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
dark lineations on warm Martian slopes that lenghthen
incrementally downslope in spring as temperatures
warm, fade during winter months, and reccur from
year-to-year [1]. An aqueous mechanism is widely believed to form RSL [2, 3]; however, despite extensive
investigation in recent years, both the mechanism of
RSL formation and the composition of putative brines
remain elusive. Theories on aqueous formation mechanisms range from discharge by freshwater or saline
aquifers [3, 4], to eutectic melting and deliquescence of
hygroscopic salts [5]. Theories on RSL composition
are similarly diverse, from calcium chloride, to perchlorate, to ferric sulfate brines [6, 7].
The Antarctic Dry Valleys (ADV) are extremely
cold and dry, and represent the best Earth analog environment for Mars. Aqueous brine flows have been
identified in the ADV that are morphologically similar
to features observed on Mars and show seasonal flow
patterns [8, 9]. Interestingly, shallow groundwater
brines in the ADV are commonly enriched in CaCl2, an
unusual composition in carbonate-rich surface waters
on Earth. Don Juan Pond (DJP) in Wright Dry Valley
contains a nearly saturated CaCl2 brine. Such brines
have low eutectic temperatures (-50°C) and are extremely hygroscopic. If present on Mars, CaCl2 brines
could remain stable despite cold and dry conditions.
Here we use a newly developed thermodynamic
model in the Na-K-Ca-Mg-Cl system [10] to explore
the potential for low-temperature chloride brines to
form water on Mars. We find that freezing-point depression (FPD), hygroscopicity, and water requirements in Ca-Mg-Cl mixtures are often greater than in
perchlorate salts, which supports the potential for CaMg-Cl brines to form aqueous flows such as RSL.
Antarctic analogs: Ca-Cl-rich compositions are rare on Earth’s surface, but are common in the ADV. In
addition to Don Juan Pond, the bottom water of Lake
Vanda in Wright Valley contains a concentrated CaMg-Na-Cl brine, as do several other ponds in Wright
and Victoria Valley. Ca-Cl enrichment is also common
in shallow subsurface flows found in Taylor and
Wright Valley. Ca-Cl-rich brines found in the ADV are
similar in composition to deep groundwaters found in
many other locations on Earth [11].
The formation of Ca-rich brines in the ADV is geochemically interesting because ADV surface waters are
rich in sulfate and carbonate; hence, evaporation and
freezing processes should cause Ca2+ to precipitate as
calcite (CaCO3) or gypsum (CaSO4·2H2O), leading to
Ca2+ deficient brines [12]. Recent analyses and modeling efforts in Taylor Valley have shown that cation
exchange reactions on mineral surfaces strongly enrich
soil solutions in Ca2+ and Mg2+ ions, leading to the
formation of Ca-Mg-Cl rich brines [13]. The exchange
mechanism governing this enrichment process is applicable to any region affected by freezing processes that
concentrate fluids in soils. Applied to Mars, Ca-Cl
enrichment processes now operating in the ADV suggest that Ca-Mg-Cl brines should be ubiquitous in the
Mastian subsurface. We hypothesize that outflows of
such near-surface Ca-Mg-Cl rich groundwater may be
responsible for RSL.
Thermodynamic model: To explore the thermodynamic properties of putative chloride brines on
Mars, we have developed a new, comprehensive thermodynamic model in the Na-K-Ca-Mg-Cl system valid
from <200 to 298.15 K [10]. This model improves over
previous models because it achieves thermodynamic
consistency by incorporating a variety of interrelated
thermodynamic properties including activity coefficients, solution heat capacities, solution enthalpies, and
salt solubilities in binary and mixed salt systems.
Ca-Mg-Cl brine properties: Freezing Point Depression. The maximum equilibrum FPD possible is
the eutectic temperature (Te). Of the chloride salts,
CaCl2 has the lowest Te (-50.1°C). This Te is similar to
Te in Mg(ClO4)2 solutions (-57°C), but somewhat higher than the metastable eutectic often observed in this
salt (-67°C). Ca(ClO4)2 has the lowest Te of any Mars
relevant salt (-75°C); however, this salt is unlikely to
form aqueous solutions in soils [14] because it is deposited surficially via atmospheric interations onto
carbonate-rich soils [15]. Possibly, Ca(ClO4)2 salts
could form as Ca-Cl-rich groundwaters discharge into
surface soils and entrain perchlorates.
Eutectics in brine mixtures are largely determined
by the salt component having the lowest T e i.e. little
additional freezing-point depression occurs in salt mixtures. For example, a CaCl2 brine in mixture with
MgCl2 (Te = -33.4°C) has a maximum FPD of T e = 50.8°C, which is nearly the same as in pure CaCl2.
Deliquescence/Hygroscopicity. The deliquescence
relative humidity (DRH) is given by the water activity
over a saturated salt solution. DRH increases with decreasing temperature (salts become less hygroscopic at
Sixth Mars Polar Science Conference (2016)
lower temperatures), primarily following changes in
solubility (Fig. 1). The DRH of salt mixtures is lower
than the DRH of any of the individual salt components.
In mixtures such as Na-Ca-Cl, the net DRH lowering
relative to pure CaCl2 or NaCl is negligible; however,
our model indicates that DRH in Ca-Mg-Cl mixtures is
substantially lower than in individual salt components,
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. Alternatively, low DRH will inhibit the evaporation of
brines exposed at the soil surface, which could explain
the seasonal persistence of brines in soils.
6005.pdf
lar or lower water requirements compared to perchlorates. With the exception of Ca(ClO4)2 salts, MgCl2
salts have the lowest water requirements of all due to
their high hydration states. MgCl2·12H2O will melt
spontaneously at 255 K, dissolving in its own hydration
water to form brines; hence, this salt does not require
any additional water at 255 K to form brine.
Fig. 2. The mass of water (g) that would need to be
added to a given salt to form 1 ml of solution.
Fig. 1. DRH modeled in various perchlorate and
chloride salt solutions, including Ca-Mg-Cl mixtures
(red line). The hydration waters of equilibrium salt
phases are indicated.
Water budget. Given the paucity of liquid water
sources on Mars, salts requiring less water to form a
given volume of brine are more favorable for the formation of aqueous flows. The mass of water needed to
form 1 ml of brine, the water requirement (WR, g ml-1),
is a function of the molality of the saturated solution
( msat ), the stoichiometric hydration waters in the salt
( x ), and the saturated solution density ( dsat ) in g cm-3:
(1.1)
 1000  18msat x 
WR  
 d sat
1000  msat M w 
where M w is the molecular weight of the salt. We derive densities of the saturated solution as a function of
concentration from FREZCHEM [16], assuming that
values at 298.15 K are temperature invariant.
Hydration waters have the strongest effect on WR
because these waters are added to solution upon salt
dissolution (Fig. 2). CaCl2 and MgCl2 salts have simi-
Conclusions: The formation of Ca-Mg-Cl rich
brines in the ADV, and recent finding on the mechanism responsible for this enrichment, suggest that CaMg-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 solutions, and are more stable than Mg(ClO4)2 solutions
above 250 K. Furthermore, the water requirement
needed to form Ca-Mg-Cl aqueous flows is less than
for perchlorates above 250 K. These properties support
the notion that RSL could be seasonal flows of Ca-MgCl rich aqueous solutions.
References: [1] McEwen, A.S., et al. (2011), Sci.,
333(6043), 740-743. [2] McEwen, A.S., et al. (2014),
Nat. Geo., 7, 53–58. [3] Stillman, D.E., et al. (2016),
Icarus, 265, 125–138. [4] Stillman, D.E., et al. (2014),
Icarus, 233, 328-341. [5] Kreslavsky, M.A. and J.W.
Head (2009), Icarus, 201(2), 517-527. [6] Chevrier,
V.F. and E.G. Rivera-Valentin (2012), GRL, 39(21), 15. [7] Ojha, L., et al. (2015), Nat. Geo., 8, 829–832.
[8] Dickson, J.J., et al. (2013), Sci. Rep., 3(1166, 1-7.
[9] Levy, J.S. (2012), Icarus, 219(1), 1-4. [10] Toner,
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, 135143. [13] Toner, J.D. and R.S. Sletten (2013), GCA,
110, 84–105. [14] Kounaves, S.P., et al. (2014), Icarus, 232, 226–231. [15] Smith, M.L., et al. (2014),
Icarus, 231, 51-64. [16] Marion, G.M. and J.S. Kargel
(2008), Berlin/Heidelberg: Springer.
Additional Information: Funding from NASA
Habitable Worlds grant (NNX15AP19G).