Submicroscopic metallic iron (SMFe) created by space weathering has strong effects on the optical properties of the lunar surface. Spectra measured in situ by the visible‐near‐infrared spectrometer (VNIS) on board the Chang'E‐3 Yutu rover were used to investigate optical maturity differences at the CE‐3 landing site caused by lander exhaust. SMFe abundances were estimated using Hapke's radiative transfer model. Analysis of the spectrum for a minimally disturbed soil indicates that it contains 0.368 wt % SMFe, corresponding to an Is/FeO maturity index of ~53 and indicating that the landing site is submature. The soil at a location that was more disturbed contains 0.217 wt % SMFe, suggesting that the material removed by the rocket blast is more weathered than the regolith that remained behind. We conclude that maturity differences related to removal of the finest, highly mature particles play a major role in the observed reflectance changes associated with rocket blast.

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Space weathering is an important surface process occurring on the Moon and other airless bodies, especially those that have no magnetic field. The optical effects of the Moon's space weathering have been largely investigated in the laboratory for lunar samples and lunar analogues. However, duplication of the pristine regolith on Earth is not possible. Here we report the space weathering from the unique perspective of the Chang ′ E-3 ′ s (CE-3) "Yutu" rover, building on our previous work (Wang et al. 2017;Wu and Hapke 2018). Measurement of the visually undisturbed uppermost regolith as well as locations that have been affected by rocket exhaust from the spacecraft by the Visible-Near Infrared Spectrometer (VNIS) revealed that the returned samples bring a biased information about the pristine lunar regolith. The uppermost surficial regolith is much more weathered than the regolith immediately below, and the finest fraction is rich in space weathered products. These materials are very dark and attenuated throughout the visible and near-infrared (VNIR) wavelengths, hence reduce the reflectance and mask the absorption features. The effects on the spectral slope caused by space weathering are wavelength-dependent: the visible and near-infrared continuum slope (VNCS) increases while the visible slope (VS) decreases. In the visible wavelength, the optical effects of space weathering and TiO 2 are identical: both reduce albedo and blue the spectra. This suggests that developing new TiO 2 abundance algorithm is needed. Optical maturity indices are composition related and hence only locally meaningful. Since optical remote sensing can only sense the uppermost few microns of regolith and since this surface tends to be very weathered, the interpretation of surface composition using optical remote sensing data needs to be carefully evaluated. Sampling the uppermost surface is suggested.

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