Previous work has shown that the strongest concentrations of lunar crustal magnetic anomalies are located antipodal to four large, similarly aged impact basins (Orientale, Serenitatis, Imbrium, and Crisium). Here, we report results of a correlation study between magnetic anomaly clusters and geology in areas antipodal to Imbrium, Orientale, and Crisium. Unusual geologic terranes, interpreted to be of seismic or ejecta origin associated with the antipodal basins, have been mapped antipodal to both Orientale and Imbrium. All three antipode regions have many high‐albedo swirl markings. Results indicate that both of the unusual antipode terranes and Mare Ingenii (antipodal to Imbrium) show a correlation with high‐magnitude crustal magnetic anomalies. A statistical correlation between all geologic units and regions of medium to high magnetization when high‐albedo features are present (antipodal to Orientale) may suggest a deep, possibly seismic origin to the anomalies. However, previous studies have provided strong evidence that basin ejecta units are the most likely sources of lunar crustal anomalies, and there is currently insufficient evidence to differentiate between an ejecta or seismic origin for the antipodal anomalies. Results indicate a strong correlation between the high‐albedo markings and regions of high magnetization for the Imbrium, Orientale, and Crisium antipodes. Combined with growing evidence for an Imbrian age to the magnetic anomalies, this supports a solar wind deflection origin for the lunar swirls.

Planetary thermal neutron fluxes provide a sensitive proxy for mafic and feldspathic terranes and are also necessary for translating measured gamma‐ray line strengths to elemental abundances. Both functions require a model for near‐surface temperatures and a knowledge of the dependence of thermal neutron flux on temperature. We have explored this dependence for a representative sample of lunar soil compositions and surface temperatures using the Monte Carlo N‐Particle Code (MCNP™)(MNCP is a trademark of the Regents of the University of California, Los Alamos National Laboratory). For all soil samples, the neutron density is found to be independent of temperature, in accord with neutron moderation theory. The thermal neutron flux, however, does vary with temperature in a way that depends on Δ, the ratio of macroscopic absorption to energy‐loss cross sections of soil compositions. The weakest dependence is for the largest Δ (which corresponds to the Apollo 17 high‐Ti basalt in our soil selection), and the largest dependence is for the lowest Δ (which corresponds to ferroan anorthosite, [FAN] in our selection). For the lunar model simulated, the depth at which the thermal neutron population is most sensitive to temperature is ∼30 g cm−2. These simulations were compared with the flux of thermal neutrons measured using the Lunar Prospector neutron spectrometer over the lunar highlands using a subsurface temperature profile that varies with latitude, λ, as Cos1/4λ. Model results assuming equatorial temperatures of 200 and 250 K are in reasonable agreement with measured data. This range of equatorial temperatures is not inconsistent with the average temperature measured below the diurnal thermal wave at the equator, Tmeas = 252 ± 3 K [Langseth and Keihm, 1977].

Neutron spectroscopy is a new way to study planetary bodies that have sufficiently thin atmospheres. This technique was demonstrated for the first time with Lunar Prospector around the Moon. Here, we report results for moderated neutrons having energies from 0 to 500keV that were measured using the anti-coincidence shield (ACS) of the gamma-ray spectrometer. We describe the detection method, followed by data reduction with an emphasis on each data processing step; most steps rely on in-flight calibrations. The behavior of the ACS is well known regarding the measurement of moderated neutrons. We present a map of 0–500keV neutrons over the whole Moon with a spatial resolution of ∼60km. Statistical errors per pixel are less than 2%. The resulting map includes information about the hydrogen content, concentrations of Fe, Ti, traces of Sm and Gd, and the atomic mass of the regolith. These data complement other neutron products of Lunar Prospector, namely thermal (0–0.4eV), epithermal (0.4–100eV) and fast (0.5–8MeV) neutrons. The previous unexplored region between 100eV and 500keV reveals several high counting rate regions that are also visible in epithermal neutron data.

Mapping and model simulations of Lunar Prospector magnetometer measurements show that the source of the strongest known magnetic anomaly on the lunar near side (42 nanoTeslas at 18.6 km altitude) coincides approximately with a high‐albedo region of the Descartes mountains centered 60 km south‐southeast of the Apollo 16 landing site. The Descartes mountains represent primary ejecta from one or more basin‐forming events (Imbrium and/or Nectaris), supporting the hypothesis that basin ejecta materials emplaced >3.8 Gyr ago are the main sources of lunar magnetic anomalies. The higher albedo of the surface at this location is consistent with a significant role for solar wind ions in the optical maturation (or “space weathering”) of the lunar surface.

Maps of thorium [Th], [FeO], the ratio of epithermal to thermal neutrons (E/T), and fast neutrons (FN) from Lunar Prospector were studied to determine their global distribution on the Moon. These distributions are compared to that of the 750 nm lunar albedo from Clementine to aid in their interpretation. All distributions were parameterized using a spherical harmonic expansion out to order l = 30. Resultant harmonic coefficients generally decrease with increasing l value as a power law in l for all five variables. The axes of all dipole components ( l = 1) cluster closely about a centroid given by +14.1° latitude and 16.4° west longitude. This location is very close to the symmetry axes of their quadrupole components ( l = 2), which cluster about a centroid given by +24.6° latitude and 25.1° west longitude. Both centroids are near the center of a suggested Procellarum basin, given by Whitaker [1981] at +23° latitude and 15° west longitude. This suggestion is strengthened by a sharp decrease of the intensities of all three variables at ∼50° from the centroids, which is close to the boundary of the putative Procellarum basin. The coincidence of the global concentration of heat‐producing elements (through [Th]) and mare deposits on the lunar surface (through [FeO], [T/E], and [FN]) with the circular outline of a putative Procellarum basin suggests that the events that were responsible for producing Oceanus Procellarum (perhaps a single giant impact) must figure importantly in shaping the global distribution of surface composition. A second, apparently older circular structure having its center near −5° latitude and 65° east longitude and a radius of 50° is also hinted at in the data.

We investigate the crustal magnetic signatures of lunar craters using Lunar Prospector (LP) electron reflectometer data. Craters of all ages often have associated magnetic lows, showing that crustal fields were present even in pre‐Nectarian times (≳3.9 Ga). Magnetic lows extend to ∼2–4 crater radii, suggesting shock rather than thermal demagnetization. Younger craters are more likely to have clear and complete demagnetization signatures, suggesting that many older magnetic lows have been subsequently obscured. No size dependence is found for craters larger than 50 km in diameter, suggesting that demagnetization effects for all craters in this size range completely penetrate the magnetized layer. If shock demagnetization is responsible, this suggests an upper limit of ∼50 km for the depth of magnetization. Evidence of edge effects due to magnetization contrasts may show that strong far‐side crustal fields are coherent at scales of ∼25 km.

Observations of electron distributions above the shadowed surface of the Moon show energy‐dependent loss cones which indicate reflection by both magnetic and electric fields. At the same time, low energy (≲100 eV) field aligned upward‐going electron beams are observed. Together, these observations imply average night‐side potential differences between the surface and the Lunar Prospector (LP) spacecraft of ∼−35 V. The lunar surface may be at an even higher negative potential relative to the ambient plasma, since LP will likely also charge negative. The potential difference is consistent with simple current balance models which include secondary emission. No clear dependence is found on surface terrane type and age, or on ambient electron density and temperature. Instead, the potential difference is found to depend strongly on the angle from the subsolar point and the angle between the magnetic field and the normal to the lunar surface.

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.

Global measurements of iron abundances on the lunar surface are presented using data from the Lunar Prospector (LP) Gamma‐Ray Spectrometer (GRS) and Neutron Spectrometer (NS). In this study, we derive relative iron abundances from the low‐altitude, high spatial resolution (∼(45 km)2) LP data using the 7.6 MeV neutron capture gamma‐ray doublet. As part of the LP‐GRS analysis, we demonstrate the importance of accounting for variations in neutron number density across the lunar surface by measuring neutron fluxes using LP‐NS data. In a first step of comparing the LP‐GRS data with previously published iron abundances inferred from Clementine Spectral Reflectance (CSR) data, we show that the existing CSR FeO data are nonlinear with respect to the LP relative iron abundances. We use the LP data to linearize the relationship between the CSR and the relative iron values then recalibrate the CSR data to iron abundance using returned soil abundances. We then correlate the CSR data, except for major anomalies, with the LP relative iron measurements to convert the LP data to absolute iron abundances. When we compare the LP‐GRS and revised CSR data sets, we find a very good correspondence. There are two locations (Mare Tranquillitatis and South Pole‐Aitken (SPA) basin) that show major discrepancies, suggesting that the CSR data are locally overestimating iron abundances. In both these regions, the discrepancies identified by the LP‐GRS/CSR comparison are possibly explained by mineralogical differences that are not accounted for in the CSR to FeO calibration. In regards to our understanding of the Moon, the LP data have found the following: (1) There exist large expanses of mare basalt in the western mare regions that have very high iron abundances (22–23 wt.% FeO) that are underrepresented but not absent from the returned sample collection and are highly unusual for mare soils, which typically contain a significant amount of highlands contamination. (2) The low iron abundances in the lunar highlands (∼5 FeO wt.%) are consistent with a previous analysis using thermal and epithermal neutrons and with the idea that the lunar crust formed by a relatively simple magma ocean process. (3) The comparison of LP and CSR derived iron abundances suggests that the material within SPA basin is similar to a norite‐type rock without an enriched mantle FeO signature. (4) A comparison of LP and CSR data at Tycho Crater shows a large discrepancy such that the CSR data show moderate iron abundances of 8–9 wt.% FeO while the LP data show very low iron abundances of 3–4 wt.% FeO. This discrepancy cannot yet be easily explained by any known process.

We use global magnetic field data and digitized geologic maps to determine the magnetic properties of lunar terranes. Average fields vary by a factor of 100 from demagnetized impact basins and craters to strongly magnetized antipodal regions. Additional information is contained in the original extended abstract.

Gamma rays are produced in the lu-nar surface by cosmic ray interactions and by the decay of radioisotopes. The gamma ray spectrum measured from orbit contains information on the abundance of major elements, including O, Si, Ti, Al, Fe, Mg, and Ca, and radioactive elements, including Th, U, and K. The Lunar Prospector mission acquired gamma ray and neutron spectra at two altitudes (30- and 100-km) over the whole moon. We have binned these spectra on equal area squares to produce data sets from which maps of elemental abundance can be determined. The Lunar Prospector gamma ray spectometer (GRS) consisted of a BGO scintillation detector with a plastic-scintillator anticoincidence shield. The pulse height resolution of the spectrometer was ~13% full-width-at-half-maximum at 662 keV. At this resolution, most spectral features included contributions from mul-tiple elements. Maps of Th have been made using the well-resolved 2.61 MeV gamma ray.[1] Maps of Fe have been made by analyzing net count rates in the region above 7.5 MeV.[2] Contributions from Al to this region were ignored, which may influence the ac-curacy of Fe abundance in regions with low Fe. Maps of Fe and Ti have been made by deconvolution of the spectrum above 5.5 MeV.[3] We have extended the deconvolution method to 700 keV, using a library least squares technique, which enables us to determine the abundance of all major- and radioactive-elements.

The Lunar Prospector Alpha Particle Spectrometer (LP APS) searched for lunar surface gas release events and mapped their distribution by detecting alpha particle?; produced by the decay of gaseous radon-222 (5.5 MeV, 3.8 day half-life), solid polonium-2 18 (6.0 MeV, 3 minute half-life), and solid polonium-210 (5.3 MeV, 138 day half-life, but held up in production by the 21 year half-life of lead-210). These three nuclides are radioactive daughters from the decay of uranium-238.

Lunar Prospector magnetometer data has been used to identify a number of nearside magnetic anomalies. Some of the features identified appear to correlate with impact ejecta, supporting a basin ejecta origin to the nearside anomalies. Additional information is contained in the original extended abstract.

Improved versions of Lunar Prospector thermal and epithermal neutron data were studied to help discriminate between potential delivery and retention mechanisms for hydrogen on the Moon. Improved spatial resolution at both poles shows that the largest concentrations of hydrogen overlay regions in permanent shade. In the north these regions consist of a heavily cratered terrain containing many small (less than ∼10‐km diameter), isolated craters. These border circular areas of hydrogen abundance ([H]) that is only modestly enhanced above the average equatorial value but that falls within large, flat‐bottomed, and sunlit polar craters. Near the south pole, [H] is enhanced within several 30‐km‐scale craters that are in permanent shade but is only modestly enhanced within their sunlit neighbors. We show that delivery by the solar wind cannot account for these observations because the diffusivity of hydrogen at the temperatures within both sunlit and permanently shaded craters near both poles is sufficiently low that a solar wind origin cannot explain their differences. We conclude that a significant portion of the enhanced hydrogen near both poles is most likely in the form of water molecules.

Lunar Prospector (LP) electron reflectometer measurements show that surface fields are generally weak in the large mare basalt filled impact basins on the near side but are stronger over highland terranes, especially those lying antipodal to young large impact basins. Between the Imbrium and Nectaris basins, many anomalies correlate with the Cayley and Descartes Formations. Statistical analyses show that the most strongly magnetic nearside terranes are Cayley‐type light plains, terra materials, and pre‐Imbrian craters. Light plains and terrae include basin impact ejecta as a major component, suggesting that magnetization effects from basin‐forming impacts were involved in their formation. The magnetization of pre‐Imbrian craters, however, may be evidence of early thermal remanence. Relatively strong, small‐scale magnetic anomalies are present over the Reiner Gamma feature on western Oceanus Procellarum and over the Rima Sirsalis rille on the southwestern border of Procellarum. Both Apollo subsatellite and LP data show that the latter anomaly is nearly aligned with the rille, though LP magnetometer and reflectometer data show that the anomaly peak is actually centered over a light plains unit. This anomaly and the Reiner Gamma anomaly are approximately radially aligned with the center of Imbrium, suggesting an association with ejecta from this basin.

TiO2. R. C. Elphic1, D. J. Lawrence1, S. Maurice2, W. C. Feldman1, B. L. Barraclough1, O. M. Gasnault1, A. B. Binder3, P. G. Lucey, and D. T. Blewett4, 1Space and Atmospheric Sciences, MS D466, Los Alamos National Laboratory, Los Alamos, NM 87545 USA (relphic@lanl.gov), 2Observatoire Midi-PyrŽnŽes, 31400 Toulouse, FRANCE, 3Lunar Research Institute, Ste 2360, 9040 South Rita Rd., Tucson, AZ, 85747, 4Hawai’i Institute of Geophysics and Planetology, University of Hawai’i, Manoa, HI USA.

With Lunar Prospector reflectometry data we now have sufficient surface coverage to allow detailed comparisons between crustal magnetism and geology. We find substantial evidence that lunar magnetism is dominated by the effects of impact processes. Additional information is contained in the original extended abstract.