Abstract
The Chang’E-4 (CE-4) spacecraft landed successfully on the far side of the Moon on 3 January 2019, and the rover Yutu-2 has explored the lunar surface since then. The visible and near-infrared imaging spectrometer (VNIS) onboard the rover has acquired numerous spectra, providing unprecedented insight into the composition of the lunar surface. However, the noise in these spectral data and its effects on spectral interpretation are not yet assessed. Here we analyzed repeated measurements over the same area at the lunar surface to estimate the signal–noise ratio (SNR) of the VNIS spectra. Using the results, we assessed the effects of noise on the estimation of band centers, band depths, FeO content, optical maturity (OMAT), mineral abundances, and submicroscopic metallic iron (SMFe). The data observed at solar altitudes <20° exhibit low SNR (25 dB), whereas the data acquired at 20°–35° exhibit higher SNR (35–37 dB). We found differences in band centers due to noise to be ~6.2 and up to 28.6 nm for 1 and 2 μm absorption, respectively. We also found that mineral abundances derived using the Hapke model are affected by noise, with maximum standard deviations of 6.3%, 2.4%, and 7.0% for plagioclase, pyroxene, and olivine, respectively. Our results suggest that noise has significant impacts on the CE-4 spectra, which should be considered in the spectral analysis and geologic interpretation of lunar exploration data.
Highlights
Visible and near-infrared (VNIR) reflectance spectra are the primary data sources used in analysis of the composition of the materials and processes on the lunar surface
The irradiance of solar energy increases with solar altitude, in conjunction with increasing signal–noise ratio (SNR) in our dataset (Figure 6c)
The SNR is lower than 25 dB for solar altitudes
Summary
Visible and near-infrared (VNIR) reflectance spectra are the primary data sources used in analysis of the composition of the materials and processes on the lunar surface. The effect of noise on observed data must be estimated in the spectral analysis of lunar surface characteristics. Noise is known to affect the estimation of spectral features such as band center and band depth. How noise of CE-4 spectral data affects the estimations of SMFe abundance needs to be quantitatively assessed. Noise in in situ hyperspectral data and its effects on the spectral features of lunar regolith can be determined by observing the same areas repeatedly. We first estimated the noise level of the VNIS measurements (i.e., the SNR of the hyperspectral images obtained) and analyzed their variations with solar altitude and incident energy. We assessed variations in mineral abundance and SMFe caused by the noise These results are critical in the interpretation of the in situ spectral data acquired by CE-4.
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