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

Abstract

[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.Citation: Peplowski, P. N., and D. J. Lawrence (2013), New insights into the global composition of the lunar surface from high-energy gamma rays measured by Lunar Prospector,

Bulk hydrogen abundances in the lunar highlands: Measurements from orbital neutron data

“…For example, certain compositions might show biases to retain hydrogen more effectively than other compositions. While the bulk iron and titanium abundances across the FHT are mostly uniformly low (Lucey et al, 2000b;Lawrence et al, 2002), other bulk composition parameters, such as magnesium concentrations, do appear to show significant variations across the FHT (Prettyman et al, 2006;Peplowski and Lawrence, 2013) and in principle could be linked to compositional effects related to hydrogen retention. More recent measures of optical maturity (Nettles et al, 2011) show different compositional variations that can be compared to the derived hydrogen concentrations.…”

Compositional variability on the surface of 4 Vesta revealed through GRaND measurements of high‐energy gamma rays

“…). Examination of LP GRS data (Peplowski and Lawrence ) found that, for gamma rays with energies between 8 and 8.9 MeV, the continuum originated predominantly from the lunar surface. Unlike the LP measurements, the GRaND gamma‐ray measurements include an appreciable background contribution from the spacecraft at all energies and therefore require a background reduction process that was not necessary for the LP HEGR measurements.…”

“…Peplowski and Lawrence () used the laboratory measured charged pion production cross sections (σ) of Cochran et al. () to express the total, element‐dependent pion production cross section σ i (π) aswhere Z i and N i are the proton and neutron numbers of element i , respectively, and the units are millibarns (1 mb = 10 −27 cm 2 ).…”

“…Under the assumption that HEGR production is directly proportional to the abundance of a given element i and its σ i (π) value, Peplowski and Lawrence () redefined the total pion production cross section (σ T ) as a new composition parameter ( C p ) to describe the dependence of HEGR production on the bulk elemental composition. C p is calculated asin units of mb, where χ i is the molar fraction for element i in the material of interest.…”

“…The HEGR continuum is normally treated as a spectral background that must be removed to characterize the full‐energy gamma‐ray peaks of interest. A recent analysis of Lunar Prospector (LP) gamma‐ray spectrometer (GRS) data found that the total count rate of gamma rays with energies between 8 and 8.9 MeV contains compositional information about the bulk, near‐surface elemental composition of the Moon (Peplowski and Lawrence ). Specifically, the HEGR count rate was observed to increase over regions with higher abundances of the heaviest major elements (e.g., Fe, Ti).…”