Investigation into the lunar regolith layer using Apollo mission active seismic data

Abstract Over the past 50 years, advances in lunar research have been made from telescopic, orbital, and surface observations. However, some fundamental scientific issues remain unclear, such as the properties of the regolith layer. There has been a debate about the thickness of the regolith since the Apollo mission seismic experiments. Studies since the 1970s Apollo missions have shown varying regolith layer depths. We use seismic profiles to study selenomorphology at Apollo ASE sites. Our tomographic models revealed an average regolith layer depth of 14.8 m at the Apollo 14 site and 22.5 m at the Apollo 16 site. At the Apollo 14 site, the regolith layer was subdivided into three segments with average velocities of up to 110 m/s (up to 5.2 m depth), up to 150 m/s (up to 8.7 m depth), and up to 200 m/s (up to 14.8 m depth). The subsequent layer, at the base, reached an average velocity of 328 m/s (up to 22.5 m depth). Similar patterns were observed for Apollo 16, with velocity divisions at depths of 12.3 m, 16.4 m, 22.5 m, and 25.8 m. Apollo 14's profiles suggested a gentle sloped interface between regolith and rocky basement. For Apollo 16, the layer's seismic wave velocity gradient is stronger, and the interface suggests higher variation in slopes. The abrupt gradual variation in these velocity values and the larger slope are possibly related to a high rate of local meteoritic impacts, suffered by the Apollo 16 site, in comparison to Apollo 14. The morphological model for the lunar surface and shallow structure explains the thicker regolith at the Apollo 16 site from eroded craters. Graphical Abstract