Adsorption isotherm of methane on tholin substrate. The blue dots show the number of adsorbed molecules scaled to the number of adsorption sites and the thick red line shows the isotherm obtained with ΔF^o = 1.519 × 10⁻²⁰ J to match the Curtis et al. (2008) isotherm for methane (their Fig. 7). The other isotherms (thin red lines) are obtained with ΔF^o = (1.519 ± 0.006) × 10⁻²⁰ J. The vertical solid and dashed lines show the condensation threshold at S = 1.07 ± 0.008.