The Effects of Viewing Geometry on the Spectral Analysis of Lunar Regolith as Inferred by in situ Spectrophotometric Measurements of Chang'E‐4

• Published in: Geophysical research letters Vol. 47; no. 8
• Main Authors: Yang, Yazhou, Lin, Honglei, Liu, Yang, Lin, Yangting, Wei, Yong, Hu, Sen, Yang, Wei, Xu, Rui, He, Zhiping, Zou, Yongliao
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 28.04.2020
• Subjects: Absorption; Absorption bands; Absorption spectra; Aerospace environments; Angles (geometry); Azimuth; Diagnostic systems; Imaging techniques; Infrared imaging; Infrared spectra; Infrared spectrometers; Lunar regolith; lunar regolith, Chang'E‐4, in situ spectrophotometric measurement, viewing geometry, spectral features, visible and near‐infrared; Lunar surface; Mineral composition; Mineralogy; Minerals; Moon; Olivine; Plagioclase; Reflectance; Regolith; Remote sensing; Spectra; Spectral analysis; Spectrophotometry; Spectrum analysis; Surface chemistry; Wavelength
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2020GL087080

The visible and near‐infrared imaging spectrometer on board the Yutu‐2 rover of Chang'E‐4 mission has conducted 2 sets of spectrophotometric measurements at two sites on its 10th lunar day. At each site, the reflectance spectra were measured on the same lunar regolith at different geometric configurations. The experiments reveal an increasing trend of the spectral reddening effect with the increasing phase angles, which may cause significant uncertainties on interpreting the maturity of the lunar regolith. Furthermore, the phase angles have significant effects on the 2‐μm band center and the 1‐μm band depth of the spectra, consequently on the estimation of mineral composition of the regolith. Finally, the derived average Hapke parameters using the in situ spectrophotometric measurements provide important ground‐truth for remote sensing observations. Plain Language Summary Olivine, pyroxenes and plagioclase, the major constituent minerals of the surface of the Moon, have diagnostic absorption features in their visible and near‐infrared reflectance spectra. These features, including the wavelength position of the absorption band center and the depth of the absorption band, can be used to identify the mineralogy of the observed surface. The slope of the reflectance spectral curve is another feature that can be used to describe the degree of how the surface materials interact with space environment. The Yutu‐2 rover on board the China's Chang'E‐4 mission has been equipped with a visible to near‐infrared spectrometer, to carry out in situ spectral measurements at varied illumination and azimuth angles. These measurements provided us an excellent chance to study the effects of measurement angles on these spectral features, which can help us better understand the mineralogical information of lunar surface. Key Points In situ spectrophotometric measurements by Chang'E‐4 Yutu‐2 rover show viewing geometry may affect band centers and depth of the spectra Phase reddening coupled with space weathering effects on the spectra may cause significant uncertainties on interpreting the maturity of lunar regolith The derived Hapke parameters from the in situ spectrophotometric measurements provide ground truth for both in situ and orbital observations