In many types of compositional remote sensing, a key problem involves separating the signals of two or more compositionally distinct materials that are mixed on a planet's surface. We investigate mixing effects in planetary gamma ray spectroscopy. Two important sources of planetary gamma ray emissions are inelastic scatter reactions induced by energetic neutrons and prompt capture reactions induced by thermal neutrons. The nature of the neutron energy spectrum at a planet's surface therefore exerts an important influence on the nature of the gamma radiation emitted and on the interpretation of compositional information from that radiation. The most important factors controlling the neutron energy spectrum are the hydrogen content xH and the macroscopic thermal neutron absorption cross section ∑ of the surface material. If a planetary surface is composed of several mixed materials with differering values of xH and/or ∑, then the emitted gamma ray spectrum can depend sensitively on the physical nature of the mixing. An important example may be Mars, where one expects to find rocks with low xH intermixed with fine-grained soil that may have a significantly higher xH due to inclusion of water of hydration. If rocky materials are mixed through the soil at a size scale that is finer than the characteristic length scale for neutron transport (tens of centimeters), then they will be immersed in the thermal neutrons produced by scattering in the H-rich soil. Such rock may be expected to generate strong prompt capture lines. However, rock segregated in bodies larger than this length scale will be shielded from the thermal neutrons, so their emission will be dominated by inelastic scatter lines. We quantify these effects by modeling gamma ray emissions from the Martian surface. Our model includes production of neutrons by cosmic ray interactions, neutron scattering, gamma ray production by inelastic scatter, thermal neutron capture, and natural radionuclides, attenuation of the gamma ray signal by passage through surface materials and the Martian atmosphere, production of the gamma ray continuum background, and the physics and statistics of gamma ray detection from an orbiting spacecraft. In order to investigate the kinds of effects that can occur, we consider mixing of “Viking soil” with rocks of “mafic” and “felsic” composition, in three geometries: intimate mixing, in which the mixing length scale is small compared to neutron transport length scales; checkerboard mixing, in which it is large; and layered mixing, in which a thin layer of soil overlies a semi-infinite layer of rock. We also consider mixing of soil and ice in these geometries. We show that mixing geometry can have a major impact on observed spectra and hence on geochemical interpretations. We also show that significant steps can be made toward characterizing the mixing geometry and disentangling the soil and rock signals if adequate independent knowledge of the composition and concentration of the soil component is available.
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