicates a Gaussian prior with mean µ and standard deviation σ. The last four columns summarize the model parameters used by the different forward models in the retrievals (✓ = used, × = unused; see Sect. 2.3.2) and the model color-coding used throughout the paper. – opaque H₂SO₄ clouds (84% H₂SO₄, 16% H₂O by weight); –transparent H₂SO₄ clouds (84% H₂SO₄, 16% H₂O by weight); – opaque H₂O clouds (100% H₂O); ℳ^CF – cloud-free.

References. ⁽¹⁾Obtained by fitting Eq. (1) to the Venus P–T profile in Fig. 1 of Mueller-Wodarg et al. (2008); ⁽²⁾Near surface abundances from NASA’s planet factsheet: https://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.html; ⁽³⁾Mean of values from Oschlisniok et al. (2012) and Krasnopolsky (2015); ⁽⁴⁾Mean values for the mode 1 cloud particles in Titov et al. (2018).