What are permanently shadowed regions, and why is it so hard to map their topography?

Permanently shadowed regions (PSRs) are areas within craters at the lunar south pole that are never exposed to sunlight. Because these regions are permanently shadowed, they’re extremely cold. Evidence suggests there are frozen volatile elements within PSRs, which would be of high utility to lunar missions and potentially future missions to Mars. PSRs aren’t exposed to sunlight because of the low solar altitude angle at the lunar south pole of ~2° (see diagram).

Digital elevation models (DEMs) on Earth rely on topography being illuminated by the sun. The “weak reflectance from nearby surfaces” of PSRs poses challenges to creating accurate, high-resolution PSR DEMs (Jia et al. 2024:245). Current DEM techniques at the lunar south pole include:

Lunar Orbiter Laser Altimeter (LOLA)

• LOLA is an instrument on the Lunar Reconnaissance Orbiter (LRO)

• Ranging and detection technology similar to terrestrial Lidar, but laser altimetry DEMs include lots of noise and interpolation issues

Lunar Reconnaissance Orbiter Camera’s Narrow-Angle Cameras (LROC NACs) 

• Stereo photogrammetry that uses images from the LROC NAC

• Stereo photogrammetry images are processed from camera images into DEMs by Ames Stereo Pipeline (ASP) free open source software (FOSS) developed by NASA

*This ASP software has a decades-long history of use with stereo cameras on various rovers on planetary/moon surfaces

DEMs created via laser altimetry and stereo photogrammatry have coarse detail, which is a problem to lunar missions with landing and science objectives that depend on accurate, high-resolution DEMs. Shadow-derived DEMs present a method for mapping PSRs accurately and with significantly more detail than laser altimetry or stereo photogrammetry alone. 

Shadow-derived DEMs

• Shape-for-shading (SfS) is a DEM technique that can be applied to existing DEM images from laser altimetry or stereo photogrammetry to zoom in and analyze shading in order to deduce pixel values

• DEMs produced via SfS feature significantly more detail than DEMs derived from laser altimetry or stereo photogrammetry alone

• ShadowCam is a new instrument on the Korea Pathfinder Lunar Orbiter that takes photos of PSRs with greater sensitivity to illumination differences than laser altimetry and stereo photogrammetry images. For example, ShadowCam is 200 times more sensitive to illumination differents than LROC NAC

Overall, shadow-derived DEMs present a novel technique to create DEMs of permanently shadowed regions of the lunar south pole that are meaningfully higher resolution than DEMs from laser altimetry (LOLA) or stereo photogrammetry (LROC NAC) alone, which matters in the context of increased lunar missions targeting PSRs.

Sources: Beyer et al. 2018 (NAC, ASP info), Jia et al. 2024 (shadow-derived info). Diagram adapted from RASC Calgary Centre and Sky Lights.

*Cool article if you want to read more about PSRs and volatiles