Photometric Normalization of Chang’e-4 Visible and Near-Infrared Imaging Spectrometer Datasets: A Combined Study of In-Situ and Laboratory Spectral Measurements

Abstract

Until 29 May 2020, the Visible and Near-Infrared Imaging Spectrometer (VNIS) onboard the Yutu-2 Rover of the Chang’e-4 (CE-4) has acquired 96 high-resolution surface in-situ imaging spectra. These spectra were acquired under different illumination conditions, thus photometric normalization should be conducted to correct the introduced albedo differences before deriving the quantitative mineralogy for accurate geologic interpretations. In this study, a Lommel–Seeliger (LS) model and Hapke radiative transfer (Hapke) model were used and empirical phase functions of the LS model were derived. The values of these derived phase functions exhibit declining trends with the increase in phase angles and the opposition effect and phase reddening effect were observed. Then, we discovered from in-situ and laboratory measurements that the shadows caused by surface roughness have significant impacts on reflectance spectra and proper corrections were introduced. The validations of different phase functions showed that the maximum discrepancy at 1500 nm of spectra corrected by the LS model was less (~3.7%) than that by the Hapke model (~7.4%). This is the first time that empirical phase functions have been derived for a wavelength from 450 to 2395 nm using in-situ visible and near-infrared spectral datasets. Generally, photometrically normalized spectra exhibit smaller spectral slopes, lower FeO contents and larger optical maturity parameter (OMAT) than spectra without correction. In addition, the band centers of the 1 and 2 μm absorption features of spectra after photometric normalization exhibit a more concentrated distribution, indicating the compositional homogeneity of soils at the CE-4 landing site.

Radiative Transfer Modeling of Chang’e-4 Spectroscopic Observations and Interpretation of the South Pole-Aitken Compositional Anomaly

“…Photometric models for local regolith at the CE-4 landing region have been obtained from VNIS data collected on the 10th lunar day when spectral measurements of the same target at various illumination geometry were done by the Yutu-2 rover. The VNIS spectra of local soils at the CE-4 landing region were corrected to REFF at standard illumination geometry (i = 30°, e = 0°, and g = 30°) using the Lommel-Seeliger (LS) model (Qi et al 2020). Although the LS model is a highly simplified solution of the Hapke RTM with the assumption of isotropic single scattering without multiple scattering, this empirical model somehow works better for photometric correction of CE-4 VNIS data sets than Hapke's RTM.…”

Dusty Mafic Rocks Along the Path of Chang'E‐4 Rover: Initial Analysis of the Image Cubes of the Onboard Visible and Near‐Infrared Imaging Spectrometer

“…However, more careful analysis is required for these in-situ data because the detected targets often exhibit more complex textural and composition characteristics than the orbital detection. For example, shadows have a pronounced effect on the in-situ measured albedo (Figure S9 in Supporting Information S1; Qi et al, 2020).…”

Effect of Lunar Complex Illumination on In Situ Measurements Obtained Using Visible and Near-Infrared Imaging Spectrometer of Chang’E-4

“…Next, the reflectance was obtained by its ratio to the solar spectral irradiance data F by using Equation (1) [18,22]. The distance d between the Sun and Moon was considered to be approximately 1 AU [23].…”