Top: size distribution of the tholin grains in the experiment of Curtis et al. (2008). The solid lines correspond to the actual published data and the dashed lines correspond to an extrapolation made in this work to complete the distribution.Bottom: measured adsorption isotherms (ratio of adsorbed molecules relative to the number of adsorption sites) of methane and ethane on tholin as a function of the saturation ratio (Curtis et al. 2008).