Lunar Prospector Gamma Ray Spectrometer Research Articles

New insights into the global composition of the lunar surface from high-energy gamma rays measured by Lunar Prospector

[1] An analysis of the lunar gamma-ray spectrum as measured by the Lunar Prospector Gamma-Ray Spectrometer has revealed that 8–8.9 MeV gamma rays contain information about the elemental composition of near-surface materials. These high-energy gamma rays are found to be primarily sensitive to the total Fe and Mg content of the surface, although other elements also contribute. This information has been used to identify several regions with unique compositions, including the Hertzsprung and Orientale basins. A method for deriving global Mg abundances from high-energy gamma-ray measurements is presented. The physical mechanism for high-energy gamma-ray production is proposed to be radiation produced during the decay of galactic cosmic ray produced pions within the lunar surface. Laboratory measurements of pion production cross sections are found to be consistent with the empirically derived relationship between the lunar Fe, Mg, and Ti abundances and the measured high-energy gamma-ray count rates.

Mapping Lunar global chemical composition from Chang'E-1 IIM data

The global distribution of the chemical composition of the lunar surface is an important factor helping us to understand the formation and evolution of the Moon. In this paper, formulas were established for deriving FeO, TiO2, Al2O3 and MnO abundances from Chang'E-1 (CE-1) Interference Imaging Spectrometer (IIM) data on the basis of the method “color ratio of UV/VIS and NIR/VIS versus VIS reflectance diagram” which was put forward by Lucey and Blewett. Global high-resolution maps (200m/pixel) of FeO, TiO2, Al2O3 and MnO were produced, and then compared qualitatively with results from Clementine UVVIS, Lunar Prospector (LP) Gamma-Ray Spectrometer (GRS) and Neutron Prospector (NS) data. The abundance ranges of the above four elements are 0–21.0wt%, 0–9.5wt%, 5.4–32.1wt%, and 0.015–0.28wt% respectively. The abundance range of FeO is consistent with the results from LP-GRS data reported by Gillis et al. (2004), and the abundance range of TiO2 is consistent with the results from LP-NS data reported by Elphic et al. (2002). Relative abundance distributions of FeO and TiO2 from Clementine and IIM data are slightly different from those from LP-GRS and LP-NS data. In map from the LP-GRS data, FeO abundances are the highest at Oceanus Procellarum and Mare Imbrium. However, in the map from CE-1 IIM data they are the highest at Oceanus Procellarum and Mare Tranquillitatis. Although the spatial resolution of these maps is high, caution must be taken when the maps in this paper are used at the crater scale because they suffer from errors owing to topographically induced shading. In future work, a high-accuracy DEM from Lunar Reconnaissance Orbiter Mission Laser Altimeter (LOLA) data coupled with a photometric model can probably be used to resolve this problem.

Regolith thickness over the lunar nearside: Results from Earth-based 70-cm Arecibo radar observations

The inversion of regolith thickness over the nearside hemisphere of the Moon from newly acquired Earth-based 70-cm Arecibo radar data is investigated using a quantitative radar scattering model. The radar scattering model takes into account scattering from both the lunar surface and buried rocks in the lunar regolith, and three parameters are critically important in predicting the radar backscattering coefficient: the dielectric constant of the lunar regolith, the surface roughness, and the size and abundance of subsurface rocks. The measured dielectric properties of the Apollo regolith samples at 450MHz are re-analyzed, and an improved relation among the complex dielectric constant, bulk density and regolith composition is obtained. The complex dielectric constant of the lunar regolith is estimated globally from this relation using the regolith composition derived from Lunar Prospector gamma-ray spectrometer data. To constrain the lunar surface roughness and abundance of subsurface rocks from radar data, nine regions are selected as calibration sites where the regolith thickness has been estimated using independent analysis techniques. For these sites, scattering from the lunar surface and buried rocks cannot be perfectly distinguished, and a tradeoff relationship exists between the size and abundance of buried rocks and surface roughness. Using these tradeoff relations as guidelines for globally representative parameters, the regolith thickness of four regions over the lunar nearside is inverted, and the inversion uncertainties caused by calibration errors of the radar data and model input parameters are analyzed. The regolith thickness of the maria is generally smaller than that of highlands, and older surfaces have thicker regolith thicknesses. Our approach cannot be applied to regions where the surface roughness is very high, such as with young rocky craters and regions in the highly rugged highlands.

The Chandrayaan-1 X-ray Spectrometer: First results

We present X-ray fluorescence observations of the lunar surface, made by the Chandrayaan-1 X-ray Spectrometer during two solar flare events early in the mission (12th December 2008 and 10th January 2009). Modelling of the X-ray spectra with an abundance algorithm allows quantitative estimates of the MgO/SiO2 and Al2O3/SiO2 ratios to be made for the two regions, which are in mainly basaltic areas of the lunar nearside. One of these ground tracks includes the Apollo 14 landing site on the Fra Mauro Formation. Within the 1σ errors provided, the results are inside the range of basaltic samples from the Apollo and Luna collections. The Apollo 14 soil composition is in agreement with the results from the January flare at the 1σ uncertainty level. Discrepancies are observed between our results and compositions derived for the same areas by the Lunar Prospector gamma-ray spectrometer; some possible reasons for this are discussed.

Thorium abundances of basalt ponds in South Pole-Aitken basin: Insights into the composition and evolution of the far side lunar mantle

[1] Imbrian-aged basalt ponds, located on the floor of South Pole-Aitken (SPA) basin, are used to provide constraints on the composition and evolution of the far side lunar mantle. We use forward modeling of the Lunar Prospector Gamma Ray Spectrometer thorium data, to suggest that at least five different and distinct portions of the far side lunar mantle contain little or no thorium as of the Imbrian Period. We also use spatial correlations between local thorium enhancements and nonmare material on top of the basalt ponds to support previous assertions that lower crustal materials exposed in SPA basin have elevated thorium abundances, consistent with noritic to gabbronoritic lithologies. We suggest that the lower crust on the far side of the Moon experienced multiple intrusions of thorium-rich basaltic magmas, prior to the formation of SPA basin. The fact that many of the ponds on the lunar far side have elevated titanium abundances indicates that the far side of the Moon experienced extensive fractional crystallization that likely led to the formation of a KREEP-like component. However, because the Imbrian-aged basalts contain no signs of elevated thorium, we propose that the SPA impact event triggered the transport of a KREEP-like component from the lunar far side and concentrated it on the nearside of the Moon. Because of the correlation between basaltic ponds and basins within SPA, we suggest that Imbrian-aged basaltic volcanism on the far side of the Moon was driven by basin-induced decompressional melting.

Estimation of elemental abundances of the lunar regolith using clementine UVVIS+NIR data

In this study we propose a regression model for the estimation of lunar elemental abundances from spectral features extracted from Clementine multispectral imagery in the visible and near-infrared domain. We extract a set of spectral features, including the continuum slope, the FWHM of the ferrous absorption trough near 1000 nm, and the wavelengths and relative depths of the absorption minima and inflection points present in the trough. As a “ground truth” for the elemental abundances we rely on the Lunar Prospector gamma ray spectrometer (LP GRS) data. With respect to the elemental abundances of the Apollo and Luna landing sites independently derived from returned samples, the best examined regression model is a second-order polynomial. The proposed regression-based approach allows an estimation of the elemental abundances of Ca, Al, Fe, Mg, and O at an accuracy of about 1 wt% and some tenths of a weight percent for Ti. We examine the influence of calibration of the Clementine UVVIS+NIR data and find that its effect on the results obtained with the regression approach is minor. Furthermore, we define a three-endmember model which allows the petrographic mapping of the lunar surface materials in terms of their Fe, Mg, and Al abundances. We examine the global distribution of Mg-rich rocks, the distribution of cryptomaria, and the occurrence of aluminous mare basalts in the Frigoris region. A possible regional compositional anomaly in northwestern Oceanus Procellarum is found, which corresponds to an extended area displaying spectral characteristics consistent with mare basalt containing significant amounts of olivine. On local scales, we examine in terms of our regression model the highland craters Proclus and Tycho, the compositionally anomalous central peaks of the craters Copernicus and Bullialdus, and the pyroclastic deposits on the floor of Alphonsus and on the northern rim of Petavius. As a general result, we show that the regression-based approach allows the detection of the main lunar terrain classes and rock types based on multispectral imagery in the visible and near-infrared domain.

Thorium abundances on the Aristarchus plateau: Insights into the composition of the Aristarchus pyroclastic glass deposits

Thorium (Th) data from the Lunar Prospector gamma ray spectrometer (LP‐GRS) are used to constrain the composition of lunar pyroclastic glass deposits on top of the Aristarchus plateau. Our goal is to use forward modeling of LP‐GRS Th data to measure the Th abundances on the plateau and then to determine if the elevated Th abundances on the plateau are associated with the pyroclastic deposits or with thorium‐rich ejecta from Aristarchus crater. We use a variety of remote sensing data to show that there is a large, homogenous portion of the pyroclastics on the plateau that has seen little or no contamination from the Th‐rich ejecta of Aristarchus crater. Our results show that the uncontaminated pyroclastic glasses on Aristarchus plateau have an average Th content of 6.7 ppm and ∼7 wt % TiO2. These Th and Ti values are consistent with Th‐rich, intermediate‐Ti yellow glasses from the lunar sample suite. On the basis of this information, we use petrologic equations and interelement correlations for the Moon to estimate the composition of the source region from which the Aristarchus glasses were derived. We find that the source region for the Aristarchus glasses contained high abundances of heat‐producing elements, which most likely served as a thermal driver for the prolonged volcanic activity in this region of the Moon.

Searching for high alumina mare basalts using Clementine UVVIS and Lunar Prospector GRS data: Mare Fecunditatis and Mare Imbrium

In the context of sample evidence alone, the high-alumina (HA) basalts appear to be an unique, and rare variety of mare basalt. In addition to their distinct chemistry, radiometric dating reveals these basalts to be among the oldest sampled mare basalts. Yet, HA basalts were sampled by four missions spanning a lateral range of ∼2400 km, with ages demonstrating that aluminous volcanism lasted at least 1 billion years. This evidence suggests that HA basalts may be a widespread phenomenon on the Moon. Knowing the distribution of HA mare basalts on the lunar surface has significance for models of the origin and the evolution of the Lunar Magma Ocean. Surface exposures of HA basalts can be detected with compositional remote sensing data from Lunar Prospector Gamma Ray Spectrometer and Clementine. We searched the lunar surface for regions of interest (ROIs) that correspond to the intersection of three compositional constraints taken from values of sampled HA basalts: 12–18 wt% FeO, 1.5–5 wt% TiO 2, and 0–4 ppm Th. We then determined the “true” (unobscured by regolith) composition of basalt units by analyzing the rims and proximal ejecta of small impacts (0.4–4 km in diameter) into the mare surface of these ROIs. This paper focuses on two ROIs that are the best candidates for sources of sampled HA basalts: Mare Fecunditatis, the landing site of Luna 16; and northern Mare Imbrium, hypothesized origin of the Apollo 14 HA basalts. We demonstrate our technique's ability for delineating discrete basalt units and determining which is the best compositional match to the HA basalts sampled by each mission. We identified two units in Mare Fecunditatis that spectrally resemble HA basalts, although only one unit (Iltm) is consistent with the compositional and relative age of the Luna 16 HA samples. Northern Mare Imbrium also reveals two units that are within the compositional constraints of HA basalts, with one (Iltm) best matching the composition of the basalts sampled by Apollo 14.

Distinguishing high‐alumina mare basalts using Clementine UVVIS and Lunar Prospector GRS data: Mare Moscoviense and Mare Nectaris

High‐alumina (HA) mare basalts are a unique group of the lunar sample collection. Sample geochemistry indicates that these basalts are derived from sources composed of late‐stage cumulates from the Lunar Magma Ocean (LMO). Their aluminous nature suggests their sources contained significant plagioclase, which has implications regarding the efficiency of plagioclase separation from earlier forming, mafic cumulates in the LMO to form the anorthositic lunar crust, and hence the heterogeneity of the lunar mantle. The Apollo and Luna missions sampled HA basalts from four different locations that are separated by 80 equatorial degrees (∼2400 km). Radiometric age dating of these samples demonstrates aluminous basaltic volcanism spanned over 1 billion years, suggesting HA basalts may be more prevalent on the Moon than implied by the sample population. Knowing their global occurrence would ultimately enhance our understanding of lunar evolution. Aluminous mare basalts occupy a unique location in Th‐FeO compositional space that suggests they can be identified using global remote‐sensing data of the Moon. We present our approach for distinguishing exposures of HA basalts using Clementine ultraviolet‐visible‐infrared (UVVIS) and Lunar Prospector Gamma Ray Spectrometer (LP‐GRS) data with constraints based on the FeO, TiO2, and Th abundances of Apollo and Luna HA samples. We identified 34 regions of interest (ROIs) where HA basalts could be a prominent component of the lunar surface. By analyzing the rims and proximal ejecta from small impacts (0.4–4 km in diameter) into the mare surface we characterized compositionally distinct basaltic units that make up the mare and thus determined which units represent HA basalt lavas. The results were used to generate maps that depict discrete mare units and classify their general basalt type. Here we focus on two ROIs: Mare Moscoviense and Mare Nectaris. Mare Moscoviense is composed of four basaltic units, two of which are HA candidates. Clementine UVVIS data of Mare Nectaris show evidence of up to three mare basalt units. One is the remnants of a mid‐Ti unit that capped earlier low‐Ti flows. The majority of the basin is filled by a compositionally indistinguishable low‐Fe, low‐Ti basalt. However, spectral profiles suggest there are two units. Regardless, the units both fit the criteria for a HA basalt.

Formation of lunar mare domes along crustal fractures: Rheologic conditions, dimensions of feeder dikes, and the role of magma evolution

In this study we examine a set of lunar mare domes located in the Hortensius/Milichius/T. Mayer region and in northern Mare Tranquillitatis with respect to their formation along crustal fractures, their rheologic properties, the dimensions of their feeder dikes, and the importance of magma evolution processes during dome formation. Many of these domes display elongated summit vents oriented radially with respect to major impact basins, and several dome locations are also aligned in these preferential directions. Analysis of Clementine UV/VIS and Lunar Prospector gamma ray spectrometer data reveals that the examined mare domes formed from low-Si basaltic lavas of high FeO and low to moderate TiO 2 content. Based on their morphometric properties (diameter, height, volume) obtained by photoclinometric and shape from shading analysis of telescopic CCD images, we derive rheologic quantities (lava viscosity during eruption, effusion rate, duration of the effusion process, magma rise speed) and the dimensions of the feeder dikes. We establish three rheologic groups characterised by specific combinations of rheologic properties and dike dimensions, where the most relevant discriminative parameter is the lava viscosity η. The first group is characterised by 10 4 Pa s < η < 10 6 Pa s and contains the domes with elongated vents in the Milichius/T. Mayer region and two similar domes in northern Mare Tranquillitatis. The second group with 10 2 Pa s < η < 10 4 Pa s comprises the very low aligned domes in northern Mare Tranquillitatis, and the third group with 10 6 Pa s < η < 10 8 Pa s the relatively steep domes near Hortensius and in the T. Mayer region. The inferred dike dimensions in comparison to lunar crustal thickness data indicate that the source regions of the feeder dikes are situated within the upper crust for six of the domes in northern Mare Tranquillitatis, while they are likely to be located in the lower crust and in the upper mantle for the other examined domes. By comparing the time scale of magma ascent with the time scale on which heat is conducted from the magma into the host rock, we find evidence that the importance of magma evolution processes during ascent such as cooling and crystallisation increases with lava viscosity. We conclude that different degrees of evolution of initially fluid basaltic magma are able to explain the broad range of lava viscosities inferred for the examined mare domes. The spectral data reveal that differences in TiO 2 content may additionally account for the systematic difference in lava viscosity between the two examined lunar regions. We show that the described mechanisms are likely to be valid also for other lunar mare domes situated near Cauchy and Arago, regarded for comparison. On the other hand, we find for the Gruithuisen and Mairan highland domes that despite their inferred high lava viscosities of 10 8 – 10 9 Pa s , no significant magma cooling in the dike occurred during ascent, supporting previous findings that the highland domes were formed during a specific phase of non-mare volcanism by highly silicic viscous lavas.

Petrologic mapping of the Moon using Fe, Mg, and Al abundances

A comparison between the abundances of major elements on the Moon determined by Lunar Prospector gamma ray spectrometer and those in returned lunar samples is performed. Lunar Prospector shows higher Mg and Al content and lower Si content in western maria in comparison with the lunar sample collection. Lunar Prospector overestimated the Mg content by about 20%. There are no elemental anomalies at the lunar poles: this is additional evidence for the presence of polar lunar hydrogen. Using Mg, Fe, and Al abundances, petrologic maps containing information about the abundances of ferroan anorthosites, mare basalts, and Mg-rich rocks are derived. This approach is useful for searching for cryptomaria and Mg-rich rocks deposits on the lunar surface. A search is implemented for rare rock types (dunites and pyroclastic deposits). Ca-rich, Al-low small-area anomalies are detected in the far side highlands.

Derivation of elemental abundance maps at intermediate resolution from optical interpolation of lunar prospector gamma-ray spectrometer data

We propose a technique that interpolates available lunar prospector gamma-ray spectrometer (GRS) data using Clementine UVVIS spectral reflectance images. The main idea is to use low resolution GRS data as a “ground truth” to establish relationships linking optical data and geochemical information maximizing the respective correlation coefficients. Then the relationships and Clementine UVVIS data are used to derive elemental abundance maps with significantly improved spatial resolution. The main limitation of the technique is its dependence on how well the abundance of the elements correlates with the Clementine UVVIS data. The technique can also be applied to analysis of coming D-CIXS/Smart-1 and AMIE/Smart-1 data to increase resolution of lunar compositional maps. As an illustration of the suggested technique, maps for the elements Fe, Ti, O, Al, Ca, and Mg with pixel size 15 km×15 km are presented. The Fe and Ti distributions resemble qualitatively to the maps obtained with the well-known technique by lucey et al. (2000a. Lunar iron and titanium abundance algorithms based on final processing of Clementine ultraviolet-visible images. J. Geophys. Res. 105, 20,297–20,306), though in our case the ranges of Fe and Ti variations are, respectively, wider and narrower than for lucey's maps. New maps for the elements Fe, Ti, O, Al, Ca, and Mg appear to be informative. For instance, the map of oxygen abundance demonstrates an anomaly in the crater Tycho. The maps of Fe and Al contents show for highland regions slight variations related to maturity degree. Reliability of this relation is confirmed with lunar sample data. The reason of the correlation between chemical composition and exposition age of the lunar surface can be the global transport of the lunar surface material due to meteorite impacts.

A three end-member model for petrologic analysis of lunar prospector gamma-ray spectrometer data

We analyze preliminary Lunar Prospector gamma-ray spectrometer data. Al–Mg and Fe–Mg petrologic maps of the Moon show that Mg-rich rocks are located in Mare Frigoris, the South Pole Aitken basin, and in some cryptomaria. Analysis of distances of Lunar Prospector pixels from three end-member plane in Mg–Al–Fe space reveals existence of Ca-rich, Al-low small-area anomalies in the farside highlands. An Mg–Th–Fe petrologic technique can be used for estimation of abundances of ferroan anorthosites, mare basalts, KREEP basalts, and Mg-rich rocks.

Lunar surface geochemistry: Global concentrations of Th, K, and FeO as derived from lunar prospector and Clementine data

Accurate estimates of global concentrations of Th, K, and FeO have an important bearing on understanding the bulk chemistry and geologic evolution of the Moon. We present empirical ground-truth calibrations (transformations) for Lunar Prospector gamma-ray spectrometer data (K and Th) and a modified algorithm for deriving FeO concentrations from Clementine spectral reflectance data that incorporates an adjustment for TiO 2 content. The average composition of soil samples for individual landing sites is used as ground-truth for remotely sensed data. Among the Apollo and Luna sites, Apollo 12 and 14 provide controls for the incompatible-element-rich compositions, Apollo 16 and Luna 20 provide controls for the feldspathic and incompatible-element-poor compositions, and Apollo 11, 15, and 17, and Luna 16 and 24 provide controls for Fe-rich compositions. In addition to these Apollo and Luna sample data we include the composition of the feldspathic lunar meteorites as a proxy for the northern farside highlands to extend the range of the calibration points, thus providing an additional anorthositic end-member composition. These empirical ground-truth calibrations for Lunar Prospector Th and K provide self consistency between the existing derived data and lunar-sample data. Clementine spectral-reflectance data are used to construct a TiO 2-sensitive FeO calibration that yields higher FeO concentrations in areas of high-Fe, low-Ti, mare-basalt-rich surfaces than previous FeO algorithms. The data set so derived is consistent with known sample compositions and regolith mixing relationships.

Gamma‐ray measurements from Lunar Prospector: Time series data reduction for the Gamma‐Ray Spectrometer

The data reduction process for the Lunar Prospector Gamma‐Ray Spectrometer (LP‐GRS) time series data is described. This process takes raw data that are received directly from the spacecraft and converts them to time series gamma‐ray spectra that are used to derive elemental maps. These processed LP‐GRS time series data are used as the fundamental data set for deriving composition measurements of the lunar surface using either energy window techniques or the full modeling of the neutron and gamma‐ray transport processes. The analyses that use these gamma‐ray spectra to derive elemental abundances are beyond the scope of this manuscript but are described elsewhere in multiple publications. The processing described here to create the time series gamma‐ray spectra includes selections to eliminate bad data as well as make corrections due to time variations in the cosmic ray flux, LP‐GRS gain, LP‐GRS orientation, and altitude above the lunar surface. Descriptions are also given of data uncertainties and how the LP‐GRS data are mapped onto the lunar surface.

Small‐area thorium features on the lunar surface

Using an improved understanding of the Lunar Prospector Gamma‐Ray Spectrometer (LP‐GRS) spatial footprint, we have derived a new map of global thorium abundances on the lunar surface. This map has a full‐width, half‐maximum spatial resolution of ∼(80 km)2 and is mapped on the lunar surface using 0.5° × 0.5° pixels. This map has allowed the identification and classification of 42 small‐area (&lt;[80 km]2) thorium features across the lunar surface. Twenty of these features, all of which are located in the nearside Procellarum KREEP terrane, show a thorium‐iron anticorrelation that is indicative of mixing between mare basalts and thorium‐rich mafic impact‐melt breccias (MIB). However, there exists at least one example of a farside location (Dewar crater) that appears to have abundances similar to the thorium‐rich MIBs. This new map has also allowed the identification of mare basalts having high thorium abundances (&gt;3 μg/g) in southwestern Mare Tranquillitatis, near the Apollo 11 landing site. With our better understanding of the LP‐GRS spatial footprint, we have been able to constrain the surface thorium abundance at the Compton/Belkovich thorium anomaly to 40–55 μg/g, which is higher than any other measured location on the lunar surface and higher than most samples. Finally, using 1 km/pixel FeO abundances from Clementine and LP‐GRS spatial footprint information, we have been able to obtain plausible thorium distributions around Kepler crater at a resolution of 1 km/pixel. The materials around Kepler crater appear to be a relatively simple mixing of thorium‐rich MIB compositions and high‐thorium mare basalts.

Lunar Prospector neutron spectrometer constraints on TiO2

Lunar Prospector neutron spectrometer measurements of the epithermal and thermal neutron leakage fluxes are used to provide constraints on TiO2 abundances in lunar surface materials. We use FeO abundance estimates based on both Clementine spectral reflectance techniques and preliminary Lunar Prospector gamma ray spectrometer determinations to first establish a model thermal neutron absorption due to all major elements except titanium. Then we remove the additional absorbing effects due to the rare earth elements gadolinium and samarium by using Lunar Prospector gamma ray spectrometer thorium abundances as a rare earth element proxy. The result can be compared to the ratio of epithermal to thermal neutron fluxes, which point to the presence of the additional thermal neutron absorber, titanium. We can derive abundance estimates of TiO2 and compare to other estimates derived spectroscopically. Our results show a significantly lower abundance of TiO2 than has been derived using Clementine data.

Lunar rare earth element distribution and ramifications for FeO and TiO2: Lunar Prospector neutron spectrometer observations

Lunar Prospector neutron spectrometer data have been used to map the global surface distribution of the incompatible rare earth elements gadolinium and samarium from the low‐altitude (30±15 km) mapping orbit. These results afford improved surface resolution and detailed views of the potassium, rare earth elements, and phosphorus (KREEP) distribution within and around Mare Imbrium and elsewhere. The Gd and Sm results serve as a complementary and independent check of the distribution of KREEP on the Moon, in contrast with the Lunar Prospector gamma ray spectrometer results for thorium. The neutron spectrometer observations reflect the presence of Fe and Ti as well as Gd and Sm. The contributions of Fe and Ti are removed using high spatial resolution Clementine spectral reflectance determinations of FeO and TiO2 abundances. Overall, the resulting Gd and Sm abundance map agrees with the Th abundance map determined using the Lunar Prospector gamma ray spectrometer. In general, the detailed features of the Procellarum/Imbrium KREEP terrane are found in both. For example, distinct highs in Gd, Sm, and Th abundances are resolved over the craters Mairan, Aristarchus, Kepler, Reinhold, Lalande, and Aristillus, over the Apennine Bench and Fra Mauro regions, and over the Montes Jura and Montes Carpatus, indicating an enhanced abundance of KREEP in these locations. The neutron observations also provide constraints on FeO and TiO2 abundances; for some high‐Ti locales, there is a significant disagreement with TiO2 abundances inferred from Clementine spectral reflectance.

Global elemental maps of the moon: the Lunar Prospector gamma-Ray spectrometer.

Lunar Prospector gamma-ray spectrometer spectra along with counting rate maps of thorium, potassium, and iron delineate large compositional variations over the lunar surface. Thorium and potassium are highly concentrated in and around the nearside western maria and less so in the South Pole-Aitken basin. Counting rate maps of iron gamma-rays show a surface iron distribution that is in general agreement with other measurements from Clementine and the Lunar Prospector neutron detectors.