The First Map of Mercury's Northern Hemisphere Near Subsurface Hydrogen from MESSENGER GRNS Data

Since the early 1990s a combination of Earth-based radar return and near-surface modelling has implied the presence of water ice in the permanently shadowed craters at Mercury's poles. Data returned by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft confirmed this decades-long hypothesis. Data from the Mercury Dual Imaging System and the Mercury Laser Altimeter has allowed improved mapping of the permanently shaded regions, which has made more convincing the overlap between the PSRs with the high radar backscatter regions. At the north pole Gamma-Ray and Neutron Spectrometer (GRNS) data revealed the presence of a hydrogen rich layer beneath a 10-20 cm thick layer that is less rich in hydrogen. However, in previously published work the GRNS-derived hydrogen distribution was available only as a latitudinal profile in the northern hemisphere. Here, we will present the results of a new analysis of the epithermal neutron data from the GRNS, which has enabled the production of a spatially resolved map of near-subsurface hydrogen in Mercury's northern hemisphere. The map, in addition to the hydrogen at the pole, reveals a correlation between the locations of Mercury's cold poles and enhancements in hydrogen. Combining the gamma-ray data from the GRNS with particle transport codes enables the absolute abundances of the hydrogen distribution to be determined. Additionally, an independent spatially resolved analysis of the GRNS gamma ray data confirms the association between Mercury's cold poles and enhanced hydrogen abundance.