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Mineral Hydration as a Source of Accessible Water on Mars to Enable Human Missions at Equatorial Sites

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29 August 2026

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31 August 2026

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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 larger amounts of water are needed to produce propellants for departing Mars for the return trip to Earth. There is evidence that significant amounts of water occur as mineral hydration of magnesium sulfates in the accessible upper layer of Mars regolith at various scattered equatorial locations. Several such magnesium sulfates occur on Mars with water content 20% to 50% of the sulfate mass. Several forms of hydrated MgSO4 are known to provide a significant share of observed water-equivalent hydrogen in the upper meter of Mars regolith. These include “Gypsum” (MgSO4⋅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 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. 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 at least as attractive as a mission to higher latitudes based on putative accessible ice.
Keywords: 
Mars
;  
ISRU
;  
water on Mars
;  
human missions to Mars
;  
hydrated sulfates
;  
Mars landing sites
Subject: 
Engineering  -   Aerospace Engineering

Introduction

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 larger amounts of water are needed to produce propellants for departing Mars for the return trip to Earth. [1,2,3] With observations of Mars from orbit over the past decade or two, optimism has developed in the Mars mission community that accessible water-ice is likely to be accessible at latitudes of about 40°N or higher, and planning for future human missions is ongoing for landing sites near 40°N. [4,5,6] While an equatorial landing is preferred for several reasons, the possibility of near-surface water-ice at higher latitudes swayed NASA planners to accept a higher latitude landing site as the consequence for (putative) accessible water. [4,5,6] Alternatively, Rapp (2025) proposed a simplified human mission to Mars that does not require indigenous Mars water that might suffice for the first landings. [3] Nevertheless, for future, larger scale Mars landings, indigenous sources of water on Mars will be required.
It is widely known that some of the minerals in the Martian regolith include water of hydration within their crystal structures, and these minerals are widely distributed across the surface of Mars. Indeed, the calibration of neutron spectrometers for H detection is based on comparison of any given pixel to a pixel in the background assumed to contain 2% water by weight, at least partly due to hydrated minerals spread across the background pixel. [7,8] Since these pixels are very large (typically 600 km x 600 km) the fact that the average water percentage is 2% across 600 km x 600 km does not preclude existence of smaller local outcrops of far more concentrated hydrated minerals where the H2O percentage might be 20% by weight or higher. Such as-yet hypothetical sources of H2O at equatorial latitudes might be even more accessible and exploitable than the putative water-ice resources at higher latitudes suggested by orbital observations. If they prove to be existent as suggested by recent observations with a collimated neutron spectrometer from orbit, that might provide preference for landing at equatorial sites. Therefore, we suggest that the rush to define missions at higher latitude might be premature until the availability of mineral hydrates at equatorial latitudes is clarified.
Gypsum is a common mineral on Mars with formula MgSO₄⋅2H₂O, containing 20.9% H2O by weight. [9] When exposed to very low humidity at the Martian surface, it can slowly lose water of hydration and transform to Bassanite MgSO₄⋅0.5H₂O. Epsomite (MgSO4·7H2O) (commonly known as “epsom salts”), hexahydrite (MgSO4·6H2O) and starkeyite (MgSO4·4H2O) provide even higher percentages of H2O than Gypsum.
There is not much pragmatic interest in occurrence of sulfates at latitudes greater than about 40°N because accessible ice is thought to occur there. But a good source of hydrated sulfates at equatorial latitudes would be a valuable alternative, allowing a more favorable equatorial landing site, and water extraction at moderate temperatures.
Figure 1 shows a comparison of observed relative concentrations of S and H in the top ~1 m of Mars regolith by the Odyssey gamma ray spectrometer. [10] The strong correlation between the S and H distributions in the near surface strongly suggests that the three red/yellow areas of higher H2O concentration in the equatorial zone are mainly due to sulfates, and probably dominated by magnesium sulfates.
Because the pixels are so large (600 km x 600 km), the distribution of H within each pixel might range from being broadly uniform to highly non-uniform, with local pockets of highly concentrated mineral hydrates. For example, a pixel with an average over the pixel of say, 3% H, might contain anywhere between (a) 100% of area with 3% H vs. (b) 10% of area with 30% H, or perhaps (c) 95% of area with 2% H and 5% of area with 22% H. Any other combination is imaginable.

Sulfates on Mars

Several papers reported existence of sulfates in the Mars surface at various locations, and others discussed the stability of sulfates on Mars, but they were mainly interested in the evolutionary history of water on Mars, rather than the utilitarian merit of sulfates as sources of water for missions. One of the many articles was:
Flahaut et al. (2015) reported: “sulfate-rich outcrops of the Burns Formation in Meridiani Planum”, (between 3.5°S to 6.5°N latitude and 8.0°W to 8.0°E longitude) and “Sulfates and clays are detected across the entire etched terrains and hematite plains of Meridiani Planum. Their widespread distribution suggests the following regional stratigraphy: a sulfate-rich bottom unit (kieserite + PHS), an intermediate clay-enriched unit (Fe and Al-rich smectites), a topmost sulfate-rich unit (PHS + jarosite, gypsum, hematite).” [11]
Hynek et al. (2019) reported extensive sulfate deposits that presumably carry with them water of hydration. [12]
David, et al. (2022) demonstrated that sulfates are the main contributor to the water content of soils and are likely to be the source of the hydrogen and sulfur measured from orbit. [13]
Feldman et. al (2004) pointed out that significant deposits of hydrogen have been observed within large areas near the equator of Mars, and equilibrium analysis suggested that water ice would not be stable, so the question was how did this hydrogen occur? [14] They decided to investigate hydrated MgSO4 as a source of equatorial hydrogen because large endowments of hydrated MgSO4 had been identified at some equatorial locations in previous work. They concluded that Epsomite (MgSO₄⋅7H₂O) (also known as “Epson salts”) “appears to be either stable or close to stability at all locations where water equivalent hydrogen (WEH) is abundant at equatorial latitudes”.
Chou and Seal (2007) concluded that magnesium sulfate probably plays a dominant role in the water cycle of Mars away from the polar ice caps due to its abundance, its occurrence in numerous hydration states, and its ability to hydrate and dehydrate rapidly. [15] New experimental studies on the metastable reaction between hexahydrite (MgSO4·6H2O) and starkeyite (MgSO4·4H2O) as a function of temperature and relative humidity, supplemented by recent investigations of the stable reaction between epsomite (MgSO4·7H2O) and hexahydrite and by phase equilibrium calculations, suggest that the most important magnesium sulfate phases involved in the Martian water cycle are MgSO4·11H2O, epsomite, starkeyite, and possibly kieserite (MgSO4·H2O).
Vaniman et al. (2004) suggested a prominent role for Mg sulfates in accounting for equatorial H2O. [16] They reported the exposure of several MgSO4·nH2O phases to varied temperature, pressure and humidity to constrain their possible H2O contents under Martian surface conditions. Magnesium sulfate salts can retain sufficient H2O to explain at least a portion of the surface hydrogen observed from orbit.
Clarke et al. (2008) were early advocates of using sulfates on Mars as a source of water for ISRU. [17] Regolith with water content 5-20% are well within the capabilities of small excavators. They suggested use of an area in South Australia (the largest known terrestrial occurrence of magnesium sulfates (mainly epsomite) as an ideal location for full scale ISRU trials of water extraction from hydrated minerals.
The so-called “MWIP study” developed scenarios for processing water from hydrated minerals. [18,19] Of particular interest was the assumed scenario where 40% of the regolith consists of gypsum (containing 21% H2O by weight) for which H2O would represent 8% of the regolith mass, with dehydration at 150°C.
Marion and Kargel (2005) claimed: “At Meridiani Planum, MgSO4 salts constitute 15 to 40 wt.% of sedimentary rocks. [20] Additional S is hosted by gypsum and jarosite. Reflectance and thermal emission spectroscopy are consistent with the presence of kieserite MgSO4·4H2O and epsomite MgSO4·7H2O. Theoretically, the dodecahydrate MgSO4·12H2O should also have precipitated.” They explored the stability of various MgSO4 hydrates on Mars and found that various hydrates are stable when water vapor is present, which might occur with buried ice well below the surface. In absence of water vapor, higher hydrates lose H2O and transform to lower hydrates.

Evidence from Highest Resolution Neutron Spectroscopy

The FREND instrument (Fine Resolution Epithermal Neutron Detector) onboard the Trace Gas Orbiter (TGO) of the Russian European ExoMars mission included a neutron collimator – a passive system significantly limiting the instrument's field of view (FOV) allowing higher resolution but requiring longer integration times. The results of higher resolution observations of wt % water equivalent hydrogen (WEH) in the top ~ 1 m of regolith were reported by Malakhov et al. (2022). [21] The observations of WEH at equatorial latitudes are most likely to be due to mineral hydrates, of which Magnesium Sulfates are the most likely candidates.
Figure 2 provides a comparison of the higher resolution map (200 km) to the omnidirectional map (~600 km) previously reported in the original neutron spectroscopy data. The hi-resolution and low-resolution maps correlate very well, and it can easily be visualized how the low-resolution map provides greater detail when the resolution is increased. It is notable that throughout the equatorial range, the highest value of WEH in the higher resolution plot is 8% with local inferences of 12%. The highest WEH in the lower resolution plot is 6% with local inferences of 8%. This shows that smaller local areas with higher WEH become diffused at lower resolution because they are averaged with areas of lower WEH. Therefore, we can expect that as the pixel size is further reduced from 200 km, the 8% – 12% areas in the high-resolution plot will show smaller areas with even higher WEH.
Malakahov et al. (2022) selected two areas in Figure 2 to focus on, and the WEH contours for these two areas are shown in Figure 3. Figure 3 shows that in Arabia Terra, local areas with WEH ≥ 10% occur at 5°N latitude, and at higher resolution, the WEH values will grow. For Medusa Fossae, values of WEH ≥ 12% occur at 12° to 15°S latitude, and at higher resolution, the WEH values will grow.
Malakahov et al. (2022) noted several local equatorial areas with greater than average WEH. They focused in on these areas and somehow achieved higher resolution. They presented two maps of WEH in local areas in the equatorial zone with much greater than average WEH that we have modified slightly for clarity. (See far left of upper figure in Figure 2). These are shown as Figure 4 and Figure 5. It seems likely that the WEH in these maps is mainly due to hydrated sulfates.
They also noted that these areas of unusually high WEH tend to follow the terrain and occur at low elevation compared to surrounding areas.

Processing

An important factor in evaluating the feasibility of using sulfate hydrates as a source of water for Mars missions is the power required to process the regolith containing the sulfate hydrate. Of course, the power required to extricate regolith, deliver it to the reactor, and remove waste regolith must also be included. But here, we only deal with power for processing.
We consider seven possible combinations of starting hydrated sulfates and various numbers of H2O molecules removed by processing. We use the notation x-y-z where x is the starting hydration number of H2O molecules in Magnesium Sulfate, y is the final hydration number of H2O molecules in Magnesium Sulfate, and z = (x – y) is the number of H2O molecules removed per molecule of hydrated sulfate. Thus, we consider the following processes:
7-6-1: Starting with MgSO4·7H2O and processing to MgSO4·6H2O losing 1 H2O
7-2-5: Starting with MgSO4·7H2O and processing to MgSO4·2H2O losing 5 H2O
7-1-6: Starting with MgSO4·7H2O and processing to MgSO4·1H2O losing 6 H2O
6-2-4: Starting with MgSO4·6H2O and processing to MgSO4·2H2O losing 4 H2O
6-1-5: Starting with MgSO4·6H2O and processing to MgSO4·1H2O losing 5 H2O
2-1-1: Starting with MgSO4·2H2O and processing to MgSO4·1H2O losing 1 H2O
1-0-1: Starting with MgSO4·1H2O and processing to MgSO4 losing 1 H2O
We begin by setting the energy required to derive water from pure hydrated sulfates as the sum of the energy required to warm from Martian temperature to the dehydration temperature, plus the energy for dehydration. For expediency, the average Mars temperature is assumed to be –70°C. The estimated actual energy to warm the sulfate is reduced to 40% of the theoretical energy, assuming 60% heat recuperation by using spent sulfate to warm incoming sulfate. Then, additional energy is allocated to warm regolith for any percentage of sulfate in the regolith.
Table 1. Basic Data.
Table 1. Basic Data.
1 2 3 4 5 6 7 8 9
Start
dot-H2O 		End
dot-H2O 		delta-H2O avg sp ht (kJ/kg-C) Max
Temperature C 		Sulfate MW start Water mass kg water mass/ sulfate mass Dehydrate energy per kg water kJ
7 6 1 1.4 50 213 18 0.085 240
7 2 5 1.2 120 213 90 0.423 230
7 1 6 1.1 150 213 108 0.507 253
6 2 4 1.1 120 195 72 0.369 223
6 1 5 1.0 150 195 90 0.462 240
2 1 1 0.9 150 123 18 0.146 370
1 0 1 0.8 250 105 18 0.171 380
Table 2. Energetics of Pure Sulfates (assuming 60% heat recoup).
Table 2. Energetics of Pure Sulfates (assuming 60% heat recoup).
1 2 3 4 5 6 7 8
Start
dot-H2O 		End
dot-H2O 		delta-H2O Water mass kg Dehydrate energy kJ warming energy kJ Total energy kJ Total energy kJ per kg H2O
7 6 1 18 4,320 14,200 18,500 1,030
7 2 5 90 20,700 19,400 40,100 446
7 1 6 108 27,400 20,600 48,000 444
6 2 4 72 16,000 16,300 32,300 449
6 1 5 90 21,600 17,200 38,800 431
2 1 1 18 6,660 9,740 16,400 911
1 0 1 18 6,840 10,800 17,600 977
Table 3. Energy requirement (kJ) per kg of water produced for regolith containing sulfates (assuming 60% heat recoup).
Table 3. Energy requirement (kJ) per kg of water produced for regolith containing sulfates (assuming 60% heat recoup).
Percent sulfate in regolith 7-6-1 7-2-5 7-1-6 6-2-4 6-1-5 2-1-1 1-0-1
10 7,160 2,390 2,320 2,670 2,490 7,410 9,040
20 3,750 1,310 1,280 1,440 1,350 3,800 4,560
30 2,620 949 930 1,030 965 2,600 3,070
40 2,050 770 757 819 774 1,990 2,320
50 1,710 662 653 696 659 1,630 1,870
60 1,480 590 583 614 583 1,390 1,580
70 1,320 538 534 555 529 1,220 1,360
80 1,200 500 496 511 488 1,090 1,200
90 1,100 470 467 476 456 991 1,080
100 1,030 446 444 449 431 911 977
Next, we calculate the power requirement to produce 3 kg/h of water from regolith containing sulfates. This figure was chosen for illustration to produce 36 metric tons of H2O in 500 days. The data can be scaled to any other requirement. We take the energy per kg of water and multiply it by 3, to get the energy to produce 3 kg of water. We allow production for one hour. Then we divide by 3,600 s/h to get the energy per second in kJ/s, which is the power in kW. The results are shown in in Figure 5.

Discussion

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 larger amounts of water are needed to produce propellants for departing Mars for the return trip to Earth. The discovery of large potential ice-rich areas on Mars around 40°N or higher [1] influenced NASA mission analysts to plan for human missions to Mars at such higher latitudes, assuming that accessible water ice would be plentiful. [4,5,6] However, Rapp and Inglezakis (2025) took a more cautious view of the potential accessibility of water ice based on these early observations from orbit. [1] It remains to be determined how accessible these putative ice deposits are. With the optimistic view of ice-rich areas on Mars around 40°N or higher adopted by much of the Mars community, interest in hydrated minerals as a source of H2O seems to have diminished.
There are many published reports on hydrated minerals on Mars, but almost all of these were concerned with the evolution of water systems on Mars from the scientific view, and only a few dealt with hydrated minerals as a pragmatic source of water for missions.
Scans using the neutron spectrometer from orbit show a remarkable correlation between occurrence of S and H in the equatorial region, indicating that hydrated sulfates are a primary source of significant H2O there. Recent higher resolution scans using a collimated neutron spectrometer from orbit show pockets of much higher H2O content over several equatorial areas. Since even these higher resolution scans still involve 200 km x 200 km pixels, this implies that much higher local concentrations of H2O almost surely exist within these 200 km x 200 km pixels. The evidence strongly suggests that local concentrations of hydrated sulfates provide significant sources of H2O in the equatorial region.
Hydrated magnesium sulfates provide the dominant source of H2O in the accessible upper layer of Mars regolith at various scattered equatorial locations. Several forms of hydrated MgSO4 are known to provide a significant share of observed water-equivalent hydrogen in the upper meter of Mars regolith. These include “Gypsum” (MgSO₄⋅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).
The power requirements to evolve H2O from a range of potential hydrated magnesium sulfates can easily be provided by typical power systems.
Practical engineering systems to exploit H2O from hydrated magnesium sulfates embedded in Mars regolith are beyond the scope of this paper. However, we briefly mention a few relevant papers.
Bertolini (2023) prepared a preliminary design of a processing plant to recover H2O from gypsum embedded in Mars regolith. [22] van Susante et al. (2020) developed an approach for obtaining H2O from gypsum based on disaggregation of the gypsum by using a water jet system inside an enclosure. [23] “The resulting small gypsum particles and water mixture, the slurry, is sucked into a gravity separation system where most of the liquid water is syphoned off and recycled back to the water jet while the gypsum particles and the remainder of the liquid water are transferred into the reactor vessel and heated to 210°C to extract all the liquid water and crystalline bound water. The water vapor is then condensed and captured to feed back into the water jet reservoir and the excess stored for further processing.”
Jamanca et al. (2023) provided a brief description of a plan to extract H2O from the Meridiani Planum on Mars along the planet's equator. [24] They assumed 25% sulfate, of which 50% was gypsum, so the water concentration was 2.9%. However, it seems likely that regions of highest H2O content will contain higher percentages of sulfate, including sulfates with much higher water content than gypsum. (See Figure 4 and Figure 5). They provided a flow chart of the system, but very few details.
A large-scale water supply on Mars would be a significant capability to enable several alternative mission concepts. For example, SpaceX proposed a mission in which a Starship would return to LEO directly from the Mars surface. Propellants for the return trip would be produced by ISRU based on atmospheric CO2 and several hundred metric tons of indigenous H2O. [25,26,27] The landing site would be near 40°N latitude.
In choosing a site for a human mission to Mars, accessibility of large amounts of indigenous H2O is the deciding factor among many factors. [1,4,5,6] The evidence for large concentrations of magnesium sulfates providing accessible H2O in some equatorial regions of Mars, while highly suggestive, has not been verified by ground truth, nor have the local geographical features been identified. In almost the same way, the evidence for accessible water ice at latitudes around 40°N or greater as based on observations from orbit, are also highly suggestive, but lack resolution, detail and ground truth. We remain in a state of uncertainty regarding the accessibility and concentration of these putative resources, and resolution of the geographical details could influence the choice of an optimum landing site, whether at ~40°N, or at near-equatorial sites. At this juncture, lacking definitive observations for both resources, we suggest that a human mission to Mars at equatorial latitudes based on hydrated sulfates as a source of water is at least as attractive as a mission to higher latitudes based on putative accessible ice.

Future Work

None of the several rover missions conducted over the past couple of decades were aimed at finding and resolving putative deposits of water ice or hydrated minerals. Instead, other scientific priorities determined the landing sites that were generally barren of accessible water.
With NASA currently burdened by the demands of the lunar enterprise, it seems unlikely that NASA will fund missions in the intermediate term to provide greater resolution of the distribution of near-surface hydrated sulfates on Mars.
A strategy based on finding the water of hydration at equatorial sites might not be optimum for immediate science return, but it would provide leverage for future scientific exploration and might be advantageous in the intermediate term. In any event, the location -8° latitude and -167° longitude is so remarkably replete with H2O (see Figure 4) that this location ought to be a top choice for the next science mission landing site (if there is one). NASA has no present plans for a Mars surface mission, and if it did, based on experience, it seems unlikely that this location would hardly be chosen as the landing site, since other science priorities are likely to drive the landing site elsewhere.
While the programmatic outlook for NASA missions is not optimistic, the European Rosalind Franklin Rover mission to be launched in 2028/2029 has chosen a landing site at Oxia Planum, located at approximately 18.3°N latitude and 24.6°W longitude, which when compared to the left figure in Figure 3, appears to be a region with some enhancement of hydrated sulfates, even though the mission description showed no interest in hydrated sulfates. Since the plan is to drill down to 2 m seeking ancient biosignatures, they might find considerable hydrated minerals at the same time?
A JPL team developed a rover-mounted hydrated mineral detector (HMD) based on a dielectric spectrometer to detect bulk subsurface hydrated minerals along the rover traverse across the Mars surface at 2m resolution. [28] Field testing demonstrated detection of buried gypsum samples. along the rover traverse irrespective of surface obscuration. The HMD (TRL 4) is a proof-of-concept instrument that can be developed and matured for future Mars rover missions. Prospects for further funding are uncertain at best.

References

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Figure 1. 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.
Figure 1. 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.
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Figure 2. Water equivalent hydrogen maps, measured by collimated FREND/DSEN. (a) Upper map is collimated for maximum resolution (200 km). (b) Lower map is omni directional (550 km). Black and white isolines correspond to WEH values. Adapted from Figure 2 of Malakahov et al. (2022). [21].
Figure 2. Water equivalent hydrogen maps, measured by collimated FREND/DSEN. (a) Upper map is collimated for maximum resolution (200 km). (b) Lower map is omni directional (550 km). Black and white isolines correspond to WEH values. Adapted from Figure 2 of Malakahov et al. (2022). [21].
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Figure 3. Enhanced segments of the global map shown in Figure 2, showing details of two areas with most water content: Arabia Terra (top left), Medusa Fossae (top right). Adapted from Figure 3 of Malakahov et al. (2022). [21].
Figure 3. Enhanced segments of the global map shown in Figure 2, showing details of two areas with most water content: Arabia Terra (top left), Medusa Fossae (top right). Adapted from Figure 3 of Malakahov et al. (2022). [21].
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Figure 4. Closeup chart of highest resolution view of a local area on Mars with unusually high WEH. The contour lines are lines of constant WEH. [21].
Figure 4. Closeup chart of highest resolution view of a local area on Mars with unusually high WEH. The contour lines are lines of constant WEH. [21].
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Figure 5. Closeup chart of highest resolution view of a local area on Mars with unusually high WEH. The contour lines are lines of constant WEH. [21].
Figure 5. Closeup chart of highest resolution view of a local area on Mars with unusually high WEH. The contour lines are lines of constant WEH. [21].
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Figure 5. Estimated power requirement to produce 3 kg/h of H2O from hydrated magnesium sulfates as a function of the percentage of sulfate in the regolith.
Figure 5. Estimated power requirement to produce 3 kg/h of H2O from hydrated magnesium sulfates as a function of the percentage of sulfate in the regolith.
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