USING MINI-RF TO IMPROVE ACCURACY OF LUNAR TiO2 DISTRIBUTION MAPS

The Mini-Radio Frequency (Mini-RF) instrument is on board the NASA Lunar Reconnaissance Orbiter, which has been in orbit around the Moon since June 2009. Mini-RF is capable of imaging in two bands, X-band (4.2-cm) and S-band (12.6-cm), at 150 m and 30 m (Zoom Mode) resolution respectively, and with an illumination incidence angle of ~48º. The majority of its observations to date have been obtained in S-Band Zoom Mode. Mini-RF was designed to map the permanently dark areas of the lunar poles and characterize the nature of the deposits. In addition, to aid global analyses of mare composition, Mini-RF has acquired coverage for most of the maria.

Mini-RF observations of geologic targets of interest are used to improve TiO₂ mapping within the lunar maria. Targets of interest include basalt flows that were estimated to contain high-TiO₂ compositions based on Clementine spectral reflectance data but had low-TiO₂ compositions as measured by Lunar Prospector neutron spectrometer data (LPNS).

Visible and near infrared spectral characteristics of lunar soils are controlled by multiple competing factors. Ilmenite, the principal oxide phase, is dark and spectrally neutral relative to the spectrally red mature lunar mare soils; thus causing soils to become spectrally bluer as ilmenite content increases. However, large uncertainties in ultraviolet-visible (UVVIS) based estimates of TiO₂ are revealed when comparing LPNS TiO₂ and Clementine UVVIS 415/750 ratio. Prime culprits identified as causing this effect are differential agglutinate formation on high-FeO flows, TiO₂ in phases other than ilmenite, and surface contamination by highlands materials.

Similar to UVVIS spectra, depolarized radar return is found to anticorrelate with titanium abundance – with higher TiO₂ abundance leading to lower radar returns. The source of the titanium affecting the radar return is thought to be ilmenite. The possibility that the radar loss tangent is modulated by other mineralogic components is unsupported. Therefore, differences between TiO₂ or FeO in silicates, which darkens them so they act opaque, and TiO₂ in ilmenite can be determined. Integrating complimentary information from these independent data sets yields encouraging results explaining observed differences between UVVIS spectral based maps of TiO₂ and LPNS TiO₂.