Gamma‐ray and neutron spectroscopy of planetary surfaces and atmospheres

The neutrons and gamma rays escaping from a planet can be used to map the concentrations of various elements in its surface. In a planet, the high‐energy particles in the galactic cosmic rays induce a cascade of particles that includes many neutrons. The γ rays are made by the decay of the naturally‐occurring radioelements and by nuclear excitations induced by cosmic‐ray particles and their secondaries (especially neutron‐capture or inelastic‐scattering reactions). After a short history of planetary γ‐ray and neutron spectroscopy, the γ‐ray spectrometer and active neutron detection system planned for the Mars Observer Mission are presented. The results of laboratory experiments that simulate the cosmic‐ray bombardments of planetary surfaces and the status of the theoretical calculations for the processes that make and transport neutrons and γ rays will be reviewed. Studies of Mars, including its atmosphere, are emphasized, as are new ideas, concepts, and problems that have arisen over the last decade, suc...