Observation of the γ-ray emission from the martian surface by the APEX experiment

THE study of γ-ray emission from planets with a thin atmosphere allows a rough estimation of the chemical composition of their surface to be made. The first attempt to make orbital measurements of the -γ-ray flux from Mars was made by the Soviet spacecraft Mars 5 (ref. 1). Here we present results from the -γ-ray APEX experiment on board Phobos 2. We have measured the y-ray albedo of the martian surface. This albedo appears as a large continuum between a few hundred keV and 8 MeV corresponding to a source intensity of 5 photons cm^-2 s^-1 2Φ^-l (estimated in the 150-1500 keV energy range). There is also evidence for a 511-keV annihilation emission and for a broad contribution of probable nuclear lines between 4 and 7 MeV.