Optimizing the spatial resolution of Martian hydrogen abundance maps derived from neutron spectrometer data

Hydrogen abundance, as calculated by proxy from Mars Odyssey Neutron Spectrometer epithermal neutron counts, is used to determine areas where water may have once existed on Mars. Epithermal neutrons, excited byproducts of cosmic ray bombardment, do not attain escape velocity in the presence of hydrogen atoms. The mean of multiple measurements from successive satellite orbits provides an approximation of the true epithermal neutron at a specific location. Spatial resolution of the hydrogen abundance raster is therefore limited by the size of bin used to average these counts, with the lower limit on bin size constrained by the number of counts per bin. This study uses a moving window, or boxcar function, to sample the epithermal neutron counts at prescribed intervals across the mapping area. The interval, or node spacing, determines the number of pixels per degree in the output raster. Window dimensions determine the number of counts at each node. Unlike binning, where each bin is discrete and individual measurements are used only once, moving window functions overlap allowing each measurement to be averaged at multiple nodes. By using circular windows with radii of 1° and 2° and a nodal spacing of 0.025° the spatial resolution of the raster increased from >1 pixel per degree (ppd) to 40 ppd. We conclude that geographic information system routines can be used to remove perceived limitations on the spatial resolution of epithermal neutron and hydrogen abundance rasters, producing 40 ppd maps depicting the past distribution of water on Mars.

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