Hosted by UK Centre for Astrobiology and Centre for Science at Extreme Conditions
When galactic cosmic rays strike the surface of Mars, they generate neutrons from nuclei present in the top several metres of the surface materials by various nuclear reactions. The neutrons lose energy by interacting with the surrounding nuclei and their flux leaking from the planetary subsurface is therefore an indication of the elemental composition of the top layer of the surface materials. Macroscopic scattering and capture cross sections is also dependent on soil and rock composition.
For instance, hydrogen is especially effective at moderating neutrons because its mass is nearly the same as the mass of the neutron. Thus, hydrogen rich surface materials create a deficit of epithermal neutrons, and consequently the leakage flux of epithermal neutrons from the subsurface is reduced in places where hydrogen concentration is significant.
The Mars Odyssey mission carries a collection of three instruments whose main aim is to determine the elemental composition of the top layers of the martian surface materials. Among them, the Neutron Spectrometer has produced a wealth of data that has allowed a comprehensive study of the overall distribution of hydrogen on the surface of Mars. In brief, deposits ranging between 20% and 100% Water-Equivalent Hydrogen (WEH) by mass are found poleward of _55_latitude, and less rich, but still significant, deposits are found at near-equatorial latitudes. However, the Mars Odyssey Neutron Spectrometer (MONS) has a FWHM of _ 550 km. Hence, if one wants to associate WEH with mineralogy observed independently, then instrumental smearing needs to be properly understood and removed. We have found that in the presence of noise, this problem necessitates the application of a statistical approach: Speedy Pixons.
An exciting prospect is to obtain more accurate WEH for certain locales where hydrous minerals have been found. This can, perhaps, help to constrain the real extent or the original volume of surface water needed to create evaporated deposits or other sedimentary units. Another interesting potential development is the study of the distribution of subsurface water ice at lower latitudes. Although water ice is not stable at such latitudes, recent impact craters have exposed buried deposits of nearly pure water ice at around 45_ latitude.
I will present the results of applying a pixon image reconstruction approach to the Mars Odyssey Epithermal Neutron Data. Some of our preliminary results identify large quantities of hydrogen in the top metre of the surface materials in both polar neighbourhoods.