Remote Chemical Sensing Using Nuclear Spectroscopy

Nuclear spectroscopy techniques are used to determine the elemental composition of planetary surfaces and atmospheres. Radiation, including gamma rays and neutrons, is produced steadily by cosmic ray bombardment of the surfaces and atmospheres of planetary bodies and by the decay of radionuclides within the solid surface. The leakage flux of gamma rays and neutrons contains information about the abundance of major elements, selected trace elements, and light elements such as H and C. Gamma rays and neutrons can be measured from high altitudes (less than a planetary radius), enabling global mapping of elemental composition by an orbiting spacecraft. Radiation that escapes into space originates from shallow depths (<1 m within the solid surface). Consequently, nuclear spectroscopy is complementary to other surface mapping techniques, such as reflectance spectroscopy, which is used to determine the mineralogy of planetary surfaces. The main benefit of gamma ray and neutron spectroscopy is the ability to reliably identify elements important to planetary geochemistry and to accurately determine their abundance. This information can be combined with other remote sensing data, including surface thermal inertia and mineralogy, to investigate many aspects of planetary science. This article provides an overview of this burgeoning area of remote sensing. The origin of gamma rays and neutrons, their information content, measurement techniques, and scientific results from the Lunar Prospector and Mars Odyssey missions are described. Nuclear reactions and radioactive decay result in the emission of gamma rays with discrete energies, which provide a fingerprint that can uniquely identify specific elements in the surface. Depending on the composition of the surface, the abundance of major rock-forming elements, such as O, Mg, Al, Si, Cl, Ca, Ti, Fe, and radioactive trace elements, such as K, Th, and U can be determined from measurements of the gamma ray spectrum. The geochemical data provided by nuclear spectroscopy can be used to investigate a wide range of topics, including the following: • Determining bulk composition for comparative studies of planetary geochemistry and the investigation of theories of planetary origins and evolution; • Constraining planetary structure and differentiation processes by measuring large-scale stratigraphic variations within impact basins that probe the crust and mantle; • Characterization of regional scale geological units, such as lunar mare and highlands; • Estimating the global heat balance by measuring the abundance of radioisotopes such as K, Th, and U;