Chang'E-5 In Situ Spectra Reveal Photometric Properties of the Lunar Surface

The Chang'E-5 (CE-5) lunar mineralogical spectrometer (LMS) obtained an in situ multiangular spectral data set, with which this work aims to study the photometric properties of the lunar surface in the CE-5 sampling area. We first converted the LMS Level 2B radiance data to reflectance, and then spliced it and processed thermal correction. We also used a millimeter-scale digital elevation model (DEM) to re-calculate local illumination and observation conditions considering the high-resolution topography at the landing site. We analyzed the multiangular reflectance data set with a Hapke modeling approach aimed at characterizing the scattering properties of the surface. In particular, we found that the surface exhibits a spectral signature typical of mafic materials and is close to the mature mare lunar soil. The position of the in situ photometric parameters in the hockey stick diagram indicates that CE-5 lunar soil particles are forward scattering, irregular and with a low density of internal scatters. Furthermore, we found that all the in situ data (including CE-3/4/5) and laboratory measurements exhibit forward scattering properties, but the orbiter remote sensing measurements are backward scattering. We attribute this discrepancy to the different resolution of the two data sets: the in situ analysis focuses on the individual particles, whereas remote sensing approach is sensitive to the particle complex. The Hapke parameters we derived can provide a reference for the further comprehensive analysis of remote sensing and in situ spectral data, especially, caution is advised when applying the photometric parameters derived from in situ or laboratory spectra to the remote sensing data. The photometric properties of particles are the external manifestation of their physical properties. The photometric parameters of particles can be retrieved by the reflectance spectra of particles, and then their physical properties can be studied. Currently, scientists have accumulated a wealth of spectral data in the study of the moon, including remote sensing detection, in situ detection, as well as laboratory measurements of lunar samples and simulants. The Chang'E-5 (CE-5) mission is China's first lunar sample return task. The lunar mineralogical spectrometer (LMS) onboard CE-5's lander collected a rare in situ multiangular spectral data set. We used this data set to analyze the photometric properties of the lunar soils at the CE-5 landing site. We found that the lunar soil particles are forward scattering, which is different from the conclusions induced from remote sensing spectral data. We attribute this discrepancy to the difference in resolution: the remote sensing spectra have a lower resolution (hundreds of meters), and its fundamental scattering unit is different from that of in situ lunar soil particles, but the particle complex composed of these particles. These findings can provide a reference for the further comprehensive analysis of remote sensing and in situ spectral data. Thermal and topographic corrections are processed on the in situ multiangular spectral data set obtained by the CE-5 Photometric analysis of the in situ spectra indicates that CE-5 lunar soil particles are forward scattering Caution is advised when applying the photometric parameters derived from in situ or laboratory spectra to the remote sensing data