Adsorption isotherm of ethane on tholin substrate. The blue dots show the isotherm obtained from experiment (Fig. 11 of Curtis et al. 2008) The thick red line shows the best match of the isotherm obtained with the BET theory (Fig. 3) and the thin red lines show the isotherms obtained with the extreme values of the 2σ confidence ellipse. The vertical solid and dashed lines show the condensation threshold at S = 1.36 ± 0.08.