Because of differences in Fresnel coefficients at the regolith-vacuum interface, unpolarized subsurface thermal radiation becomes linearly polarized upon crossing the interface. Radiation with the electric field aligned with the plane of emission has higher transmissivity than the orthogonal polarization. Hence radio thermal emission from smooth planetary surfaces contains a linearly polarized component aligned with the plane of emission (i.e. oriented everywhere along the radial direction).

Note that the degree of polarization increases towards the limb, as expected from the behavior of Fresnel coefficients with emission angle. The degree of polarization can be used to measure dielectric constants.