 GEOLOGICAL WATER RESOURCES FOR HUMANS ON MARS: CONSTRAINTS FROM ORBITAL SPECTRAL MAPPING AND IN SITU MEASUREMENTS. Scott L. Murchie1, Bethany L. Ehlmann2,3, and Raymond E. Arvidson4. 1Applied Physics Laboratory, 11100 Johns Hopkins Rd., Laurel, MD 20723 (scott.murchie@jhuapl.edu); 2Division of Geological & Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 (ehlmann@caltech.edu), 3Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109; 4Department of Earth & Planetary Sciences, Washington University in St. Louis, MO 63130.  Introduction:  Scenarios for in situ resource utilization by humans on Mars include exploitation of either water ice or mineral water resources. Here we review information on the types of hydrated mineral and mineraloid resources on Mars and estimate their recoverable water. Orbital infrared spectral mapping of H2O and OH by Mars Express/OMEGA and MRO/CRISM [1-5] and landed measurements by Mars Phoenix/TEGA & MECA [6,7] and MSL/SAM [8, 9] place limits on abundance of water in regolith and different geologic formations and on the energy required for extraction (Table 1). Such "geological" water resources are relatively widespread [1,2], including at the surface near the equator in the most thermally benign operating environments; yield 1 wt% H2O by heating to 500°C in all cases; and in select deposits may have several times more water, much of it releasable at temperatures 250°C or lower. Typical Soils: Nearly all of Mars' surface is at least partially covered by "regolith", silt- and sand-sized basaltic "soils", and a surface component of partially oxidized, micrometer-sized "dust", that are variably cemented by salts [5]. Orbital mapping shows that dust and soils have an ubiquitous 3-µm absorption resulting from OH in mineral lattices plus H2O bound in minerals, incorporated into amorphous material, and adsorbed to grain surfaces. Strength of the absorption varies latitudinally, with lowest strengths in regions equatorward of 45° latitude that are nearly invariant seasonally [3-5]. Estimates from near-infrared spectral data suggest near-equatorial H2O abundances of 2-4 wt%, depending on assumptions about grain size [5,10]. Strength of the 3-µm band increases with latitude poleward of this range and varies seasonally, with band strength moderately correlated with the relative humidity predicted by climate models, yielding estimates of up to 10 wt% H2O at the optical surface. This latitudinal/seasonal variation suggests adsorbed H2O that cycles between surface and atmosphere [5].  H2O contents measured in situ in both latitude zones were similar, ~1 wt%, and smaller than predicted from orbital data. Approximately 1 wt% H2O was measured by Phoenix in soils above solid ice at 68°N, with minor H2O release at 295°C and major release at 735°C [6,7]. At 4.5°S, 1.5-3 wt% H2O was measured by MSL/SAM in the upper centimeters of a sand ripple  [8]. Most (~75%) H2O was released by SAM during heating from 100-500°C, with the remainder being released at tempertures up to 800°C. These results suggests that most soil H2O is incorporated in amorphous material or minerals, rather than adsorbed and easily released [6,8], and that recoverable H2O may be estimated conservatively at 1 wt% after heating to ~400°C. The discrepancy with estimates from nearinfrared orbital data likely arises from orbital measurements detecting molecules-thick adsorbed H2O that accounts for little bulk content [11]. An additional consideration is that soils contain sulfate, perchlorate, carbon, and and other species that may complicate processing and purification post-extraction [8,12].  Rock Formations with Hydrated Minerals: Geologic units with phyllosilicates (including clays), chlorides, sulfates, silica, and carbonates have been mapped across Mars [1,2 and refs. therein] and show higher concentrations of hydrous minerals than soils. Layered Phyllosilicates consist of a layer of Mg/Fe-rich smectite-bearing material 10s to 100s of meters thick, commonly overlain by a layer rich in Alphyllosilicate and accessory minerals meters to 10s of meters thick. Abundance modeling of smectite from CRISM spectra of well-exposed bedrock suggests up to ~50 wt% smectite [13]. Assuming this abundance, a Fe smectite (nontronite) composition (~15 wt% H2O [10]), and ~1 wt% H2O in the background basalt, a total water content of ~8% might be expected. Values of 7-9% are derived from spectral modeling of CRISM data [3-5].However, given likely decreased hydration of clays under Mars water vapor partial pressures over geologic time, and the prevalence of adsorbed water at the optical surface, this estimate is an upper bound. Crustal Phyllosilicates: A diverse set of phyllosilicates and other hydrous minerals (mostly smectite and chlorite, with serpentine, prehnite, silica and zeolite also common) occur within basalt and are exposed by craters and escarpments. Much of the phyllosilicate is thought to have formed by subsurface alteration, an interpretation supported by in situ investigation of exposures in the rim of Endeavor crater by MER/Opportunity [15]. Comparison with terrestrial analogs suggests ~25% or less clay [16], consistent with the ~5% smectite suggested by landed measurements from MER/Opportunity [14]. Following the above methods, assuming 5 wt% smectite plus basalt, enhancement of only 1 wt% H2O above the background basalt is likely. If the smectites are nontronite, most water is recoverable by heating to 450°C. If saponite or montmorillonite, half is recoverable by 300°C (dehydration) with the remainder beginning at 600°C (dehydroxylation) [17]. Although these deposits are widespread, the difficulty of processing rock vs. regolith may reduce benefits of a 1% enhancement.  Sedimentary Phyllosilicates occur in fans and lakebeds across Mars, containing phyllosilicates transported from drainage basins as well as formed in situ. Smectite abundances of select deposits modeled from orbital data are 10-45% [13]. In situ measurements at Gale crater by MSL/CheMin found  ~20 wt% saponite in sedimentary rocks of the Yellowknife Bay formation [18]. However these had not been detected from orbit. Using above methods this could represent ~4 wt. % H2O. In contrast MSL/SAM measured 1.7-2.5 wt.% H2O, indistinct from soils, with a portion requiring high release temperatures consistent with dehydroxylation. This result suggests that clay minerals at this location are mostly dehydrated. Measurements will continue to be made of other rock units, where hydration has been detected by orbital data, as Curiosity continues its exploration of Gale crater.  Layered Sulfates are light-toned, horizontally layered deposits superposed unconformably on underlying eroded bedrock. Spectrally they resemble a mix of dust and basaltic sand, with an increased content of crystalline hematite and hydrated sulfates. Two spectral types of hydrated sulfates occur, monohydrated and polyhydrated, with Mg or sometimes Fe as the cation. Monohydrated sulfates in general appear prevalent [15]; polyhydrated sulfate may commonly occur in the form of starkeyite (with 4 H2O's) [19]. From sulfur measurements by Opportunity/APXS of the Burns Formation in Meridiani Planum, sulfate content is up to 45% but usually less [20]. Following the above methods and conservatively assuming 25 wt% monohydrated sulfate mixed with basaltic sediment, an enhancement of ~3 wt% H2O above background basalt is likely. If starkeyite is assumed, then an enhancement of ~9 wt% is likely. Release of water by hydrated sulfates is expected by heating to 100°C [21]. The relatively large content of H2O recoverable at low temperatures makes hydrated sulfates attractive as a water resource. However potential issues are high contents of S and halogens. Summary & Key Outstanding Questions: Typical soils yield at least 1 wt% H2O, recoverable by heating to ~450°C. Some geologic formations formed in aqueous environments may have up to 10 wt%  H2O enhancements, recoverable in some cases (e.g. from hydrated sulfate, or some clays) by heating to only 100°-250°C. However, these type of deposits have not yet been explored in situ with payloads critical to a ground truth wt% H2O measurement. The other compounds present in the rocks and soils drive a need for methods to purify extracted waters.  References: [1] Murchie, S. et al. (2009) JGR, 114, E00D06. [2] Ehlmann, B. & C. Edwards (2014) Ann. Rev. Earth Planet. Sci., 42, 291-315. [3] Milliken, R. et al. (2007) JGR, 112, E08S07. [4] Jouglet, D. et al. (2007) JGR, 112, E08S06 [5] Audouard, J. et al. (2014) JGR, 119, 1969-1989. [6] Smith, P. et al. (2009) Science, 325, 58-61. [7] Zent et al. (2010) JGR, 115, E00E14 [8] Leshin, L. et al. (2013) Science, 341, 1238937. [9] Ming et al., 2014, Science 343, 1245267 [10] Milliken, R. & J. Mustard (2007) Icarus, 189, 574-588. [11] Arvidson, R. E., et al. (2009) J. Geophys. Res., 114, E00E02. [12] Hecht, M. et al. (2009) Science, 325, 64-67.JGR [13] Poulet, F. et al. (2014) Icarus, 231, 65-76. [14] Frost et al. (2000) Thermochim. Acta 346, 63-72.  [15] Arvidson, R. et al. (2015) JGR Planets, 120, 429-451. [16] Ehlmann, B. et al. (2013) Space Sci. Rev., 124, 329-364. [17] Milliken & Mustard (2005) 110, E12001. [18] Vaniman et al., (2014) Science 343, 1243480. [19] Liu, Y. et al. (2012) JGR, 117, E00J11 [20] Clark, B et al. (2005) Earth Plan. Sci. Lett,  240, 7394 [21] Chipera, S. & D. Vaniman (2007) Geoch. Cosmoch. Acta, 71, 241-250.   Table 1. Summary of properties of Martian selected, common hydrated mineral resources. Class of  deposit Reference area General description Assumed composition  Possible recoverable water  Comments Loose soils Gale crater, Phoenix landing site  Powdered rock, salts, amorphous materials Mix of plagioclase, olivine, pyroxene, npFeOx, amorphous phase 1-3% from heating to ~450°C  Sulfate and perchlorate salts may be common Layered phyllosilicate Mawrth Vallis (orbital est.) Stratified deposits rich in smectite Mix of up to 50% smectite clay with basaltic sediment  <7-9 wt% from heating to 450°C Upper kaolinite, silica layers lower in water content  Crustal phyllosilicate Endeavor crater rim Smectite clays in basaltic groundmass Mix of 5% smectite with basalt ~2 wt% from heating to 450°C Fractured bedrock Sedimentary phyllosilicates Jezero fan bottomset beds Smectite clays with fine-grained basaltic particles, carbonate Mix of 10-45% smectite with basalt Up to 2-8 wt% from heating to 450°C Other parts of fans dominated by basaltic fragments Sulfate-rich layered deposits Burns Form- ation, Meridiani Planum  Dust + sand, variable content and type of sulfate cement Mix of 25% monohyd./polyhyd. sulfate with basaltic sediment Up to ~3-9 wt% from heating to >100°C Halogen, S content may be issue  
