Water on Mars

Abstract

A milestone in the understanding of the Martian hydrological evolution and water cycle is being marked by the Gamma-Ray Spectrometer (GRS) onboard Mars Odyssey ([1], [2], [4]). These gamma rays are supposed to be produced during inelastic collision and/or capture of secondary neutrons which were produced in the soil by cosmic rays. The signals are indicative of the presence of a large amount (at least 35 wt %) of hydrogen below a dry layer in this region. The Neutron Spectrometer [2] and the High Energy Neutron Detector (HEND) [4] measured leakage fluxes of fast, epithermal and thermal neutrons produced in the same way. As hydrogen has a strong ability to slow down the neutrons, depression of neutron fluxes, especially of faster ones, can be interpreted by a large abundance of hydrogen and hence H₂O. Both instruments detected a large region around the south pole southward of 60°S (with an area of 10⁷ km²) with strongly reduced epithermal fluxes indicative of large water abundance. In general the upper 15–20 cm of the soil at low and mid latitudes was inferred to be water-poor dust layer underlain by a slightly water-richer regolith [2]. The global map of fast neutrons (indicative of the upper layer) showed a somewhat different picture, particularly those measured by HEND. The area with a large deficit of fluxes was larger at northern high latitudes (10⁷ km²) than at southern latitudes (4 x 10⁶ km²). In all maps the global distribution is not symmetric about the equator and some distinct longitudinal variation is also discernible.