High Resolution Map of Water in the Martian Regolith Observed by FREND Neutron Telescope Onboard ExoMars TGO

Studies of water content in the Martian subsurface by means of neutron spectroscopy is a well‐known technique, very sensitive to water abundance. The FREND instrument onboard the ExoMars TGO is the latest experiment of that kind. Its major characteristic and advantage compared to the predecessors is its capability of high spatial resolution measurements – massive collimator shields FREND's detectors, allowing for a narrow field of view. In this study we present a global map of Water Equivalent Hydrogen (WEH) in the upper meter of the Martian subsurface, which is built based on the FREND data. We show that it contains more local features and reveals more structure than analogous maps from omnidirectional experiments, available previously. Further analysis shows that local water‐rich regions can be located through the analysis of the FREND data, some of these containing about 20 wt% of WEH – a very unusual amount for regions at moderate latitudes, where free water and water ice are thought to be unstable in the shallow subsurface. Plain Language Summary Neutron spectroscopy is a well‐known technique to study water content in the upper meter of a celestial body. The FREND instrument onboard ExoMars TGO is the latest experiment of that kind. Its major characteristic is its collimator that shields detectors within a narrow field of view, thus allowing to create maps with with high spatial resolution. This study presents the global map of water content in the Martian soil, down to one m of the subsurface, based on the FREND data. This map shows a number of details that were not observed previously with omnidirectional neutron spectrometers. Further analysis reveals locations with increased water abundance, some of which contain about 20 wt% of water equivalent hydrogen. This amount is very unusual for moderate latitudes of Mars, where free water and water ice are usually unstable. Key Points FREND is a high spatial resolution neutron telescope mapping water content in the Martian subsurface onboard ESA's ExoMars Trace Gas Orbiter Global map of water content is presented for the equatorial region between latitudes 50{degree sign} north and south Some local areas with unusually high water content, up to 20 weight percent are located and discussed