Anthropogenic Contamination of the Lunar Surface: Characterizing the Effects of the Chang’e-4 Landing on Lunar Surface Properties

Abstract:

Surface alteration by the exhaust plumes of lunar landers is a major concern when interpreting measurements acquired at the lunar surface [1–3]. For example, because the spectral and angular properties of lunar-emitted particles depend on regolith structure and mineralogy [4–8], lander-induced surface contamination is expected to affect particle emissions over several hundred meters around the landing site. However, the effects of lander-induced regolith disturbances on lunar particle emissions remain poorly constrained due to the lack of dedicated in situ investigations.

Using energetic neutral atom (ENA) measurements from the Advanced Small Analyzer for Neutrals (ASAN) [9] onboard the Chang’e-4 rover Yutu-2, we investigate variations in the spectral and angular properties of backscattered hydrogen ENAs as a function of distance from the Chang’e-4 lander. Excluding periods when the precipitating solar wind was disturbed by the South Pole–Aitken magnetic anomaly [10], we obtain 30 energy spectra acquired at distances of 30–930 m from the Chang’e-4 lander over a 4.5-year interval beginning one months after landing. For each spectrum, we applied a Bayesian inference method to derive the hydrogen surface binding energy and angular scattering function using an empirical hydrogen ENA energy spectrum model [6].

Our analysis shows that the surface binding energy inferred from ASAN observations is lower, at 5.0 ± 1.8 eV, within the first 150 m from the lander, compared to 6.2 ± 2.9 eV at larger distances. This transition distance of 150 m is comparable to the size of blast zones produced by previous lunar landers, suggesting that the same surface alteration processes responsible for the visible blast zone also modify the energy spectrum of lunar-emitted particles. In contrast, we find no significant dependence of the scattering function on distance from the lander. This result is contrary to previous expectations, namely that lander plume–induced disturbances in the micrometer-scale structure of the regolith inferred from photometric measurements would affect the ENA angular scattering function [1,4]. These results motivate future investigations of surface alteration using rover-borne particle instruments, particularly during the first lunar day, when transient contamination is expected [2] but not covered by the ASAN dataset.

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