Lawrence Berkeley National Laboratory

Abstract 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, 1,500 –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. Plain Language Summary The Moon's surface, unprotected from the space environment, is bombarded with solar wind ions and micrometeoroids. These interactions are part of a process known as space weathering, which changes the physical and optical properties of lunar soils and asteroid surfaces on a microscopic scale. Technological hurdles have hindered our understanding of the connection between the physical changes caused by space weathering and the optical properties thought to result from them. Using synchrotron infrared nanospectroscopy, we examined how various weathering processes affect the infrared spectral characteristics of lunar soil grains. With these insights, we can develop better space weathering models to predict how different surfaces may be affected. The data from this investigation can also be used to calibrate laboratory analog studies of space weathering and help interpret observations of bodies similar to the Moon. Key Points Lunar soils were studied with spatially resolved near‐field spectroscopy in the mid‐infrared Spectral effects of space weathering were observed to vary continuously over a depth of 500 nm Direct experimental evidence supports a connection between microstructural/chemical changes and mid‐infrared effects in weathered lunar soil