Three spectrometers onboard Lunar Prospector...

Lunar Prospector (LP), which was launched on January 6, 1998, carries an integrated suite of three spectrometers. A Gamma-Ray Spectrometer (GRS) and a Neutron Spectrometer (NS) are providing global maps of the major and trace elemental composition of the lunar surface, with special emphasis on the search for polar water-ice deposits, implied by the H abundance. An Alpha Particle Spectrometer (APS) is being used to determine the frequency and locations of gas-release events.

The LP spectrometer system consists of a common electronics package located on the bus, a gamma-ray detector mounted on one science boom and the neutron and alpha-particle detector assembly mounted on a second boom. The spatial resolution of these spectrometers is 150 km at the nominal LP mapping altitude of 100 km and will be improved by lowering the altitude to 35 km in the extended mission starting in December 1998. It is expected that the extended mission will last until July 1999.

Gamma-ray, neutron and alpha-particle fluxes from the Moon are very low. Long integration times are therefore required to obtain statistically meaningful data. Several months to over a year are required to reach the global compositional mapping goals of the GRS. In contrast, the neutron data indicated the presence of water ice within a couple of months after Lunar Prospector achieved its lunar mapping orbit.

NS and GRS Science Objectives

Global mapping of elemental abundances by the LP GRS and NS will impose major new constraints on the bulk composition of the lunar crust, on compositional variations over the lunar surface, and on the existence of lunar resources including polar water ice.

The bulk composition of the lunar crust is important for estimating the composition of the Moon as a whole, a fundamental constraint on the origin of the Earth-Moon system. LP will determine surface concentrations of key elements, which when combined with models of lunar differentiation, will impose significant new constraints on the lunar bulk composition.

A determination of lunar surface composition variability will significantly improve our understanding of the evolution of the highland crust as well as of the duration and extent of basaltic volcanism. For example, one important component of lunar rocks whose abundance will be accurately mapped is KREEP (K-potassium, Rare Earth Elements, and Phosphorus). KREEP is enriched in incompatible elements and therefore, probably represents the last crystallization product of the putative lunar magma ocean [Warren and Wasson, 1979]. From limited Apollo data, it is known that the distribution of KREEP is highly variable. There is evidence that KREEP contaminated most lunar magmas as they made their way to the surface from sources in the mantle or lower crust [Binder, 1982]. Major basin excavations and lateral variations of crustal composition may also have contributed substantially to the distribution of KREEP at the surface. Thus KREEP is a tracer for understanding the evolution of the crust by volcanism and impact excavation/deposition.

Other specific objectives of GRS and NS include: 1) Identifying and delineating basaltic units in the maria; 2) Determining the composition of ancient or "cryptic" mare units found in the highlands, and searching for more of these units using mainly the Fe and Ti data; 3) Identifying and delineating highland petrologic units; 4) Searching for anomalous areas with unusual elemental compositions that might have resource potential; and 5) Determining the solar wind implanted H and ³He concentration to determine the degree of soil maturity and the distribution of these potential lunar resources.

Finally, maps of elemental composition will determine if significant quantities of water ice exist in permanently shadowed areas near the lunar poles [as conjectured by, e.g., Arnold, 1979]. Although returned samples show that the Moon is essentially devoid of intrinsic water, cometary and carbonaceous chondrite meteorite impacts have brought water to the Moon during its history. Clementine observations indicate that permanently shadowed regions do exist and may contain significant quantities of water ice [Nozette, et al., 1996] - though the latter conclusion is disputed on the basis of ground-based radar observations [Stacy et al., 1997].

APS Science Objectives

Using radioactive radon and polonium as tracers, the Apollo 15 and 16 orbital alpha-particle experiments obtained evidence for the release of gases at several sites, especially in the Aristarchus region [Gorenstein and Bjorkholm, 1972]. Aristarchus crater had previously been studied by ground-based observers as the site of transient optical events [Middlehurst, 1977]. The Apollo 17 surface mass spectrometer showed that ⁴⁰Ar is released from the lunar interior every few months, apparently in concert with some of the shallow moonquakes that are believed to be of tectonic origin [Hodges and Hofman, 1975]. The latter tectonic events could be associated with very young scarps identified in the lunar highlands [Schultz, 1972; Binder and Gunga, 1985] and believed to indicate continued global contraction. Thus, one goal of the APS observations is to determine whether at least some outgassing sites correlate with locations where relatively recent tectonic activity may be occurring. Because the youngest observed scarps may indicate continued global contraction, these observations will further constrain the lunar thermal history as well as sources of the Moon's surface-boundary exosphere. Finally, the data should provide information on the locations of potential sources of N₂, CO₂, and CO for lunar utilization.

The LP APS will map the distribution and temporal variability of outgassing sites on the Moon. Resultant data will determine the rate and temporal variability of at least one major source of the tenuous lunar atmosphere and could have significant resource applications as well. Specific objectives of the APS are to: 1) Determine the rate of outgassing of the lunar interior as a possible major source of the lunar atmosphere; 2) Determine the distribution of outgassing sites and their correlation with young impact craters and tectonic features; 3) Determine the global rate of tectonic events that may be responsible for shallow moonquakes and their relation to young scarps identified in the lunar highlands; 4) Determine whether the lunar tidal cycle influences the timing of the outgassing events; and 5) Provide an assessment of lunar volatiles for possible resource utilization.

GRS : How it works ?

The GRS sensor consists of a 7.1 cm diameter by 7.6-cm long bismuth germanate (BGO) crystal, placed within a 12-cm diameter by 20-cm long, well-shaped borated-plastic (BC454) anticoincidence shield (ACS), as shown in Figure 1. Both scintillators are viewed by separate photomultiplier tubes, and enclosed within a cylindrical, graphite-epoxy laminate housing. This housing material was chosen to maximize strength per unit weight, yet provide minimal background that could interfere with determination of lunar surface abundances. The gamma-ray energy range of the GRS extends between 0.3 and 9 MeV with a sampling resolution of 17.6 keV. Its spectral time resolution is 32 s, corresponding to a ground track distance of about 50 km.

The sensor housing is wrapped with a thermostat-controlled heater, placed inside a thermal blanket that will provide a stable operational environment of -28^o C for at least 1.5 years. Laboratory tests show that reduction of the BGO temperature from 21^o C to -28^o C improves its spectral resolution from 12.2% to 9.6% full width at half maximum, for ¹³⁷Cs 662 keV gamma rays.

The ACS of the GRS provides several functions. First is to tag and eliminate penetrating charged-particle events from the accepted gamma-ray spectrum. Next is to tag gamma rays that escape from the BGO to reduce the continuum portion of the BGO response function. The spectrum of this continuum (the rejected spectrum) is separately telemetered to ground to allow a clean-up of the accepted spectrum through ground-based post-processing. Laboratory tests show that subtraction of four times the rejected spectrum from the accepted spectrum removes most of this continuum.

A last function of the ACS is to detect neutrons using the boron content of the BC454. Events due to neutrons are tagged by the coincident detection of charged particles in the BC454 with a 478 keV gamma ray in the BGO, both resulting from the ¹⁰B(n,a)⁷Li* reaction. Detection of such an event in isolation results from a thermal or epithermal neutron. Detection of a time-correlated pair of events where the first of the pair is a BC454 interaction only, and the second consists of the foregoing BC454-BGO coincidence, signifies a fast neutron [Feldman et al., 1991a]. The ratio of these two count rates provides a sensitive measure of hydrogen [Feldman et al., 1991b].

The LP GRS is, as a function of energy, 2 to 8 times more sensitive than the Apollo GRS but several times less sensitive than a high resolution, germanium spectrometer. It should provide maps that can distinguish distinct surface compositional units of U, Th, and K in less than 2 months, Ti in about 3 months, Fe, Al, and solar wind implanted H in about 6 months, O in about 9 months, Si in about 1 year, Mg in about 1.2 years and Ca in about 1.8 years.

NS : How it works ?

Achievement of the NS scientific objectives requires separate measurements of thermal, epithermal, and fast neutrons that leak from the lunar surface. Maximum specificity to H and minimum sensitivity to variations in regolith chemistry is achieved by normalizing both the thermal and epithermal fluxes to the fast neutron flux [Feldman et al., 1991b].

The NS consists of two identical 5 cm diameter by 20 cm long ³He-filled gas proportional counters. One is covered with a 0.75 mm thick Cd shield, and so is only sensitive to epithermal neutrons having energies above 0.25 eV. The second counter is covered with a 0.75 mm thick Sn shield and so is sensitive to both thermal and epithermal neutrons. The difference in their counting rates yields a measure of the flux of thermal neutrons (0 to 0.25 eV). Both counters are mounted on a chassis shared with the APS detector, as shown in Figure 2. Fast neutrons will be measured using the ACS of the GRS, as explained previously.

Maps of H near the surface should be possible within 1 month at the poles, and within about 6 months elsewhere. Its sensitivity to spatially confined H deposits will be enhanced by a factor of 10 by lowering the LP orbital altitude.

APS : How it works ?

The APS searches for gas release events and maps their distribution by detecting alpha particles produced by the decay of gaseous ²²²Rn (an early daughter in the ²³⁸U decay series, half-life = 3.8 days), and solid ²¹⁰Po (a late daughter in the ²³⁸U decay series, half-life = 138 days, but is present on the surface for many decades because of the 20 year half-life of its immediate parent nucleus, ²¹⁰Pb).

The APS sensor consists of five pairs of 3 cm by 3 cm square ion-implant silicon detectors, each pair placed on one face of a cube as shown in Figure 2. Each detector is fully depleted to a depth of 55 m, which is sufficiently thin to reduce the proton background in the prime energy range of Rn-decay alpha-particle lines (between 4.1 MeV and 6.6 MeV) to manageable levels. They are covered by thin, Al-coated polypropylene foils to exclude sunlight, and collimated to a 90^o field of view (FOV) at half maximum. The combination of foil thickness, detector dead layer, and electronic noise gives a spectral resolution of about 100 keV at 5.5 MeV. The combined FOV of the 5 faces provides nearly 3p sr coverage, the only blind spot is in the direction of the spacecraft bus.

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