Phaethon’s regolith grain-size. Horizontal lines indicate the derived values of the thermal conductivity, following Eq. (1), for the different volume filling factors ϕ of the material and for the nominal thermal inertia value of 600 J m⁻² s^−0.5 K⁻¹ (from top to bottom: ϕ = 0.6, 0.5, 0.4, 0.3, 0.2, and 0.1). The curves represent the thermal conductivity of a regolith with average thermophysical properties of CC meteorites (see Opeil et al. 2010) as a function of the regolith grain size again for the different volume filling factors ϕ. The intersection of the curves with the horizontal lines (for the same ϕ) give the inferred characteristic grain size of the regolith at the abscissa.