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Introduction Measuring Water Content in Marstian Regolith Through Neutron FluxWater in the upper meter of Martian regolith is found practically everywhere on the planet using remote neutron and gamma ray sensing methods. It is most abundant in the polar regions above 60° latitude, where its content measured as Water Equivalent Hydrogen is higher than 40 wt% (WEH is a measure of water in weight percent (wt%) that the subsurface material contains in case all hydrogen is attributed to water molecules, H 2 O). Such large amounts of WEH most probably mean presence of subsurface water ice. On the other hand, moderate latitudes between 40° north and south only show several wt% WEH (Boynton et al., 2002;Feldman et al., 2002;Mitrofanov et al., 2002). In this case hydrogen is thought to be attributed to either adsorbed water or water chemically bound in hydrated minerals. Some studies (e.g., Byrne et al., 2009) suggested that water ice might be present in the subsurface even in some equatorial areas with favorable conditions. In any case, water content mapping is of scientific interest, providing insights to hydrologic history of Mars. Moreover, its present conditions are an invaluable resource for planning future robotic and human exploration.The mapping of water mentioned above was carried out through neutron and gamma ray sensing instruments. This technique is widely known and was performed on Martian orbit by three instruments in the past (Boynton et al., 2002;Feldman et al., 2002;Mitrofanov et al., 2002). It was described in details, for example, in (Drake et al., 1988;Masarik & Reedy, 1996). Galactic Cosmic Rays (GCRs) bombard the surface of a celestial body with no or thin atmosphere, penetrating its regolith to a depth of about 2 m. As a result, particles of GCRs collide with the soil constituting nuclei and produce fast neutrons, some of which leak back toward the surface and can be detected by an orbital or lander instrument. As neutrons leak, they are moderated (loose energy

<p>Fine Resolution Epithermal Neutron Detector (FREND) is an instrument onboard ExoMars' Trace Gas Orbiter. Its measurements of epithermal neutron flux on orbit provide data on hydrogen (and thus, water) content in the 1-m thick near-surface regolith layer. Similar experiments have been performed before, neutron sounding is a well-established technique for estimating water content in the celesital body's soil. FREND's chatacteristic feature is its collimator - a massive body surrounding detectors and narrowing their field of view substantially, thus providing for very high spatial resolution, around 60 to 200 km, depending on measurement conditions. Such spatial resolution allows identifying local water-rich features with relief and other geomorphological features, assess water content in small ellipses of future landing sites.</p> <p>In this study we present latest findings based on FREND data, containing a number of surprisingly "wet" local features in the equatorial band. Water or water ice is not stable at the surface of Mars, in the equatorial regions especially, that is why locating areas with enhanced subsurface hydrogen or water is of much interest both to scientists and for the purpose of planning future exploration missions. </p>

<p>Fine Resolution Epithermal Neutron Detector, FREND, is an instrument onboard ExoMars’ Trace Gas Orbiter (TGO). It uses neutron measurements to detect hydrogen (and thus water) variations in the shallow subsurface of the Martian soil. Similar experiments have been performed in the past on Mars, but FREND’s main characteristic is its neutron collimator that significantly narrows down the field of view (FOV) to 28° full cone which corresponds to a 60-200 km diameter spot on the surface. This is considerably smaller than the spatial resolution of previous experiments and thus allows us to peek inside local features of hydrogen variations.</p><p>The instrument has been measuring for almost one full Martian year currently so what we present is a result of continuous observations of shallow subsurface water between May 2018 and present. A technique to locate the most prominent local spots, either very “dry” or very “wet”, was developed to analyze the planetary surface from 70° North down to 70° South. It yielded several such local spots of interest that are presented, characterized and associated with particular geomorphological features or/and with the selected landing sites candidates.</p><p>It is known that water or water ice is not stable at the surface of Mars, especially closer to equator, thus locating areas with enhanced subsurface hydrogen or water is of much interest both scientifically and in terms of future exploration. FREND is most sensitive to water in the shallow subsurface of about 1 m deep, which makes such deposits easily accessible and valuable.</p>