Optical polarimetric and photometric analysis of the Martian surface with the instrument VPM on board the Orbiter MARS-5 produced information about the grain size of the Martian soil. The results were compared with the thermal inertia coefficients obtained at Mars' surface with the same spacecraft and with the Viking orbiters. Photopolarimetry is sensing the grain-size dp of the top surface soil. Radiometry characterizes larger fragments below the surface, or the presence of stones, rocks, or slabs imbedded into the dust layer or coated with the dust. At the resolution of 20 km, the top surface appears to be everywhere covered with small grains within the size range 20 to 45 μm, except on some very localized areas, usually related to tectonics. Below this coating, local densifications, or coarser structures, stones, or rocks are observed.