Abstract
Water is a scarce commodity on Mars, yet large amounts of water are needed for crew life support, and in most human mission scenarios, even greater amounts of water are needed to produce propellants for departing Mars for the return trip to Earth. Evidence suggests that significant amounts of water occur as mineral hydration of magnesium or calcium sulfates in the accessible upper layer of Mars regolith at various scattered equatorial locations. Several sulfates occur on Mars with water content 20% to 50% of the sulfate mass.
Several forms of hydrated sulfates are known to provide a significant share of observed water-equivalent hydrogen in the upper meter of Mars regolith. These include “Gypsum” (CaSO₄⋅2H₂O) containing 20.9% H2O by weight, and “Epsomite” (MgSO4·7H2O) (commonly known as “epsom salts”) containing 51% H2O by weight, as well as hexahydrite (MgSO4·6H2O) and starkeyite (MgSO4·4H2O).
Scans using the neutron spectrometer from orbit show a remarkable correlation between the occurrence of S and H in the equatorial region, indicating that hydrated sulfates are a primary source of H2O there. Recent higher-resolution scans using the collimated neutron spectrometer show significant pockets of higher H2O content. This implies that even higher local concentrations of H2O almost surely exist within those areas, most likely as hydrated sulfates.
The power requirements to evolve H2O from a range of potential hydrated magnesium sulfates are moderate. We suggest that a human mission to Mars at equatorial latitudes based on hydrated sulfates as a source of water is a potential competitor to a mission at higher latitudes based on putative accessible ice.
![Comparison of relative H and S concentrations in the upper ~1 m of Mars regolith as observed by the gamma ray spectrometer on the Odyssey spacecraft [10]. The pixels in both cases were about 600 km x 600 km.](https://www.igminresearch.com/articles/figures/igmin363/igmin363.g001.png)
![Estimated WEH % at a local area on Mars by reanalysis of Mars Odyssey neutron spectrometer data [21].](https://www.igminresearch.com/articles/figures/igmin363/igmin363.g002.png)
![Estimated WEH % at a local area on Mars by reanalysis of Mars Odyssey neutron spectrometer data [21].](https://www.igminresearch.com/articles/figures/igmin363/igmin363.g003.png)
![Water equivalent hydrogen maps, measured by collimated FREND/DSEN. (a) Upper map is collimated for maximum resolution (200 km). (b) Lower map is omnidirectional (550 km). Black and white isolines correspond to WEH values. Adapted from Figure 2 of Malakahov et al. (2022) [22].](https://www.igminresearch.com/articles/figures/igmin363/igmin363.g004.png)
![Enhanced segments of the global map shown in Figure 4, showing details of two areas with the most water content: Arabia Terra (top left), Medusa Fossae (top right). Adapted from Figure 3 of Malakahov et al. (2022) [22].](https://www.igminresearch.com/articles/figures/igmin363/igmin363.g005.png)
![Close-up chart of the highest-resolution view of a local area on Mars with unusually high WEH. The contour lines are lines of constant WEH [22].](https://www.igminresearch.com/articles/figures/igmin363/igmin363.g006.png)
![Close-up chart of the highest-resolution view of a local area on Mars with unusually high WEH. The contour lines are lines of constant WEH [22].](https://www.igminresearch.com/articles/figures/igmin363/igmin363.g007.png)
