Investigation of Lunar Regolith Chemistry by X-ray Absorption and Emission Spectroscopy
Understanding surface weathering processes is critical to understanding the history and continuing evolution of the moon. Bombardment by micrometeorites and solar wind leave traces in regolith chemical composition. Redox active metals have been used as metrics of the oxidizing vs reducing environment, while IR and other methods have revealed the presence of hydroxyl and peroxo groups. We propose to examine the chemical speciation of major constituent elements using X-ray absorption (XAS) and emission (XES) spectroscopy and correlate these changes with regolith maturity. The oxidation/spin states and coordination environment of transition metals will be investigated. The local coordination environments of light elements will be studied to observe changes in their speciation. O will be directly probed by O XAS and XES spectroscopy. Surface exposure (i.e., maturity) indices of lunar soils are a measure of residence time of soil in the upper millimeter of the regolith. We will use the Ferromagnetic Resonance index Is/FeO, as a quantitative index to place the XAS/XES analyzed Apollo soils within a maturity context that was used for previously analyzed surface, core, and trench soils from Apollo and Luna missions. In conjunction with Is/FeO, SEM backscatter imagery will also be used to characterize grain size and morphology (e.g., mineral or agglutinate), and EDS will be used to measure grain major element composition. To complement XAS analyses we will also analyze a handful of individual grains by electron energy loss spectroscopy (EELS). EELS is performed in a transmission electron microscope and is the electron equivalent of XAS: Electron energy loss due to absorption processes in the sample can be used to determine oxidation state, electronic structure and coordination geometry over far smaller volumes of sample. This will permit assessment of regolith sample uniformity within single samples with a given maturity index.
XAS involves the excitation of core electrons to unoccupied molecular orbitals or the ejection of core electrons. Transitions to molecular orbitals provide insight into electronic and geometric structure, while ejected electrons scatter off nearest neighbors to provide further geometric information. Low energy XAS is inherently surface sensitive, only probing the top 2 - 10 nm of sample. The high energy excited states generated in XAS rapidly decay, and this decay can be monitored via XES. XES provides information on occupied molecular orbitals, and is thus a compliment to XAS. XES provides further information on electronic structure in addition to information on bound atoms in terms of identity and charge state (i.e. O vs C, OH vs OOH). This allows for more direct characterization of point defect species. For example, a peroxo signal in O XAS can be related to a peroxo signal in Si XES to identify the peroxo containing species. Both XES and XAS are generally applicable to any element, allowing for a holistic view of the speciation in the regolith.
The observed chemical speciation will be used to address a number of questions in planetary science. The nature of bound hydroxyl observed by IR will be studied in detail. The effects of space weathering on chemical speciation will be investigated, as well as how different signatures can be used to inform on the history of a regolith sample. Using a multielement approach allows for a nuanced look at the physical chemistry of the system beyond determining redox environment. This project will greatly benefit from access to pristine samples stored under vacuum or helium atmospheres, as the targeted chemical signatures may be altered by oxygen or moisture. XAS and XES can be done on a wide scope of sample types in a non-destructive way in inert environments, so that samples will still be useable by other participants in the ANGSA program after our study is concluded.Contact Information: Richard C Walroth, [email protected]