Hebe’s regolith grain size. Horizontal lines indicate the derived values of the heat conductivity, following Eq. (B.1), for the different volume-filling factors of the material and for a thermal-inertia value of 40 J m^-2 s^-0.5 K^-1 and a surface temperature of 180 K. The curves represent the thermal conductivity of a regolith with thermophysical properties of a H5 meteorite as from Opeil et al. (2010) as a function of the regolith grain size. The intersection of the curves with the horizontal lines gives the grain size of the regolith.