Lunar and Planetary Science XXIX 1359.pdf

APOLLO LANDING SITE VALIDATION OF LUNAR PROSPECTOR FE AND TI MEASUREMENTS USING HIGH RESOLUTION CLEMENTINE MULTISPECTRAL IMAGING, Paul G. Lucey, G. Jeffrey Taylor, and B.Ray Hawke, Ha- waii Institute of Geophysics and Planetology, University of Hawaii at Manoa, 2525 Correa Rd. Honolulu, HI 96822, USA ([email protected]).

Calibration and verification of Prospector-derived ele- Table 1 compares the average values of Fe and Ti of mental measurements through comparison with the returned the relatively uniform sample return sites derived from lunar lunar samples is complicated by the fact that the sample sites samples [8] to Prospector-convolved Clementine averages. It are compositionally heterogeneous at the spatial scale sam- is evident that most of the simulated Prospector landing site pled by Prospector. Because its γ-ray detector is has a field values are bunched at intermediate values owing to the con- of view of 4π steradians, at an altitude of 100km half of the tamination of mare sites by highland signal and vice versa. lunar elemental signal received by Prospector is emitted from Even Apollo 16 has some influence of mare signal from an area 200km in diameter below the spacecraft, but Pros- Tranquilitatis to the northeast. pector will receive the other half of its signal from an annu- lus with an inner diameter of 200 km and an outer diameter Table 1. Sample and Simulated Prospector FeO of over 1100 km (the lunar horizon) [1]. In the Apollo era and TiO2 for seven sample return sites. the gamma-ray data were calibrated by assuming that rela- tively uniform large areas near the landing sites could be Sample Remote Sample Remote assigned reliable compositions [2,3]. It is far from certain FeO FeO TiO2 TiO2 that this assumption is correct. For example, it has been Apollo 11 16.34 12.2 7.9 4.1 shown by Pieters [4] that vast areas of mare basalt, even near Apollo 12 17.24 14.6 2.6 2.8 the landing sites, are unsampled. Apollo 14 10.55 13.9 1.6 2.7 Among the elements to be mapped by Lunar Prospector Apollo 16 5.02 6.4 0.53 1.0 are Fe and Ti. Recently, Blewett et al. [5] showed that the Luna 16 16.72 12.4 3.3 2.2 correlation of spectrally derived Fe and Ti with Fe and Ti Luna 20 7.46 11.7 .50 2.1 measured for lunar samples was very high, with 1σ scatter in Luna 24 19.55 10.9 1.0 1.7 predicted FeO or TiO2 to be 1-2 wt.%. A reasonable objec- tion to the use of the lunar sample calibration of Clementine We conclude that centering the calibration on the multispectral data for global Fe and Ti mapping is that the landing sites themselves probably would not give sufficient quality of the correlation may be coupled to the mineralogy “throw” in the calibration as all the sites excepting Apollo 16 of the return sample sites. Areas far from the landing sites give about the same Fe values. The situation with Ti is not could exhibit far different mineralogies and thus not be ap- much better. Moving the calibration areas to less mixed propriate to the calibration based on the landing sites. How- areas improves the contrast in perceived Fe, but greatly in- ever, Lucey et al. 1995 [6] found that the error in estimating creases the uncertainty of the estimated Fe or Ti values. A Fe contents from spectra of a wide variety of pure minerals more sophisticated calibration would be to directly correlate was slightly more than 2 wt.% for Fe contents less than 30 and regress properly filtered Clementine elemental images wt.% Fe. Despite this test, (conducted on Fe only), the min- with the Prospector data for the entire region encompassing eralogic effects of the technique are a legitimate concern. By the landing sites. the same rationale however, the Clementine-derived ele- References: 1) Metzger, A.E. et al, in Geochemical mental data is very likely to be reliable in the vicinity of the Analysis, Pieters and Englert, Eds, Cambridge, 1993; 2) landing sites. Thus, averages of Clementine elemental data Metzger, A.E. et al, Proc. Lunar Planet. Sci. Conf. 8th, p949, using a reasonable model of the Prospector footprint should 1977; 3) Davis, P.D., JGR 85, B6, p3209, 1980; 4) Pieters, be useful to validate, or even calibrate, the Prospector Fe and C.M., Proc. Lunar Planet. Sci. Conf. 9th, p2825, 1978; 5) Ti data. Blewett, D.T. et al., JGR Planets, 102, p16319, 1997; 6) Using the γ-ray resolution function [1] we Lucey, P.G. et al. Science 268, p1150, 1995; 7) Lucey, P.G. weighted high resolution Fe and Ti data [7] to Prospector et al., JGR Planets, in press, 1998; 8) Lunar Sourcebook, resolution centered on the sample-return sites in order to Heiken et al, eds, Cambridge, 1991. determine the distribution of Fe and Ti values within the Prospector footprint. Figure 1 shows Fe and Ti histograms weighted to reflect the Prospector spatial response function. With the exception of Apollo 16, all of the landing sites are bimodal in Fe within the field of view of Prospector, which is expected as most of the landing sites are near mare- highland boundaries. Many of the sites are multi-modal in Ti as well. Lunar and Planetary Science XXIX 1359.pdf

VALIDATION OF PROSPECTOR WITH CLEMENTINE: P. G. Lucey, G. J. Taylor, and B.R. Hawke

Figure 1. Feo and TiO2 histograms for 8 of the 9 sample return sites derived from Clementine multispectral FeO and TiO2 images encompassing the Prospector field of view assuming 100km orbital altitude. The regions are centered on the indicated sample return sites and the spatial response has been weighted to simulate the elemental signal contribution to be received by Prospector. FeO is the solid line and TiO2 is the dot-dash line.