A REEVALUATION OF SPECTRAL RATIOS FOR LUNAR MARE TIO2 MAPPING

The empirical relation established by Charette et al. [1974] between the 400/560 nm spectral ratio of mature mare soils and weight percent TiO2 has been used extensively to map titanium content in the lunar maria [e.g., Johnson et al., 1977; Johnson et al., 1991b]. Relative reflectance spectra of mare regions show that a reference wavelength further into the near-IR, e.g., > 700 nm, could be used in place of the 560 nm band to provide greater contrast (a greater range of ratio values) and hence a more sensitive indicator of titanium content. An analysis of 400/730 nm ratio values derived from both laboratory and telescopic relative reflectance spectra suggests that this ratio provides greater sensitivity to TiO2 content than the 400/560 nm ratio. The increased range of ratio values is manifested in higher contrast 400/730 nm ratio images compared to 400/560 nm ratio images. This potential improvement in sensitivity encourages a reevaluation of the original Charette et al. [1974] relation using the 400/730 nm ratio.