The Moon as Case Study, 2: Examining the Relationship Between Surface Chemical, Mineralogical, and Physical Properties

The relationship between lunar iron abundance determined from Apollo broad-band Gamma-ray (AGR) (1) and modeled from Clementine spectral reflectance (CSR) (2) orbital measurements (3) is explored here on a pixel by pixel basis, in preparation for future missions with such instruments where no ground truth exists. Both datasets have a bimodal distribution: the low iron mode corresponds to highland, and the high iron mode to mare compositions. (A filter was applied to the CSR data to allow comparison to the lower spatial resolution AGR data in areas of overlapping coverage.) The CSR low mode is lower (4% as opposed to 5 to 6% for AGR)), and the high mode higher (16% as opposed to 9 to 10% for AGR). AGR data show more variation in all terrains. In the farside highlands, the presence of basins always results in AGR increases in iron. The AGR mare iron values are far more variable as well, consistent with the variations reported in mare soil and rock samples (4). Previous work (5) has demonstrated that soils with soils with very different optical properties do not show the same relationship between the depth of the iron band and iron abundance. Other work has that demonstrated anomalies in an otherwise good correlation between albedo and chemical composition occur in localities with differences in soil morphology which affect optical scattering properties (6). We propose that attempts to remove variation in maturity from CSR data, involving the systematic use of reflectance at 750 nm, has resulted in systematic underestimating of iron abundance in highland soils with more blocky ejecta, and overestimating of iron in mare soils, which have more opaques. We are presently confirming that these predicted albedo variations can seen on the farside. REFERENCES: (1) Davis (1980) JGR 85, 3209- 3224; (2) Lucey, Taylor, Malaret (1995) SCIENCE 268, 1150-1153; (3) Clark, McFadden (1996) LPS XXVII, 227-228; (4) Clark, Hawke, Basu (1990) PROC LPSC 20, 147-160; (5) Fischer, Pieters (1994) ICARUS, 111, 475-488; (6) Clark, Andre, Adler, Podwysocki, Weidner (1976) GRL 3, 8, 421-424.