Research and prospect of ground-penetrating radar on the Moon

Abstract Ground-penetrating radar (GPR) has become a pivotal tool in lunar geological exploration, providing direct insight into the Moon's subsurface structure. This review provides a focused summary of GPR applications in the Apollo 17, Kaguya, and Chang'e lunar missions, synthesizing their key scientific findings and technological advancements. In terms of scientific discoveries, GPR observations have revealed a lunar subsurface far more complex than previously thought, characterized by a multi-layered stratigraphy of volcanic deposits, impact ejecta, and ancient buried regolith. Furthermore, GPR has identified numerous unique subsurface features, including lava tubes, buried impact craters, and bifurcated structures. Notably, radar data reveal that the lunar farside's regolith exhibits significantly lower attenuation of radio waves than the nearside's, due to its lower iron and titanium oxide content, which allows for greater penetration depths. The paper also highlights recent advances in lunar GPR data processing, including forward modeling, improved dielectric property estimation through methods such as hyperbolic fitting and direct comparison with returned samples, and the application of advanced migration and machine learning-based imaging methodologies. These techniques have been crucial for converting radar travel times to depth, enhancing image resolution, and accurately characterizing lunar materials. Different GPR systems offer a trade-off between penetration depth and resolution. Orbital sounders can see deep into the subsurface with low resolution, whereas surface rovers provide high-resolution data limited to shallow depths. Therefore, a comprehensive understanding of the lunar subsurface requires a multi-frequency, data-fusion approach. Looking ahead, GPR will be indispensable for future resource-prospecting missions, such as mapping water ice deposits, and for the selection of sites for lunar bases.