Ratio between the observed fluxes and the corresponding TPM predictions as a function of phase angle (negative: after opposition; positive: before opposition). The model calculations are based on a thermal inertia of 600 Jm^-2 s^-0.5 K^-1 and ρ = 0.2 for the surface roughness. The VISIR data point can only be matched by assuming a higher surface roughness (see also Sect. 4.2 for further discussions).