Speciation and Distribution of Ti in Lunar Regolith: Implications for Future Exploration

Much has been done to couple the petrography of lunar regolith with spectroscopic data from the laboratory and lunar orbit in order to use remote data to determine mineralogy and geochemistry of the Moon's surface. Efforts have been made to understand and improve upon the “Charette relationship” between the Ti content of lunar soil and UV/VIS slope. To first order, this relationship provides a useful correlation that has been validated using landing-site soils and orbital remote sensing. However, discrepancies remain between datasets based on reflectance spectroscopy and gamma-ray or neutron spectroscopy, and inconsistencies exist within the reflectance dataset for numerous mare regions including Luna 16 & 24, and Apollo 11. One possibility is that the relationship has a practical limit at intermediate to low TiO₂ concentrations (<4 wt%). Nonetheless, striking variations among basalt flows are observed using 610/360 nm (Whitaker, The Moon 4, 1972) and 502/250 nm (Hubble; Robinson et al., GRL 34, 2007). LRO will make the first global measurements using UV filters centered at 315 and 360 nm, in addition to 415 (to couple to Clementine) and four additional filters between 415 and 700. Use of the UV filters will increase sensitivity to ilmenite. Although Ti-rich regolith tends to be correspondingly rich in ilmenite, the relationship is complicated by other Ti-bearing species, including pyroxene, agglutinates and other impact glasses, volcanic glasses, and other Fe-Ti oxides. Depending on maturity and volcanic or impact-glass content, Ti-rich soil may have as much as 50% of its Ti hosted by materials other than ilmenite. Significant differences occur in the habit of ilmenite, ranging from coarse and blocky or lath-like to finely acicular, skeletal, and poikilitic forms. These variations are also important to document for consideration of in-situ resource utilization focused on ilmenite as a feedstock for oxygen and solar-wind hydrogen production.