Statistics
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufm.p12c..07g&link_type=abstract
American Geophysical Union, Fall Meeting 2003, abstract #P12C-07
Statistics
1020 Composition Of The Crust, 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 5410 Composition, 5464 Remote Sensing, 6225 Mars
Scientific paper
This study intends to identify and characterize the major provinces at the surface of Mars. We used the data from the Neutron Spectrometer (NS) and the Gamma-ray Sensor Head (GSH) aboard Mars Odyssey. NS data help to define broad provinces that should present uniform composition. GSH data can be then summed over these provinces to derive their chemical compositions with good statistics. At the present stage, we manage to conduct the first step that identifies the provinces, while the second step is still under progress to characterize them. Variations in NS epithermal and fast neutron data are largely dominated by the distribution of hydrogen and carbon at the surface or at shallow depths. Neutrons are also affected by the presence of an atmosphere. To limit these effects, we chose frost free data (i.e. measured during the summer at high-latitudes), between -60 and +60 degrees latitude, and corrected for atmospheric thickness (normalization to 16 g/cm2). On the other hand, it has been demonstrated that neutron fluxes can be used as a proxy for composition: thermal neutrons are strongly attenuated by absorbing elements such as iron, titanium, chlorine, gadolinium, and samarium; on the contrary, the fast neutron flux increases in presence of high-atomic mass elements such as iron or titanium. For a dry and airless body it is known that the ratio of epithermal over fast neutron counts is correlated to the ratio of thermal over fast neutron counts. This correlation leads to a unique parameter, which characterizes the various soil compositions. A similar behaviour has been observed within our subset of NS data. Effects of hydrogen and carbon are quite obvious, allowing us to focus on secondary components, which are driven by soil composition. From there, we derived four provinces besides the poles. The first province is made of H-rich equatorial regions (Arabia Terra and south of Lucus Planum). The second and third provinces are found north and south of the crustal dichotomy corresponding to lowlands and highlands, respectively. In particular the second province includes Acidalia Planitia and Utopia Planitia (Amazonis Planitia is an exception as it falls into the third province). Finally the fourth province covers the highest terranes (above 5-6 km): Tharsis Montes, Olympus Mons and Alba Patera. One possibility to explain these distinctive neutron signatures can be variations of iron and chlorine contents in the soil. Indeed, iron and chlorine are the neutron absorbers having the most significant abundance on Mars. Preliminary GSH analysis reveals that the second province (lowlands) is richer in iron, as well as in potassium and thorium, then the other provinces. The fourth province (high elevations) is richer in chlorine than the others [see J. Keller abstract, same session]. This analysis is still under progress, and more datasets may be included as they will become available.
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