Nanometer-scale Chemical Mapping of Space Weathered Lunar Soil: a New View

Introduction: The term " space weathering " refers to the cumulative effects of several processes operating at the surface of any solar system body not protected by a thick atmosphere. These processes include cosmic and solar ray irradiation, solar wind im-plantation and sputtering, as well as micrometeorite bombardment [e.g. 1 and references therein]. In lunar soils the most voluminous weathering products are agglutinates, glass welded aggregates created via mi-crometeorite bombardment. In addition, space weathering produces thin (60-200nm) amorphous rims surrounding individual grains in mature soils [2]. Although less voluminous than agglutinates, amorphous rims are surface-correlated, and thus play a larger role in altering the spectral properties of soils [3]. Weathered rims are created by both depositional and erosional processes. Material is deposited by impact generated melts and vapors as well as through sputtering. Erosional processes also include sputtering , as well as irradiation. Rims are often characterized by inclusions of nanophase iron (npFe 0). These nanometer-scale inclusions of metallic iron are also observed in agglutinitic glass. The abundance and distribution of npFe 0 is the primary cause of spectral alteration of lunar soils [4]. The basic processes and products underlying space weathering and the optical changes associated with the phenomenon are becoming better understood, but there are still many open questions about the rates and relative importance of various processes and the composi-tional dependence of the various components of the weathering process. New instrumentation and techniques allow us to examine these weathering products with unprecedented detail, providing fresh insights and an improved understanding of space weathering processes. Methods: For these analyses, we used the recently installed JEOL 2500SE 200 keV field-emission scanning transmission electron microscope (FE-STEM) at JSC. The FE-STEM is equipped with a large-area, thin window energy-dispersive X-ray (EDX) spectrometer to obtain spectrum images of grain rims in ultramicro-tome thin sections (~50 nm thick) of Apollo 11 soil 10084 (sub-20 µm sieve fraction). Each pixel of a spectrum image contains a full EDX spectrum, enabling the determination of quantitative element abundances on the scale of individual pixels. Spectrum images of the lunar grains were acquired with a 4 nm incident probe whose dwell time was minimized to avoid beam damage and element diffusion during mapping. Successive image layers of each rim were acquired and combined in order to achieve suitable counting statistics for major elements (e.g. Mg, Al, Si, Ca, Ti, and Fe) in each pixel.