Shape-from-shading with secondary illumination for 3D reconstruction of permanently shadowed regions at the lunar south pole

Permanently shadowed regions (PSRs) at the lunar south pole are garnering growing interest for exploration due to their potential to harbor water ice, as the consistently low temperatures may allow for its sequestration. High-resolution digital elevation models (DEMs) of PSRs are crucial for their exploration and scientific research. However, existing datasets, such as the Lunar Reconnaissance Orbiter Narrow Angle Camera (LRO NAC) dataset, do not visualize the interior of PSRs, while the Lunar Orbiter Laser Altimeter (LOLA) data are insufficient to capture adequate topographic details. ShadowCam is a newly deployed camera onboard the Korea Pathfinder Lunar Orbiter (KPLO), with a highly sensitive imaging ability for PSRs based on faint secondary illumination. This paper presents a novel approach, termed shape-from-shading with secondary illumination (SfS-SI), for 3D surface reconstruction of PSRs using ShadowCam images. The approach takes monocular ShadowCam images, a corresponding low-resolution DEM, and information on the direct solar illumination as input. A view factor-based method is incorporated into the approach to reconstruct the secondary illumination from the surrounding environment. The output is a high-resolution DEM of the same resolution as the input ShadowCam images. The proposed approach has been validated using ShadowCam images covering the PSR of the Shackleton crater at the lunar south pole. The results indicate that the approach can reconstruct pixel-wise 3D topographic details in PSRs and achieve a geometric accuracy of 0.34–1.56 m compared with the direct LOLA measurements. The proposed approach enables high-quality 3D surface reconstruction of PSRs from ShadowCam images, and can assist future exploration missions targeting these PSRs at the lunar south pole for water ice and support related scientific research.