R. C. Elphic, ^* D. J. Lawrence, W. C. Feldman, B. L. Barraclough, S. Maurice, A. B. Binder, P. G. Lucey

The Lunar Prospector neutron spectrometer data correlate well with iron and titanium abundances obtained through analysis of Clementine spectral reflectance data. With the iron and titanium dependence removed, the neutron spectrometer data also reveal regions with enhanced amounts of gadolinium and samarium, incompatible rare earth elements that are enriched in the final phases of magma crystallization. These regions are found mainly around the ramparts of the Imbrium impact basin but not around the other basins, including the much larger and deeper South Pole-Aitken basin. This result confirms the compositional uniqueness of the surface and interior of the Imbrium region.

R. C. Elphic, D. J. Lawrence, W. C. Feldman, B. L. Barraclough, Space and Atmospheric Sciences, MS D466, Los Alamos National Laboratory, Los Alamos, NM 87545, USA. S. Maurice, Observatoire Midi-Pyrénées, 31400 Toulouse, France. A. B. Binder, Lunar Research Institute, Gilroy, CA 95020, USA. P. G. Lucey, Hawai'i Institute of Geophysics and Planetology, University of Hawai'i, Honolulu, HI 96822, USA.
^*   To whom correspondence should be addressed. E-mail: relphic{at}lanl.gov

A major step toward a global assessment of lunar surface chemistry was provided by analysis of spectral reflectance data returned by the Clementine mission. Comparison of the spectral reflectance properties and chemical compositions of lunar soils returned by the Apollo and Luna missions, and remote measurement of the spectral properties of the Apollo and Luna sample collection sites by Clementine, led to the development of algorithms that derive the abundance of FeO and TiO₂ from spectral properties of lunar soils and surface units with 1 to 2 weight % accuracy. By applying these algorithms to Clementine global imaging, it has been possible to infer the quantitative abundance of FeO and TiO₂ within ±80° latitude (1-4) at resolutions approaching 100 m. The approach was calibrated with lunar samples returned from a relatively small area of the lunar nearside, and it is possible that areas distant from the landing sites, such as the farside, might have different mineralogies. Thus, the inferred FeO and TiO₂ values might be spurious there (5). Here, we test the validity of the Clementine spectral reflectance (CSR) method with independent analyses of Fe and Ti, using data from the Lunar Prospector (LP) neutron spectrometer (6, 7).

The neutron spectrometer measures neutrons at thermal (0.001 to 0.3 eV), epithermal (0.3 eV to 500 keV), and fast (500 keV to 8 MeV) energies. Lunar neutrons are created by the interaction of galactic cosmic rays with the nuclei in the lunar regolith; they are produced in the fast regime at high energies as a direct result of spallation. These neutrons inelastically scatter off of other nuclei in the soil, losing energy as they pass through the epithermal regime. When their energies approach that corresponding to the temperature of the ambient regolith (thermal regime), the neutrons are captured by nuclei that have large cross sections for thermal neutron absorption (8). Iron and titanium are the most abundant elements with large absorption cross sections; consequently, they have considerable influence on thermal neutron fluxes. In addition, Fe and Ti evidently produce more fast neutrons than elements with lower atomic numbers (9). Consequently, when LP is above the Fe- and Ti-rich maria, the neutron spectrometer detects a higher fast neutron flux and a lower thermal neutron flux (6, 7). We report on neutron data acquired during the first 6 months of the LP mapping mission, beginning 16 January 1998.

The neutron spectrometer measures the net production of fast neutrons from the elements in the regolith below LP and the net flux of thermal neutrons resulting from production, moderation, and absorption due to all species. Consequently, it is not possible to directly infer the abundances of Fe and Ti separately with neutron spectrometer data alone. Instead, we use the most recently generated CSR Fe and Ti maps from (4) to predict how much neutron production and absorption should be seen and compare that prediction to what was actually observed. Any differences must be due to either a surplus or a lack of neutron absorbers, that is, Fe or Ti in unseen mineral phases, or other possible neutron absorbers.

To compare the CSR maps [which have a surface resolution of 0.25° (7.5 km)] to the neutron observations from LP, we convolved the maps with the effective surface response functions of the neutron detector. For thermal neutrons, the relevant surface area has an effective footprint diameter of about 700 km for a 100-km orbit. The fast neutron footprint is smaller, 350 km in diameter.

The fast neutron flux should correlate with the Fe and Ti content in the regolith. The maps shown in (7) are in qualitative agreement with the published CSR Fe and Ti abundances (4). Overall, the correlation coefficient between the fast neutron count rate and the CSR Fe+Ti weight % values is 0.811. For a more restricted latitude range (where possible problems associated with lighting in the CSR data are reduced), ±60°, the correlation improves to 0.887. For a region including the eastern nearside maria and some farside highlands (40° to 180°E longitude, ±60° latitude), the correlation coefficient is 0.930, whereas for the nearside maria region alone (90°W to 90°E longitude, 30° to 60° latitude), it is 0.941.

LP detected a low flux of thermal neutrons over the maria and South Pole-Aitken basin (7), indicative of the presence of high concentrations of Fe and Ti. The net thermal neutron absorption in a given volume of regolith is related to the macroscopic absorption cross section

(1)

where f_i, _ai, and A_i are the weight fraction, thermal neutron absorption cross section, and atomic mass of element i, respectively, and N_A is Avogadro's number. Table 1 shows the contribution of the various elements to the total macroscopic absorption cross section for three bulk regolith compositions: an Apollo 11 high-Ti mare soil, an Apollo 16 low-Fe highlands soil, and an Apollo 14 KREEP basalt (KREEP is a component of some lunar rocks with a composition enriched in potassium, K, rare earth elements, REE, and phosphorus, P). Fe and Ti are the biggest contributors to absorption in the mare soil; Fe is also the chief absorber (despite its low abundance) in the highlands soil, but Ca is almost as important. Silicon, Al, and the other major elements play a lesser, but not negligible, role in thermal neutron absorption.

Table 1. Contributions to eff, thermal neutron absorption. Ap, Apollo mission. Compound  a (b)*  a f NA/A (×104 cm2/g) Ap 11  Ap 16  Ap 14  SiO2 0.16 6.84 7.29 8.59 TiO2 5.8 32.30 1.75 7.29 Al2O3 0.23 1.97 3.64 2.05 FeO 2.53 32.36 11.85 23.15 MnO 13.2 2.24 1.12 2.17 MgO 0.06 0.73 0.59 0.56 CaO 0.46 5.54 7.17 4.50 Na2O 0.51 0.29 0.29 0.40 K2O 2.07 0.13 0.13 1.67 Sm  7,900 2.72 1.58 12.66 Gd 14,900 5.99 3.42 29.68 Total eff 91.40 38.97 88.34 * b, 1028 m2 per nucleus.

Because Ca abundance is so variable on the moon, we must include its effects in our calculations. We make use of an observed inverse correlation between CaO and FeO in lunar samples to do this (10). In samples with very low FeO abundances, CaO reaches about 20% by weight; as FeO approaches about 12%, CaO abundance drops linearly to about 10% and stays at about that level in materials with higher FeO abundances. We assume that the net absorption due to Si, Al, and other major elements is approximately constant at 18 × 10^-4 cm²/g.

Certain REEs have anomalously high cross sections for absorption of thermal neutrons because of nuclear resonances (11). Consequently, the effects of these rare earth elements can be disproportionate to their low chemical abundances. Gadolinium and Sm, in particular, can significantly affect _eff in regions where they are abundant (>5 µg/g; see Table 1). Gadolinium and Sm are most abundant in KREEP, along with other incompatible elements such as Th, K, and U. These elements are enriched in the final phases of magma crystallization, and for this reason KREEP is thought to be derived from the lower crust. In the Apollo 14 KREEP basalts, the Gd and Sm contribution to neutron absorption is comparable with the total from all other elements and increases _eff from 50.8 × 10⁴ to 93.2 ×10⁴ cm²/g (12). The contributions of Sm and Gd to the Apollo 11 mare and Apollo 16 highlands _eff values are about 10 and 13%, respectively (Table 1). We have not included the effects of Gd and Sm in our estimates of _eff based on CSR-derived values of Fe and Ti abundances.

Simulations of neutron transport in lunar and martian regoliths have shown that the ratio of the fluxes of fast to thermal neutrons j_f/j_th is linearly related to _eff (12). This is what would be expected: the more neutron absorbers in the regolith, the fewer the number of thermal neutrons observed per fast neutron created. Thus, j_f/j_th is the neutron measurement proxy that directly relates to the macroscopic absorption cross section _eff.

The macroscopic absorption cross section _eff inferred from the CSR Fe and Ti results (Fig. 1) is high over western Oceanus Procellarum (50°W 15°N), Mare Imbrium (25°W 35°N), and Mare Tranquillitatis (25°E 10°N). The value of _eff is also high in the South Pole-Aitken basin mainly because of the basin's high Fe content. The ratio of the fast to thermal neutron fluxes, j_f/j_th, as observed by LP, shows similar patterns but also some differences, particularly around the periphery of Mare Imbrium (Fig. 2). To compare these two data sets, we plot the neutron flux ratio j_f/j_th versus the inferred CSR macroscopic absorption cross section _eff (Fig. 3). The observed absorption of thermal neutrons is often greater than would be expected on the basis of CSR Fe, Ti, and Ca.

The overall global correlation between _eff calculated with the CSR Fe and Ti values and the proxy for macroscopic absorption coefficient, j_f/j_th, is 0.849. By restricting the latitude range to ±60° (black points in Fig. 3), the correlation improves to 0.903. A still more limited region covering the eastern maria and some farside highlands, 20°E to 180°E longitude and ±30° latitude (red points in Fig. 3), yields a correlation coefficient of 0.978. Thus, our calculated values of _eff based on CSR Fe and Ti abundances correlate well with j_f/j_th in some regions but not as well in others.

A map of the difference _eff between this linear relation and the calculated _eff points (Fig. 3) reflects the implied abundance of neutron absorbers other than Fe and Ti (Fig. 4). The highest values are found over the rim of the Imbrium basin, including the Apennine mountains in the east up through the Alps, across to the Jura mountains in the west, down through the Aristarchus plateau and through the Fra Mauro formation to the south. LP gamma-ray spectrometer maps show that these regions also have high Th and K concentrations (13). This similarity implies that the absorbing species are affiliated with the incompatible elements found in KREEP; the correlation coefficient between the gamma-ray spectrometer Th data (13) and _eff is 0.93. The values of _eff are consistent with the range (10 × 10⁴ to 42 × 10⁴ cm²/g) of Gd and Sm contributions listed in Table 1. The K in KREEP plays only a very minor role in _eff (Table 1). Therefore, we conclude that _eff reflects primarily the concentration of Gd and Sm and, thus, is a tracer for KREEP.

The estimated thermal neutron macroscopic absorption coefficient that would be expected on the basis of concentrations of Fe and Ti derived from CSR data, coupled with an estimate of Ca concentrations, is in reasonable agreement with the LP neutron spectrometer results. Discrepancies arise in regions that have significant levels of KREEP, where Gd and Sm are major thermal neutron absorbers. Thus, the CSR method appears to be a reliable technique for obtaining FeO and TiO₂ abundances moon-wide. The inferred KREEP-rich regions form a ring around the Imbrium impact site and are directly related to either excavation of this lower crustal chemistry or to volcanism that extruded KREEP-rich lava on the surface. On the other hand, the much larger, deeper South Pole-Aitken impact basin shows little KREEP enhancement (14). This result appears to confirm the uniqueness of the Imbrium lower crustal chemistry and suggests that the moon may have considerable regional compositional heterogeneity at depth.

REFERENCES AND NOTES

1. P. G. Lucey, G. J. Taylor, E. Malaret, Science 268, 1150 (1995) [Abstract/Free Full Text] .
2. P. G. Lucey, D. T. Blewett, J. R. Johnson, G. J. Taylor, B. R. Hawke, Lunar Planet Sci. XXVII, 781 (1996) .
3. D. T. Blewett, P. G. Lucey, B. R. Hawke, B. L. Jolliff, J. Geophys. Res. 102, 16319 (1997) [CrossRef].
4. P. G. Lucey, D. T. Blewett, B. R. Hawke, ibid. 103, 3679 (1998).
5. P. E. Clark and A. Basu, Proc. Lunar Planet. Sci. Conf. 29, 1501 (1998) . Olivine is the likeliest mineral to be underrepresented in the results from analysis of spectral data.
6. W. C. Feldman et al., Science, 281 1496 (1998).
7. W. C. Feldman et al., ibid., p. 1489 (1998).
8. D. M. Drake, W. C. Feldman, B. M. Jakosky, J. Geophys. Res. 93, 6353 (1988) .
9. R. C. Reedy et al., Meteorit. Planet. Sci. 33 (suppl.), A127 (1998).
10. L. A. Haskin and P. H. Warren, in Lunar Sourcebook, G. H. Heiken, D. T. Vaniman, B. M. French, Eds. (Cambridge Univ. Press, New York, 1991), pp. 367-474, figure 8.3. The inverse correlation between FeO and CaO is principally due to variations in the abundance of anorthite, a Ca-rich and Fe-poor plagioclase. At higher FeO contents where the correlation flattens, the rocks are typically mare basalts, which contain less anorthite but more calcic pyroxenes.
11. R. Lingenfelter, E. H. Canfield, V. E. Hampel, Earth Planet. Sci. Lett. 16, 355 (1972) .
12. W. C. Feldman, R. C. Reedy, D. S. McKay, Geophys. Res. Lett. 18, 2157 (1991) .
13. D. J. Lawrence, et al., Science 281, 1484 (1998) [Abstract/Free Full Text] .
14. Note added in proof: The fast neutron data suggest that the CSR FeO abundances in South Pole-Aitken basin may be overestimated. If so, then it is likely that Gd and Sm abundances are higher there than we have estimated, and the basin is richer in incompatible elements than we have suggested.
15. This research was partially supported by NASA through a subcontract from Lockheed-Martin Corporation. We thank R. Reedy and D. Vaniman for discussions, P. Spudis and another referee for helpful and thorough reviews, and D. Thomsen for maps of lunar surface features. This work was performed under the auspices of the U.S. Department of Energy.