Near-Surface Stratigraphic Structure of the Regolith Revealed Using Chang'e-2 Microwave Brightness Temperature Data

The stratigraphic structure of the lunar regolith was essential for studying the formation and evolution of the lunar shallow layer within the lunar crust, which was constrained by previous lunar missions through spectral remote sensing, laboratory analyses, and in situ radar, such as the lunar penetrating radar on Chang'e missions. However, the stratigraphy of the vertical shallow 3&#x2013;5 m layer remains unclear to date. In this work, we proposed a method to stratify the shallow lunar regolith (<inline-formula><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula>5 m) using the microwave radiometer (MRM) onboard China's Chang'e-2 (CE-2). The averaged subsurface rock abundance (RA) at depths (<inline-formula><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula>0.5&#x2013;5 m) was extracted from CE-2 microwave brightness temperature at four channels: 3.0, 7.8, 19.35, and 37 GHz. Variations of RA with depth were estimated based on the differing penetration depths of the MRM. The near-surface regolith at the Chang'e-3 and Chang'e-4 landing sites is divided into three layers: layer I, strongly weathered regolith with RA <inline-formula><tex-math notation="LaTeX">$&lt; $</tex-math></inline-formula> 0.20; layer II, weakly weathered ejecta with RA <inline-formula><tex-math notation="LaTeX">$&gt;$</tex-math></inline-formula> 0.50; layer III, moderately weathered paleoregolith with RA between 0.20 and 0.50. We speculated that impact events during the late Copernican period (approximately 50 million years ago) created a crater (approximately 450 m diameter). The ejecta layer exhibits high RA detectable by 7.8 GHz but not by 19.35 GHz.