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Soluble sulfate in the martian soil at the Phoenix landing site
Samuel P. Kounaves,
1
Michael H. Hecht,
2
Jason Kapit,
1,3
Richard C. Quinn,
4
David C. Catling,
5
Benton C. Clark,
6
Douglas W. Ming,
7
Kalina Gospodinova,
1,8
Patricia Hredzak,
1
Kyle McElhoney,
1
and Jennifer Shusterman
1
Received 19 January 2010; revised 18 March 2010; accepted 29 March 2010; published 1 May 2010.
[1] Sulfur has been detected by X‐ray spectroscopy in
martian soils at the Viking, Pathfinder, Opportunity and
Spirit landing sites. Sulfates have been identified by
OMEGA and CRISM in Valles Marineris and by the
spectrometers on the MER rovers at Meridiani and Gusev.
The ubiquitous presence of sulfur has been interpreted as a
widely distributed sulfate mineralogy. One goal of the Wet
Chemistry Laboratory (WCL) on NASA’s Phoenix Mars
Lander was to determine soluble sulfate in the martian soil.
We report here the first in‐situ measurement of soluble
sulfate equivalent to ∼1.3(±0.5) wt% as SO
4
in the soil. The
results and models reveal SO
4
2−
predominately as MgSO
4
with some CaSO
4
.Ifthesoilhadbeenwetinthepast,
epsomite and gypsum would be formed from evaporation.
The WCL‐derived salt composition indicates that if the soil
at the Phoenix site were to form an aqueous solution by
natural means, the water activity for a dilution of greater
than ∼0.015 g H
2
O/g soil would be in the habitable range of
known terrestrial halophilic microbes. Citation: Kounaves,
S. P., et al. (2010), Soluble sulfate in the martian soil at the Phoenix
landing site, Geophys. Res. Lett.,37, L09201, doi:10.1029/
2010GL042613.
1. Introduction
[2] Sulfur has been detected by X‐ray spectroscopy in
martian soils at the Viking [Clark, 1993], Pathfinder [Wänke
et al., 2001], Opportunity [Clark et al., 2005] and Spirit
[Rieder et al., 2004] landing sites. Spectroscopic detection
of sulfur minerals from orbital and landed missions has
added to the evidence for potentially widespread occurrence
of sulfates on Mars [Christensen et al., 2004; Bibring et al.,
2007]. Sulfates have been identified by OMEGA in Valles
Marineris [Gendrin et al., 2005], Meridiani [Arvidson et al.,
2005], and in a large dune field approximately 700 km north
of the Phoenix landing site where gypsum (CaSO
4
·2H
2
O)
was detected [Langevin et al., 2005]. The Compact Recon-
naissance Imaging Spectrometer for Mars (CRISM) has
identified sulfates in numerous sites including thin stratified
deposits at several locations and km‐deep sulfate‐rich
canyons and mounds [Bishop et al., 2009; Murchie et al.,
2009]. The Mars Exploration Rover (MER) Spirit in the
Columbia Hills area of Gusev Crater has identified shallow
soils enriched in Mg‐sulfates [Yen et al., 2005; Wang et al.,
2006; Gellert et al., 2004] as well as other soils containing
mixtures of Fe‐,Ca‐, and Mg‐sulfates [Johnson et al., 2007;
Yen et al., 2008; Ming et al., 2006]. The MER Mini‐TES and
Mössbauer instruments have also both detected sulfates
[Glotch et al., 2006; Morris et al., 2006]. The ubiquitous
presence of sulfur in soils has been interpreted as a widely
distributed sulfate mineralogy [Yen et al., 2005].
[3] One goal of the Wet Chemistry Laboratory (WCL)
[Kounaves et al., 2009a] on board NASA’s 2007 Phoenix
Mars Lander [Smith et al., 2009] was to measure and directly
confirm the identity and solubility of the sulfur species in the
martian soil. The earlier analysis of the data for the acquired
samples showed the presence of several ionic species with
average solution concentrations of 3.3 (±2) mM Mg
2+
,
2.4 (±0.5) mM ClO
4
−
, 1.4 (±0.3) mM Na
+
, 0.6 (±0.3) mM Ca
2+
,
0.5 (±0.1) mM Cl
−
, and 0.4 (±0.1) mM K
+
, with a moderate
pH of ∼7.7 (±0.3), and an average conductivity of ∼1.4
(±0.5) mS/cm. The charge balance, calculated ionic strength,
and conductivity, showed a discrepancy, suggesting that the
solution contained unidentified anionic species at a minimum
of several mM [Hecht et al., 2009; Kounaves et al., 2010].
[4] We report here for the first time the presence of soluble
sulfate, its concentration, and possible phases, in the soil at
the Phoenix landing site. Calculations based on the results of
the soil salt composition indicate that the water activity of
brines formed at this location would be tolerable for ter-
restrial microbes.
2. Analytical Methodology
[5] The Phoenix WCL received three ∼1cm
3
soil samples
on mission sol 30 (cell‐0, surface, Rosy Red), sol 41 (cell‐1,
subsurface, Sorceress‐1), and sol 107 (cell‐2, subsurface,
Sorceress‐2), where “sol”measures Martian solar days
elapsed from the landing, and the names refer to the samples
and sampling sites assigned by the Phoenix team. Phoenix did
not measure soil density; however, density of the Phoenix
soils was estimated at 1.0 g/cm
3
based on the Viking‐2 data
for the bulk density of the martian fine granular material of
1.1 g/cm
3
[Clark et al., 1977]. The two subsurface samples
were sublimation lag scraped off the ice table at ∼5cm
depth. The location, acquisition, and delivery of samples by
the robotic arm has been previously described [Arvidson
1
Department of Chemistry, Tufts University, Medford, Massachusetts,
USA.
2
Jet Propulsion Laboratory, California Institute of Technology,
Pasadena, California, USA.
3
Now at Woods Hole Oceanographic Institution, Woods Hole,
Massachusetts, USA.
4
SETI Institute, NASA Ames Research Center, Moffett Field, California,
USA.
5
Department of Earth and Space Sciences, University of Washington,
Seattle, Washington, USA.
6
Space Science Institute, Boulder, Colorado, USA.
7
NASA Johnson Space Center, Houston, Texas, USA.
8
Now at Massachusetts Institute of Technology, Cambridge,
Massachusetts, USA.
Copyright 2010 by the American Geophysical Union.
0094‐8276/10/2010GL042613
GEOPHYSICAL RESEARCH LETTERS, VOL. 37, L09201, doi:10.1029/2010GL042613, 2010
L09201 1of5
et al., 2009]. Each soil sample was added to 25 mL of a
leaching solution in a WCL cell and analyzed for solvated ionic
species, pH, and solution electrical conductivity [Kounaves
et al., 2010].
[6] Analysis of soluble SO
4
2−
in the martian soil after
addition to the WCL solution was performed by precipitation
with Ba
2+
added as BaCl
2
. Each WCL cell was equipped
with a reagent dispenser that could release up to three indi-
vidual ∼0.11 g additions of powdered BaCl
2
, each contained
within a miniature cylindrical crucible. The dissolution of
the BaCl
2
allows the Ba
2+
to react with the soluble SO
4
2−
in
the sample and precipitate as BaSO
4
[Kounaves et al.,
2009a; Lukow, 2005]. Each BaCl
2
addition allowed for
determination of SO
4
2−
up to 5 wt %, or for a total of 15 wt %
if all three additions were used.
[7] The concentrations of the added Ba
2+
and Cl
−
are
monitored by their respective ion selective electrodes (ISEs)
and the Cl
−
also by chronopotentiometry (CP). Under the
analytical conditions existing in the solution during the
analysis on Mars, the BaSO
4
precipitation technique is rapid
and highly selective for SO
4
2−
(solubility product, K
sp
,at
7°C ∼5×10
−11
) with no interference from BaCO
3
. Thus,
monitoring [Ba
2+
] indicates when there is no longer any SO
4
2−
remaining to precipitate the Ba
2+
, while monitoring the [Cl
−
]
gives the total amount of BaCl
2
added. The Ba‐ISE response
is not used for quantifying the Ba
2+
, but solely as an indi-
cator of when the end point has been reached. The amount
of SO
4
2−
precipitated is equal to half the difference between
the concentrations of Cl
−
just before the addition and when
the endpoint is reached. As described below, the addition
process for the BaCl
2
unexpectedly deviated from the
original plan, but the overall analytical methodology can be
applied to understanding the sulfate abundance in the soil.
3. Results
[8] Over the course of the WCL analyses, the chloride
concentration slowly increased at a rate of ∼1.5 × 10
−3
mol
L
−1
h
−1
(Figure 1a and 1b) in all samples, except for cell‐0,
where it stabilized by the end of the sol. This was initially
thought to result from the sample itself releasing chloride.
However, no other cation was observed to increase simulta-
neously with the chloride. During several attempts to deliver
sample to the fourth WCL cell (cell‐3) on sol 96, the soil
became lodged on top of the delivery funnel and none was
dispensed into the solution. The analysis run on cell‐3 thus
constituted a “blank”. As shown in Figure 1c, the Ba
2+
and
Cl
−
sensors both showed an increase in concentration at a
Ba:Cl ratio of 1:2. Similar increases were not seen for other
ions. The independent CP analysis for Cl
−
also confirmed
the increase.
[9] The most consistent model for explaining the above
results, is that the powdered BaCl
2
, contained in three sepa-
rate containers (crucibles) in the WCL reagent dispenser and
intended for the second sol analysis, leaked into the WCL
solution during the first sol. The high relative humidity
present in the WCL, after the leaching solution was dis-
pensed, would most likely have caused water to condense
on the reagent dispenser and allowed the BaCl
2
to creep or
drip into the WCL cell. The same is seen to occur during the
other analyses, not only on sols 30 and 107, but also on the
following sols (34 & 116) where sulfate analyses were to
take place. Unfortunately, the Ba
2+
sensor in cell‐1 failed
and thus no analyses were possible on sols 41 and 43.
[10] After the initial solution and calibrants were added, a
delay was observed, suggesting that BaCl
2
leakage did not
begin in cells 0, 1 and 3 until after soil addition. Fur-
thermore, there was no observed increase indicated by the
ISE, CP, or conductivity sensors during the pre‐sample
calibration periods. The barium chloride leakage was not seen
in pre‐flight testing, and we are currently investigating pos-
Figure 1. Concentrations of the barium and chloride in
the leached soil samples as measured by the Ba
2+
ISE (▴),
Cl
−
ISE (▪), and chronopotentiometric Cl
−
(•)ineachof
the WCLs. Results are shown for (a) “Rosy Red”sample
in cell 0 on sols 30 and 34, (b) Sorceress‐2 sample in cell 2
on sols 107 and 116, and (c) “blank”in cell 3 on sol 96. In
all cases it’s clear that although BaCl
2
was being added to
the solution, the concentration of Ba
2+
remained at or below
it’s calibration level.
KOUNAVES ET AL.: SULFATE AT THE PHOENIX LANDING SITE L09201L09201
2of5
sible causes. However, at present the results of the analysis
along with the consistent behavior from cell to cell, gives us
confidence that the values obtained for the sulfate are reliable.
[11] Figures 1a and 1b show the titration curves used to
determine the total soluble sulfate, [SO
4
2−
]
T
, present in the
WCL cell‐0 (sols 30 and 34) and cell‐2 (sols 107 and 116)
after delivery of martian soil. The addition of the BaCl
2
is
indicated by and proportional to half the increasing Cl
−
concentration. The [Ba
2+
] remains relatively constant until
the second sol when the crucible containing additional
powdered BaCl
2
was added. A short time after this addi-
tion, both the [Ba
2+
] and [Cl
−
] rapidly increase, indicating
that the Ba
2+
was no longer being precipitated by SO
4
2−
.
Thus the total sulfate present, [SO
4
2−
]
T
, is equal to D[Cl
−
]/2
(i.e., the change in [Cl
−
] from immediately after the sample
addition response to the start of the Ba
2+
increase that
indicated all the SO
4
2−
had been titrated). For cell‐0 this
gives [SO
4
2−
]
T
= 4.8 (±1.5) mM in solution. Assuming a 1 cm
3
sample with a density of 1 g/cm
3
, and that all the SO
4
2−
was
dissolved, this is equivalent to 1.2(±0.5) wt % SO
4
2−
in the
soil. For cell‐2 this gives [SO
4
2−
]
T
= 5.9 (±1.5) mM equiv-
alent to 1.4(±0.5) wt % SO
4
2−
. On average this is equivalent
to approximately 1.1(±0.5) wt % sulfur reported as SO
3
in
the soil. Results of analyses are summarized in Table 1.
4. Discussion
[12] With minor exceptions [Clark et al., 2005; Ming et al.,
2006], soils at previous landing sites have been reported to
contain 4 to 8 wt % SO
3
[Clark, 1993; Wänke et al., 2001;
Clark et al., 2005; Rieder et al., 2004], and have a nearly
uniform S/Cl molar ratio of ∼4:1. Based on the previous
data for the S/Cl ratio, one would predict that given a total
of 2.9 mM Cl in the WCL solution, it should then have
contained ∼12 mM SO
4
2−
, assuming all of the SO
4
2−
was
soluble. The molar ratio of sulfur (as SO
4
2+
) to total chloride
(Cl
−
+ ClO
4
−
) as measured by the WCL for the Phoenix soils
is ∼2:1. This factor of 2 discrepancy may be due to: (1) some
of the sulfur measured by XRF in previous missions is in a
form that is non‐soluble, or only sparingly soluble, within the
time frame of the WCL analyses; or (2) the Phoenix soil is
simply different from those analyzed at other locations and
sulfate or perchlorate are lower or higher, respectively, in
these soils.
[13] The Phoenix Thermal and Evolved Gas Analyzer
(TEGA) analyses found that the Phoenix soil contained
3–5% CaCO
3
[Boynton et al., 2009; Kounaves et al.,
2009b], however, as of this point in time the TEGA data
with respect to possible evolution of SO
2
is still being eval-
uated. Such types of analyses may be complicated by inter-
mediate product reactions during the pyrolysis process and
also highly dependent on temperature and sulfate phases
present.
[14] There are several plausible soluble mineral phases
that may be responsible for the SO
4
2−
measured by WCL.
These include a variety of K‐,Na‐,Fe‐,Mg‐, and Ca‐
sulfates. However, several can be excluded from being
present in any significant amounts. Soluble Fe‐sulfates are
eliminated since pre‐flight characterization tests clearly
showed that the presence of >0.1 mM Fe
2+/3+
would have
poisoned several of the ion selective electrode (ISE) sensors
and would have been detected by cyclic voltammetry (CV).
In addition, Fe‐sulfates would have buffered the solution at a
more acidic pH. Since none of the above responses were
observed with any sample, the presence of soluble Fe‐sulfate
at >0.1 mM in the WCL samples is not likely. Both K and
Na forms are plausible, but with only 0.4 mM K
+
and
1.4 mM Na
+
present in the sample solution, they would
account for only a fraction of the sulfate species. This leaves
MgSO
4
and CaSO
4
as the most probable phases present in
the soil.
[15] In order to further constrain the sulfate mineral phase
(s) present in the soil samples, we performed equilibrium
calculations using Geochemist’s Workbench (GWB)®. To
obtain the measured concentrations observed for the WCL
analysis [Hecht et al., 2009; Kounaves et al., 2010] using
GWB requires that SO
4
2−
was initially <0.1 mM and was
dissolving during the analysis at a rate greater than the
addition of the BaCl
2
. If [SO
4
2−
]≥6 mM, then [Ca
2+
] and
[Mg
2+
] would need to be ≥3mMor≥7 mM, respectively,
concentrations 3–10 times greater than actually measured.
The GWB model calculations show that the addition of
Table 1. Summary of Results for All WCL Cells
a
WCL
Cell
Sol(s)
Analyzed Sample
SO
4
2−
in
Solution
(mM)
%asSO
3
in Soil
0 30, 34 Rosy Red 4.8 (±1.5) 1.0 (±0.3)
1 41, 43 Sorceress‐1 n.p.
b
n.p.
b
2 107, 116 Sorceress‐2 5.9 (±1.5) 1.2 (±0.3)
3 96 Blank ‐‐
a
Concentration in soil assumes delivery of a 1 g sample with a density of
1 g/cm
3
.
b
Barium sensor failed thus sulfate analysis was not possible.
Table 2. Concentration of Species Likely Present in Solution
After Adding 1 g of the Phoenix Mars Soil into 25 mL of Pure
H
2
O, and the Amounts of the Minerals or Species Required in the
Soil to Give the Measured and Calculated Ionic Concentrations
a
Species
Equilibrium
Concentration
in Solution
(mM)
Concentration
in Soil
(wt %)
CaCO
3
(calcite) Saturated 3 −5
b
MgCO
3
(magnesite) Saturated ≥1.8
c
MgSO
4
(epsomite) Dissociated 3.3
d
ClO
4
−
2.5 0.6
Na
+
1.4 0.08
Cl
−
0.40 0.04
K
+
0.40 0.04
Mg
2+
6.4 –
SO
4
2−
3.9 –
HCO
3
−
5.4 –
MgSO
4
(aq) 1.2 –
Ca
2+
0.75 –
CaSO
4
(aq) 0.17 –
a
Equilibrium calculated using GWB React at 7°C and a 4 mbar CO
2
headspace. Composition differs from that previously reported in that it
corrects for BaCl
2
leakage. Addition of Ba
2+
precipitated SO
4
2−
and
shifted the equilibrium to values different than if the soil had been added
to pure water. The rate of Ba
2+
addition appears to have been sufficient
in all analyses to maintain [SO
4
2−
] < 0.5 mM and fully dissociate all SO
4
2−
.
b
As determined by TEGA and WCL.
c
Minimum required by model to give saturated Mg
2+
in 25 mL of
solution.
d
Equivalent to 5.3 mM total SO
4
2−
in solution. At such concentrations,
other hydrates give similar values.
KOUNAVES ET AL.: SULFATE AT THE PHOENIX LANDING SITE L09201L09201
3of5
BaCl
2
during the sol, coupled with the dissolution of SO
4
2−
,
would result in an increase in [Mg
2+
] and a decrease in
[Ca
2+
] only if a MgSO
4
phase was being added to the WCL
solution. This was clearly observed during the sol 107 anal-
ysis, though present but less so, during the sol 30 and
41 analyses [Kounaves et al., 2010]. The addition of soluble
CaSO
4
would have caused an increase in [Ca
2+
] and no
change in [Mg
2+
], which is not observed. This result suggests
that a major fraction of SO
4
2−
was added as a MgSO
4
phase.
[16] TEGA and WCL results suggest that the soil may
have been wet in the past because the relatively large
quantity of carbonate detected is difficult to form under dry
conditions [Boynton et al., 2009; Kounaves et al., 2009b]. If
the soil was once wet, then salts deposited from evaporating
the WCL solution (Table 2) could provide a guide to minerals
present in the soil. Evaporation models over temperature
ranges of 0–25°C and partial pressures of CO
2
(P
CO2
)of
0.004‐1 atm, showed that the evaporites are dominated by
calcite (CaCO
3
), magnesite (MgCO
3
), epsomite (MgSO
4
·
7H
2
O), gypsum (CaSO
4
·2H
2
O), KClO
4
, and NaClO
4
.
Depending on the process of evaporation, T, and P
CO2
,
epsomite exceeds gypsum precipitation by 3 times to 3 orders
of magnitude. We have not considered the formation of
phases at temperatures of <0°C, but this would result in other
possible species such as meridianite (MgSO
4
· 11H
2
O)
[Marion et al., 2010]. While by themselves these results
show only the possible candidates for the hydrated sulfate
phases, these equilibrium model calculations are consistent
with the current chemical and mineralogical data obtained
by other landers and orbiters.
[17] The presence of soluble sulfate at the Phoenix
landing site has implications for the geochemical history
and potential past habitability of Mars. With the gypsum
dune fields and the edge of the polar ice cap only 700 km
to the north, and the Alba Patera volcano 1700 km to the
south, the Phoenix site is located between significant sources
of SO
x
and H
2
O. Nearby volcanic eruptions could have
provided large quantities of SO
2
and H
2
S which would have
been atmospherically oxidized to H
2
SO
4
[Settle, 1979] and
that would have reacted with carbonates and other minerals to
produce CaSO
4
and MgSO
4
. Subsequently, if liquid water
ever occurred, such minerals may have undergone trans-
formations through aqueous speciation. Alternatively, the
sulfates may have been brought to the Phoenix site by
wind‐blown dust, after a volcanic or aqueous origin else-
where on Mars.
[18] The findings of the Phoenix WCL, and the levels of
the dominant salts specifically, have a direct bearing on the
question of whether under the right conditions, water activity
(a
H2O
) on Mars could have been sufficient to support life.
Previous calculations of the maximum water activity at
Meridiani Planum and other sites where salts precipitated
from martian brines, indicate that it was often below levels
tolerated by any known terrestrial organisms [Tosca et al.,
2008]. Table 2 shows our best estimate for the chemical
composition of a solution consisting of 1 g of the average
Phoenix site soil in 25 mL pure H
2
O. These values were used
with the GWB React software package to calculate the pre-
cipitation of minerals and water activity (a
H2O
) as a function
of decreasing water on evaporation. Figure 2 shows the
results for the evaporation of water with the concentrations
given in Table 2. Calcite, hydromagnesite, and KClO
4
, pre-
cipitate at a water activity (a
H2O
) > 0.97, while epsomite
precipitates at ≈0.9, gypsum at ≈0.78, and Mg(ClO
4
)
2
·6H
2
O
at ≈0.55. In the past, changes in the obliquity and longitude
of perihelion of Mars have caused summer surface tem-
peratures to exceed 273 K at the latitude of Phoenix
[Richardson and Michna, 2005]. Thus, our findings suggest
that if a small portion of the Phoenix soil was wetted, for
example by a melting snowpack [Clow, 1987; Christensen,
2003], the water activity for a dilution of greater than
∼0.015 g H
2
O/g soil (assuming no aqueous interactions
with the soil and already fully hydrated salts) would be in
the habitable range (a
H2O
≥0.75) of terrestrial halophilic
microbes [Grant, 2004].
[19]Acknowledgments. WethankallwhohelpedmaketheWCL
experiments and science possible, C.A. Cable, B. Comeau, A. Fisher,
P. Grunthaner, P‐C. Hsu, S. R. Lukow, J‐M. Morookian, R.V. Morris,
S. Stroble, X. Wen, S. West, S.M.M. Young, the Phoenix Robotic Arm
team, and other contributors to MECA and Phoenix. We’dalsoliketo
thank J. L. Bishop, W. V. Boynton, D. C. Golden, and B. Sutter for their
comments on the manuscript. The Phoenix Mission was led by P. Smith
(PI) at the University of Arizona, Tucson, on behalf of NASA and was
managed by NASA’s Jet Propulsion Laboratory, California Institute of
Technology, Pasadena, California. The spacecraft was developed by
Lockheed Martin Space Systems, Denver, Colorado.
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KOUNAVES ET AL.: SULFATE AT THE PHOENIX LANDING SITE L09201L09201
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