Statistics
Scientific paper
May 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agusm.p41a..09j&link_type=abstract
American Geophysical Union, Spring Meeting 2007, abstract #P41A-09
Statistics
1027 Composition Of The Planets, 6225 Mars
Scientific paper
The Gamma Subsystem (GS) of the Gamma Ray Spectrometer (GRS) suite of instruments on board the 2001 Mars Odyssey spacecraft has detected the signature of hydrogen on Mars. This signature is the 2223 MeV gamma ray given off by hydrogen when its nucleus is excited by capture of a thermal neutron. The number of such gamma rays emitted is not only a function of the concentration of hydrogen, but also of the number of cosmic rays, the atmospheric composition and thickness, and the composition of the martian regolith, all of which are involved in producing and moderating the thermal neutrons responsible for raising the hydrogen nucleus to an excited state. Fortunately, we are able to take these parameters into account and normalize our results since silicon produces gamma rays by both inelastic scatter and thermal capture, is relatively evenly distributed, and has been measured in situ. We report the results of this detection as the equivalent wt% of water in the martian regolith for the equatorial region (~±45°). Values range from a low of 1.5 (± 0.3) up to 7.5 (± 0.6) wt%. Results assume that any water is uniformly distributed within the top meter or so of the surface, the depth to which gamma ray spectrometry is sensitive. Uncertainties in the data are seen to be relatively small, on the order of ±10% relative, and are dominated by counting statistics. Thus the regions of greatest uncertainty are those areas where the atmospheric thickness is greatest, e.g. Hellas Basin, and are well known. Our results for water concentration are generally lower than those reported by the Neutron Spectrometer (NS) aboard the same spacecraft. It has been suggested that variation in water content with depth, having a wetter layer buried below a dryer surface layer, might account for this discrepancy. However, such a distribution would result in our measured gamma ray flux returning a lower derived value for the concentration, exacerbating the problem. Hydrogen is the most variable of any of the elements that the GS has mapped to date, ranging over a factor of 5. High concentrations of water occur in Arabia Patera and Apollinaris Patera. The distribution of water generally correlates with that of chlorine particularly within Apollinaris Patera and to a lesser extent in Arabia Terra. There are no significant correlations of water with the other elements GS has mapped to date: iron, silicon, potassium and thorium.
Janes Daniel M.
Team G.
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