Centimeter-Scale Roughness Characteristics of Lunar Geological Structures and Permanently Shadowed Regions: Insights From Mini-RF SAR Data

Topographic roughness is a critical parameter in lunar exploration and scientific research due to its scale-dependent properties. Synthetic aperture radar (SAR) is a powerful tool for characterizing lunar surface physical properties. This study investigates the centimeter-scale roughness characteristics of lunar terrain using S-band Mini-RF SAR data, including seven types of geological structures at the mid-latitude regions and permanently shadowed regions (PSRs) at the lunar poles. First, a novel roughness parameter, single scattering trace (SST), is proposed based on matrix trace statistics to quantify lunar surface and near-surface roughness from radar echoes. The effectiveness of SST is validated using high-resolution Narrow Angle Camera (NAC) Digital Terrain Model (DTM) data. Then, by comparing the radar observations, topographic roughness, topographic slopes, and rock abundance of different geological structures, we find that radar observations can reveal wavelength-scale textural variations of geological structures that are not apparent in larger-scale topographic roughness measurements. Furthermore, the roughness characteristics of PSRs and non-PSRs at the lunar polar regions are analyzed. Results show that PSRs exhibit slightly lower roughness than non-PSRs at the south pole, while the opposite trend is observed at the north pole. The inter-polar roughness differences can be attributed to the distinct roughness characteristics of non-PSRs rather than variations within PSRs. Overall, our results demonstrate that the proposed SST parameter is an effective metric for assessing lunar surface roughness.