Speaker
Description
In light of the renewed interest in landed missions to the Moon, here we empirically evaluate the effect of meter-scale rock-related roughness on radar backscatter by combining monostatic S-band (13 cm) radar measurements by the Lunar Reconnaissance Orbiter's (LRO) Miniature Radio Frequency (Mini-RF) instrument with Diviner-derived meter-scale rock abundance (RA) [1] over the lunar surface, and use numerical simulations to infer the physical properties of regolith in more detail. Polarimetric radar backscatter provides a wealth of knowledge that informs geologic studies and hazard assessment, including wavelength-scale topographic roughness, scatterer size and morphology, and dielectric properties within the radar penetration depth. Nevertheless, the multiple contributors to backscatter motivate a quantitative assessment of its use as a diagnostic of surface properties.
In detail, we investigate the backscattered Stoker parameters (I, Q, U, V) (for details on processing, see [2]) as a function of RA. The parameters Q and U describe the echo power in different orientations of linear polarization and the parameter V in linear polarization. We identified three scattering regimes over which backscattered I, -V/I, and Q/I are (1) insensitive to RA when RA < 0.3 %, (2) increase strongly over 0.3 % ≤ RA ≤ 1 %, and (3) plateau or decrease for RA > 1 %. In category 1, the lunar regolith is assumed to be dominantly fine-grained with few cm-to-dm scale particles, where geometric optics provides a sufficient approximation for surface scattering. In category 2, the regolith has a volumetric balance between fine-grained regolith and cm-to-dm scale rubble, where the resonance-regime scattering must be included. In category 3, the high RA enables significant multiple scattering between cm-to-dm scale rubble and increased dihedral scattering between larger rocks. Further, because Diviner derives the rock abundance using thermal observations, i.e., only through the thermal skin depth, whereas radar observes the subsurface through several decimeters, we also assess possible discrepancies between the visible surface RA and invisible but radar-observable subsurface RA. This approach, which requires using both empirical and numerical modeling, allows us to advance the interpretation of radar polarimetry of different types of planetary surfaces.
We use Advanced Discrete-Dipole Approximation (ADDA) code [3] to simulate radar scattering by realistic cm-to-dm scale rocky particles with the goal to infer the Diviner-derived RA with respect to the cm-to-dm scale particles. This is a crucial step to more general interpretation of radar scattering in terms of regolith size distribution and observation geometry, as both parameters play a significant role in the scattering properties as can be shown both empirically and theoretically. In this presentation, the most recent advances of this work are discussed.