Determination of the elemental composition of martian rocks from Phobos 2

PREVIOUS orbiter and lander pictures have given an indication of the general morphology of the surface of Mars and of some relatively small features on the surface. Here we have determined the elemental composition of martian soil from Phobos 2 using orbital γ-ray spectrometry. This technique is used to analyse the surfaces of planets which either do not have an atmosphere or which have only a low-density atmosphere (that is, there is negligible absorption of γ-rays). The γ-radiation is caused by the decay of radioactive elements excited by the interaction of solar (SCR) and galactic (GCR) cosmic rays with the surface materials. Measurements of this radiation by a -γray spectrometer enable the elemental composition of the near-surface rocks to be determined. The first successful measurements of γ-radiation were conducted from Luna 10 (ref. 1). The method was used subsequently during studies of the Moon from Apollos 15 and 16 (ref. 2), and Mars 5 (ref. 3).