Abstract We performed spectral measurements on mixtures of ilmenite powder (<48 μ m) and clinopyroxene or gabbro powder (<155 μ m) to investigate the effect of ilmenite content on the spectral characteristics in the ultraviolet (UV), visible (Vis), and near-infrared (NIR) ranges. We found that increasing ilmenite content leads to (1) a darkening of Vis and NIR reflectance, (2) a decrease of the UV slope, (3) a weakening of band depths around 950–1050 nm and 1250–1300 nm, and (4) a neutralization of the Vis–NIR slope. For lunar mare regions, which have ∼7wt% ilmenite, it was estimated that the albedo in the Vis–NIR reflectance would decrease by 25%–29%, the Vis–NIR slope would decrease by 9%–26%, and the UV slope would decrease by 4%–5%. We also examined the effect of the addition of clinopyroxenes and gabbro on the spectral characteristics of ilmenite. The results indicate that most diagnostic features of ilmenite were masked, except for a distinct peak near 900 nm, which can thus serve as a reliable indicator of the presence of ilmenite in mixed compositions. Furthermore, we evaluated existing methods for estimating TiO 2 content in lunar mare regions using spectral band ratios. The results indicate that ratios that incorporate UV reflectance are more sensitive to ilmenite content than those that incorporate reflectance only at Vis and/or NIR wavelengths longer than 415 nm. This suggests that UV reflectance is more reliable for the estimation of ilmenite abundance on the lunar surface.
Matsuoka et al. (2026) studied this question.