Thermal contrast detectable inside the VIRTIS field of view as a function of thermal inertia when no roughness is assumed. The filled region represents the minimum and maximum temperatures observed by VIRTIS, while the up and down triangles are the expected maximum and minimum observable temperatures for a given thermal inertia Γ respectively. Values of Γ lower than 100    J   m^-2   K^-1   s ^− 1/2 and higher than 150 J   m^-2   K^-1   s^−1/2 are inconsistent with the data.