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

Abstract

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

Unveiling the Subsurface of Late Amazonian Lava Flows at Echus Chasma, on Mars

“…Permittivity values from the Moon can also be used as a reference to provide an estimate of the density, as the lunar layers are typically dry, and have been compared with measurements of samples brought back to Earth [39]. According to data derived from the FREND workgroup [40], the water content of the shallowest subsurface is apparently low, which allows us to handle this material as dry volcanic rocks.…”

Evaluating Atmospheric and Surface Drivers for O2 Variations at Gale Crater as Observed by MSL SAM

“…Brine stability modeling by Chevrier et al (2020) found subsurface brines to mostly be frozen at depths greater than the annual thermal skin depth of ∼1 m. Brines with low water activities (such as the Mg/Ca-perchlorate brines) can remain liquid and evaporation can occur, but evaporation rates are below 1 mm per Martian year, too low to contribute significant O 2 fluxes into the atmosphere. Within the top few meters of the surface, neutron measurements by the Mars Odyssey Neutron Spectrometer (MONS; Feldman et al 2004;Wilson et al 2018), the Curiosity Dynamic Albedo of Neutrons (DAN) (Mitrofanov et al 2014), and the Trace Gas Orbiter (TGO) Fine-Resolution Epithermal Neutron Detector (FREND; Malakhov et al 2022) also found less than 10 wt% water equivalent hydrogen (WEH) in the Gale crater region. With such low wt% WEH values, the volume of water contained in the top meter of the surface would be an order of magnitude too small to dissolve the required amount of O 2 .…”

Scientific Instrumentation Complex for the ExoMars-2022 Landing Platform

“…The information on water distribution can be obtained from radar sounding data, but outside the polar ice caps, in low and middle latitudes, the neutron monitoring data serve as a main source. Long-term orbital measurements of the neutron flux induced by cosmic radiation showed the presence of hydrogen nuclei in the upper (1-2 m) layers of regolith, corresponding to a mass fraction of water from units of percent to 20-40% (see, e.g., Malakhov et al, 2022). To detect ice at equatorial latitudes is possible only in deep canyons (Mitrofanov et al, 2022a).…”