The Formation of Liquid Water on Present-Day Mars: Calcium-Magnesium Chloride Brines in the Antarctic Dry Valleys As a Mars Analog

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Sixth Mars Polar Science Conference (2016) 6005.pdf

THE FORMATION OF LIQUID WATER ON PRESENT-DAY MARS: CALCIUM-MAGNESIUM CHLORIDE BRINES IN THE ANTARCTIC DRY VALLEYS AS A MARS ANALOG. J. D. Toner and D. C. Catling, University of Washington, Dept. Earth & Space Sciences and Astrobiology Program, Seattle, WA 98195, USA. (e-mail: [email protected])

Introduction: Recurring Slope Lineae (RSL) are freezing processes should cause Ca2+ to precipitate as dark lineations on warm Martian slopes that lenghthen calcite (CaCO3) or gypsum (CaSO4·2H2O), leading to incrementally downslope in spring as temperatures Ca2+ deficient brines [12]. Recent analyses and model- warm, fade during winter months, and reccur from ing efforts in Taylor Valley have shown that cation year-to-year [1]. An aqueous mechanism is widely be- exchange reactions on mineral surfaces strongly enrich lieved to form RSL [2, 3]; however, despite extensive soil solutions in Ca2+ and Mg2+ ions, leading to the investigation in recent years, both the mechanism of formation of Ca-Mg-Cl rich brines [13]. The exchange RSL formation and the composition of putative brines mechanism governing this enrichment process is appli- remain elusive. Theories on aqueous formation mecha- cable to any region affected by freezing processes that nisms range from discharge by freshwater or saline concentrate fluids in soils. Applied to Mars, Ca-Cl aquifers [3, 4], to eutectic melting and deliquescence of enrichment processes now operating in the ADV sug- hygroscopic salts [5]. Theories on RSL composition gest that Ca-Mg-Cl brines should be ubiquitous in the are similarly diverse, from calcium chloride, to per- Mastian subsurface. We hypothesize that outflows of chlorate, to ferric sulfate brines [6, 7]. such near-surface Ca-Mg-Cl rich groundwater may be The Antarctic Dry Valleys (ADV) are extremely responsible for RSL. cold and dry, and represent the best Earth analog envi- Thermodynamic model: To explore the thermo- ronment for Mars. Aqueous brine flows have been dynamic properties of putative chloride brines on identified in the ADV that are morphologically similar Mars, we have developed a new, comprehensive ther- to features observed on Mars and show seasonal flow modynamic model in the Na-K-Ca-Mg-Cl system valid patterns [8, 9]. Interestingly, shallow groundwater from <200 to 298.15 K [10]. This model improves over brines in the ADV are commonly enriched in CaCl2, an previous models because it achieves thermodynamic unusual composition in carbonate-rich surface waters consistency by incorporating a variety of interrelated on Earth. Don Juan Pond (DJP) in Wright Dry Valley thermodynamic properties including activity coeffi- contains a nearly saturated CaCl2 brine. Such brines cients, solution heat capacities, solution enthalpies, and have low eutectic temperatures (-50°C) and are ex- salt solubilities in binary and mixed salt systems. tremely hygroscopic. If present on Mars, CaCl2 brines Ca-Mg-Cl brine properties: Freezing Point De- could remain stable despite cold and dry conditions. pression. The maximum equilibrum FPD possible is Here we use a newly developed thermodynamic the eutectic temperature (Te). Of the chloride salts, model in the Na-K-Ca-Mg-Cl system [10] to explore CaCl2 has the lowest Te (-50.1°C). This Te is similar to the potential for low-temperature chloride brines to Te in Mg(ClO4)2 solutions (-57°C), but somewhat high- form water on Mars. We find that freezing-point de- er than the metastable eutectic often observed in this pression (FPD), hygroscopicity, and water require- salt (-67°C). Ca(ClO4)2 has the lowest Te of any Mars ments in Ca-Mg-Cl mixtures are often greater than in relevant salt (-75°C); however, this salt is unlikely to perchlorate salts, which supports the potential for Ca- form aqueous solutions in soils [14] because it is de- Mg-Cl brines to form aqueous flows such as RSL. posited surficially via atmospheric interations onto Antarctic analogs: Ca-Cl-rich compositions are ra- carbonate-rich soils [15]. Possibly, Ca(ClO4)2 salts re on Earth’s surface, but are common in the ADV. In could form as Ca-Cl-rich groundwaters discharge into addition to Don Juan Pond, the bottom water of Lake surface soils and entrain perchlorates. Vanda in Wright Valley contains a concentrated Ca- Eutectics in brine mixtures are largely determined Mg-Na-Cl brine, as do several other ponds in Wright by the salt component having the lowest Te i.e. little and Victoria Valley. Ca-Cl enrichment is also common additional freezing-point depression occurs in salt mix- in shallow subsurface flows found in Taylor and tures. For example, a CaCl2 brine in mixture with Wright Valley. Ca-Cl-rich brines found in the ADV are MgCl2 (Te = -33.4°C) has a maximum FPD of Te = - similar in composition to deep groundwaters found in 50.8°C, which is nearly the same as in pure CaCl2. many other locations on Earth [11]. Deliquescence/Hygroscopicity. The deliquescence The formation of Ca-rich brines in the ADV is geo- relative humidity (DRH) is given by the water activity chemically interesting because ADV surface waters are over a saturated salt solution. DRH increases with de- rich in sulfate and carbonate; hence, evaporation and creasing temperature (salts become less hygroscopic at Sixth Mars Polar Science Conference (2016) 6005.pdf

lower temperatures), primarily following changes in lar or lower water requirements compared to perchlo- solubility (Fig. 1). The DRH of salt mixtures is lower rates. With the exception of Ca(ClO4)2 salts, MgCl2 than the DRH of any of the individual salt components. salts have the lowest water requirements of all due to In mixtures such as Na-Ca-Cl, the net DRH lowering their high hydration states. MgCl2·12H2O will melt relative to pure CaCl2 or NaCl is negligible; however, spontaneously at 255 K, dissolving in its own hydration our model indicates that DRH in Ca-Mg-Cl mixtures is water to form brines; hence, this salt does not require substantially lower than in individual salt components, any additional water at 255 K to form brine. by up to 11 % DRH. The DRH of Ca-Mg-Cl mixtures is much lower than for Mg(ClO4)2·6H2O above 250 K (by up to 20 % DRH). Although we have not yet modeled chloride-perchlorate mixtures, such mixtures have the potential for extremely low total DRH. Absorption of atmospheric water vapor in low DRH brines and salts could be a mechanism for RSL formation. Alter- natively, low DRH will inhibit the evaporation of brines exposed at the soil surface, which could explain the seasonal persistence of brines in soils.

Fig. 2. The mass of water (g) that would need to be added to a given salt to form 1 ml of solution.

Conclusions: The formation of Ca-Mg-Cl rich brines in the ADV, and recent finding on the mechanism responsible for this enrichment, suggest that Ca- Mg-Cl rich brines should be pervasive in the Martian subsurface. Our analysis of FPD and DRH indicates that Ca-Mg-Cl brines could be stable on the Martian surface. Such brines are more stable than NaClO4 so- Fig. 1. DRH modeled in various perchlorate and lutions, and are more stable than Mg(ClO4)2 solutions chloride salt solutions, including Ca-Mg-Cl mixtures above 250 K. Furthermore, the water requirement (red line). The hydration waters of equilibrium salt needed to form Ca-Mg-Cl aqueous flows is less than phases are indicated. for perchlorates above 250 K. These properties support the notion that RSL could be seasonal flows of Ca-Mg- Water budget. Given the paucity of liquid water Cl rich aqueous solutions. sources on Mars, salts requiring less water to form a References: [1] McEwen, A.S., et al. (2011), Sci., given volume of brine are more favorable for the for- 333(6043), 740-743. [2] McEwen, A.S., et al. (2014), mation of aqueous flows. The mass of water needed to Nat. Geo., 7, 53–58. [3] Stillman, D.E., et al. (2016), form 1 ml of brine, the water requirement (WR, g ml-1), Icarus, 265, 125–138. [4] Stillman, D.E., et al. (2014), is a function of the molality of the saturated solution Icarus, 233, 328-341. [5] Kreslavsky, M.A. and J.W. Head (2009), Icarus, 201(2), 517-527. [6] Chevrier, ( msat ), the stoichiometric hydration waters in the salt ( x ), and the saturated solution density ( d ) in g cm-3: V.F. and E.G. Rivera-Valentin (2012), GRL, 39(21), 1- sat 5. [7] Ojha, L., et al. (2015), Nat. Geo., 8, 829–832. [8] Dickson, J.J., et al. (2013), Sci. Rep., 3(1166, 1-7.  1000 18mxsat [9] Levy, J.S. (2012), Icarus, 219(1), 1-4. [10] Toner, (1.1) WR  dsat 1000  mMsat w J.D. and D.C. Catling (submitted), GCA. [11] Garrett, D. (2004), Amsterdam: Elsevier Academic Press. [12] Lyons, W.B. and P.M. Mayewski (1993), AGU, 135- where Mw is the molecular weight of the salt. We de- rive densities of the saturated solution as a function of 143. [13] Toner, J.D. and R.S. Sletten (2013), GCA, concentration from FREZCHEM [16], assuming that 110, 84–105. [14] Kounaves, S.P., et al. (2014), Ica- rus, 232, 226–231. [15] Smith, M.L., et al. (2014), values at 298.15 K are temperature invariant. Icarus, 231, 51-64. [16] Marion, G.M. and J.S. Kargel Hydration waters have the strongest effect on WR (2008), Berlin/Heidelberg: Springer. because these waters are added to solution upon salt Additional Information: Funding from NASA dissolution (Fig. 2). CaCl and MgCl salts have simi- 2 2 Habitable Worlds grant (NNX15AP19G).