Updated Mapping of the Hydrogen Distribution in the Lunar Polar Regions

It is well known that methods of nuclear physics allow one to study distribution of hydrogen-bearing compounds in the upper 1–2 m subsurface soil layer of atmosphereless celestial bodies or planets with thin atmospheres like Mars by measuring neutron spectra leak from the surface. For this study one needs not only to measure neutron spectra but to perform also a set of numerical simulations of the neutron production by the Galactic Cosmic Rays (GCRs) in subsurface soil, leakage of these neutrons from the surface, their transport to the neutron spectrometer on the orbit and processes of neutron interactions with the instrument’s detectors. These simulations make possible a model dependent deconvolution of the measured data to obtain the hydrogen concentration and/or other soil properties at a particular region of the planet.

Currently a number of numerical codes are being used for simulations of the neutron production and transport in planetary applications. All these codes provide a reasonable precision both in modeling of laboratory experiments and nuclear planetology tasks. However, the gravitational field description appropriate for simulation of a neutron propagation on planetary scales is not well addressed. For the planetary scales, it is not just enough to implement a uniform gravitational field (this option is available in some numerical codes). The planetary gravity should be described as a full-scale central force field with its potential depending from the distance from the center of planet.

We have developed a method of accounting effects of lunar gravity force and finite neutron lifetime on the spectral and angular distributions of neutron flux at different altitudes above Moon surface. This method was implemented to reprocess the data gathered by the collimated detectors of LEND instrument operated onboard NASA LRO spacecraft. The gravitational field description appropriate for simulation of a neutron propagation on planetary scales was not well addressed earlier.

As the result of the updated LEND data reprocessing with the discussed method, we obtained a new estimations of Water Equivalent Hydrogen (WEH) abundance in the lunar regolith and new maps of WEH distribution in the lunar polar regions. It is shown that difference of new derived values of WEH is about 0.08 wt% larger in comparison with the previously estimated value. The updated polar maps shows slightly different WEH distribution over the polar regions in comparison to the early published. The new polar maps will be used to select the landing sites of future landers.