Detecting sub-micron space weathering effects in lunar grains with synchrotron infrared nanospectroscopy

Space weathering processes induce changes to the physical, chemical, and optical properties of space-exposed soil grains. For the Moon, space weathering causes reddening, darkening, and diminished contrast in reflectance spectra over visible and near-infrared wavelengths. The physical and chemical changes responsible for these optical effects occur on scales below the diffraction limit of traditional far-field spectroscopic techniques. Recently developed super-resolution spectroscopic techniques provide an opportunity to understand better the optical effects of space weathering on the sub-micrometer length scale. This paper uses synchrotron infrared nanospectroscopy to examine depth-profile samples from two mature lunar soils in the mid-infrared, 1500–700 cm-1 (6.7–14.3 µm). Our findings are broadly consistent with prior bulk observations and theoretical models of space weathered spectra of lunar materials. These results provide a direct spatial link between the physical/chemical changes in space-exposed grain surfaces and spectral changes of space-weathered bodies.

Methods
We used an iterative, non-linear, and robust peak fitting (deconvolution) procedure to determine peak parameters for SINS phase spectra. The procedure uses a trust-region minimization algorithm to fit a linear combination of Lorentzians with a local linear background to the data. Robustness is ensured by minimizing the residuals’ summed square and using bi-square weighting to reduce the impact of outliers. For more detail, see the Supporting Information for the associated publication.

太空风化过程会改变暴露于太空环境的土壤颗粒的物理、化学与光学特性。针对月球而言，太空风化会导致其可见光及近红外波段的反射光谱出现红化、变暗以及对比度降低的现象。而引发这些光学效应的物理与化学变化，其尺度小于传统远场光谱技术的衍射极限。近年来新兴的超分辨率光谱技术，为我们在亚微米尺度上深入解析太空风化的光学效应提供了全新可能。本文采用同步辐射红外纳米光谱（Synchrotron Infrared Nanospectroscopy, SINS）技术，在1500–700 cm⁻¹（6.7–14.3 µm）的中红外波段，对两份成熟月球土壤的深度剖面样品开展了分析。本研究结果与此前针对月球物质太空风化光谱的整体观测结果及理论模型大体一致。上述成果直接建立了暴露于太空的颗粒表面物理/化学变化与太空风化天体光谱变化之间的直接空间关联。

**方法**
我们采用迭代、非线性且稳健的峰拟合（反褶积）流程，以获取SINS相位光谱的峰参数。该流程借助信赖域最小化算法，将洛伦兹线型的线性组合与局部线性背景相结合，对实验数据进行拟合。通过最小化残差平方和并采用双平方权重以降低异常值的影响，保障了拟合过程的稳健性。如需了解更多细节，请参阅相关发表论文的支持信息。