Remote sensing of the Moon's subsurface with multifrequency microwave radiometers: A numerical study

Within the renewed interest in the study of the Moon, in 2006 the European Space Agency approved a feasibility study for the European Student Moon Orbiter (ESMO) mission. In order to accomplish the ESMO mission objectives, a Microwave Radiometric Sounder (MiWaRS) was selected as a possible payload for flight on the ESMO satellite. This work summarizes the results of a numerical analysis of MiWaRS sounding capabilities. An (inhomogeneous) multilayer model of the microwave emission from the Moon's subsurface is presented, focusing the attention on the Moon's morphological, thermal, and dielectric properties. These properties have been determined and parameterized after a thorough investigation of available measurements and models. To this end, a radiative transfer numerical model, neglecting volume scattering, is coupled with a nonlinear thermal equation to simulate the microwave emission of the Moon's subsurface. Numerical simulations between L and Ka bands are performed to investigate the capability of MiWaRS to sound the characteristics of the Moon's regolith subsurface and detect the presence of rocks and ice under the near-surface regolith layer. Under these forward model assumptions, results show that the Moon's brightness temperature response allows the detection of discontinuities within regolith media down to 2 and 5 m depth when channels at 3 and 1 GHz are used, respectively. Lunar near-surface temperature may be also estimated within an accuracy less than a few kelvins. The discrimination of ice from rock by MiWaRS is hardly practicable and is limited to the presence of ice in the upper layers (with a depth less than 20 cm) beneath the lunar crust.