UV-VIS-NIR Reflectance Spectra of Synthetic Opaque Minerals – Implications for Remote Mapping of TiO2 on the Surface of the Moon and Mercury

Returned lunar samples show wide variations in TiO₂ abundance. Remote mapping of TiO₂ abundance on the lunar surface will enable detailed studies of lunar crustal composition, surface volcanism, and temporal and spatial variations in mantle composition. Substantial effort has been made to infer TiO₂ composition of the lunar surface using color ratios in the UV-VIS-NIR. The MESSENGER mission to Mercury promises similar opportunities to map TiO₂ on that surface. We have obtained reflectance spectra of 6 synthetic Ti-Fe-bearing opaque minerals (ferrospeudobrookite FeTi₂O₅, armalcolite (Mg,Fe)Ti₂O₅, armalcolite-ansovite 0.85[(Mg,Fe)Ti₂O₅]0.15[Ti₃O₅], ulvöspinel Mg_0.4Fe_1.6TiO₄, Mg-ulvöspinel Mg_1.2Fe_0.8TiO₄ and ilmenite FeTiO₃) that occur in returned lunar samples and may be important constituents of the lunar and mercurian surface. We find that the UV-VIS slopes (below ~500nm) of all measured Ti-Fe-bearing opaque minerals are negative and so similar that they are not diagnostic of the specific minerals in remote multispectral observations on planetary regoliths. This is not a surprising result since the UVVIS is controlled by metal-metal and metal-oxygen charge transfers. These charge transfers saturate at low levels of transition metals, thus the Fe-Ti-bearing opaque minerals are similar in the UVVIS. This may have important implications for the remote detection of TiO₂ abundance on the lunar surface and supports the observation of the MESSENGER MDIS team of small dark deposits on Mercury suggestive of the presence Ti-bearing opaque mineral. Our results indicate that negative UV-VIS spectral slopes are indicative of Fe-Ti bearing opaque minerals and but are not diagnostic of a specific mineral species.