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Soluble sulfate in the Martian soil at the Phoenix landing site

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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 . If the soil had been wet in the past, 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.
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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 Xray 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 NASAs Phoenix Mars
Lander was to determine soluble sulfate in the martian soil.
We report here the first insitu 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 WCLderived 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 Xray 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 kmdeep sulfaterich
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 Mgsulfates [Yen et al., 2005; Wang et al.,
2006; Gellert et al., 2004] as well as other soils containing
mixtures of Fe,Ca, and Mgsulfates [Johnson et al., 2007;
Yen et al., 2008; Ming et al., 2006]. The MER MiniTES 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 NASAs 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 (cell0, surface, Rosy Red), sol 41 (cell1,
subsurface, Sorceress1), and sol 107 (cell2, subsurface,
Sorceress2), where solmeasures 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 Viking2 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.
00948276/10/2010GL042613
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 BaISE 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 cell0,
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 (cell3) on sol 96, the soil
became lodged on top of the delivery funnel and none was
dispensed into the solution. The analysis run on cell3 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 cell1 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 presample
calibration periods. The barium chloride leakage was not seen
in preflight 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 Redsample
in cell 0 on sols 30 and 34, (b) Sorceress2 sample in cell 2
on sols 107 and 116, and (c) blankin cell 3 on sol 96. In
all cases its clear that although BaCl
2
was being added to
the solution, the concentration of Ba
2+
remained at or below
its 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 cell0 (sols 30 and 34) and cell2 (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 cell0 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 cell2 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 nonsoluble, 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
35% 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 Fesulfates are
eliminated since preflight 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, Fesulfates 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 Fesulfate
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 Geochemists 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 3mMor7 mM, respectively,
concentrations 310 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 Sorceress1 n.p.
b
n.p.
b
2 107, 116 Sorceress2 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 025°C and partial pressures of CO
2
(P
CO2
)of
0.0041 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
windblown 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, PC. Hsu, S. R. Lukow, JM. 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. Wedalsoliketo
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 NASAs 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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... Cette observation est encore une fois cohérente avec le bilan de masse car CheMin n'a détecté aucune phase cristalline riche en soufre. Les candidats soufrés les plus plausibles et les plus couramment observés dans les sols martiens correspondraientà des sulfates Mg/Fe/Ca/Al (e.g., Wang et al., 2006b;Kounaves et al., 2010).À partir de la dispersion tir-à-tir de l'aire du pic du soufreà 564 nm, il apparaît que la présence de sulfate de magnésium soit majoritaireétant donné que les points riches en soufre sont associésà de fortes concentrations en Mg (Figure 3.22.a). Des sulfates de calcium semblentégalement présents (Figure 3.22.b) ...
... En revanche, tous les sulfates de calcium se décomposentà des températures trop Tel que décrit dans la section 1.3.3, les sulfates de magnésium en particulier, ainsi que des sulfates de calcium et de fer, ontété abondamment détectés dans les sols de Mars (e.g., Soderblom et al., 2004;Wang et al., 2006b;Kounaves et al., 2010;Karunatillake et al., 2014;Hood et al., 2019). Nos résultats tendentà montrer que les solsà différentes localisations sur la planète possèdent effectivement des similitudes de composition chimique, maiségalement d'ordre minéralogique, et que les sols de Gale n'y font pas exception. ...
... Although the goal of our experiments is not to reproduce the martian soil mineralogy, magnesium sulfate 286 was used because it was observed in other martian soils (e.g., Kounaves et al., 2010;Soderblom 287 et al., 2004;Wang et al., 2006) and in Gale crater bedrock (e.g., Fraeman et al., 2016; 288 , and consequently could be a potential candidate for amorphous phases. Magnesium 289 sulfates are also easily soluble in water, facilitating the production of coating for our experiments (see 290 section 2.1.2). ...
Thesis
Les conditions paléo-environnementales de la surface de Mars sont accessibles à travers les enregistrements géologiques de la planète, et plus particulièrement via les minéraux d'altération qu'ils contiennent. Ces phases secondaires peuvent constituer de véritables marqueurs géochimiques des conditions durant l'altération aqueuse. L'instrument ChemCam, à bord du rover Mars Science Laboratory (Curiosity) est le premier instrument de spectroscopie sur plasma induit par laser déployé à la surface de Mars. Il offre un moyen inédit de caractériser la géochimie des roches du cratère Gale, à une échelle d'analyse submillimétrique. Ce travail de thèse s'intéresse à la caractérisation des phases d'altération ainsi qu'aux processus responsables de leur formation dans les sols et les roches sédimentaires de Gale. L'approche utilisée combine des expériences de laboratoire avec une réplique de ChemCam ainsi que l'interprétation des données de l'instrument de vol. Ces études ont permis de tester la capacité de ChemCam à identifier des argiles dans les roches sédimentaires et être ainsi un outil diagnostique de ces phases. Elles ont également aidé à mieux comprendre les paramètres influençant la mesure LIBS dans les milieux granulaires, ainsi qu'à montrer que les sulfates de magnésium amorphes pourraient être des phases majeures dans les sols martiens et responsables de leur hydratation. Par ailleurs, nous avons également proposé une nouvelle quantification du fer pour les données ChemCam, qui a permis d'étudier une structure géomorphologique particulière de Gale. Cette dernière est associée à une forte signature spectrale d'hématite depuis l'orbite. L'étude de la variabilité en fer sur ce terrain a permis de mieux appréhender son mécanisme de formation, en mettant en évidence une mobilité de cet élément durant la diagénèse, en lien avec des processus d'oxydo-réduction. Ces résultats indiquent que les sédiments de Gale ont subi une histoire géologique post-dépôt complexe, impliquant probablement de multiples épisodes aqueux aux propriétés différentes.
... Another problem appears for hydraulic structures and irrigation channels which are presented by the leaching problem for gypseous soil below these structures [4]. The presence of water in these structures and the difference in heads between the upper and lower stream of dam structures causes seepage of water, also it causes washing of the dissolved gypsum between soil particles [5]. This washing process led to what we call cavities, inside the soil mass below the heavy hydraulic structure which is filled with water and the dissolved gypsum. ...
... With continuous feeding of water, the excitation of saturated gypseous soil model rearrange soil particles and increase footing settlement for soaking and leaching tests in addition that the pores between soil particles increase with continuous flooding of water because gypsum dissolution causes sudden collapse of gypseous soil. Figure 5 shows the hydraulic conductivity (k) relation with leaching time for the gypseous soil model subjected to different vibration time amplitudes (5,10,15,20,25, and 25 s). At the beginning of the leaching test, high values of coefficient of permeability were recorded this may be due to the pressure release of collected water from the previous soaking stage and not from chemical changes .An increase in shaking time for the laboratory model subjected to a dynamic vibration reduces permeability due to an increase in the pore water pressure for saturated gypseous soil due to the disturbance movement of particles inside soil specimen so the permeability decreases. ...
Article
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This study is concerned with the soaking and leaching of gypseous soil at both static and dynamic conditions. The soil used was Natural gypseous soil with 50% gypsum. Three parameters were studied (deformation ratio, dissolve gypsum salts, and hydraulic conductivity) in both static and dynamic conditions..20 tests were carried using laboratory model. A platform base connected to the loading frame was designed in a manner that allows free movement provided for dynamic test, as an earthquake. Results of experimental work revealed that the deformation ratio S/B (settlement /footing width) for the sample subjected to 10 seconds vibration was twice that of sample without vibration, while the deformation ratio was 15 times that of the sample without vibration when subjected to 30 seconds. On the other hand, 70% of hydraulic conductivity was achieved at the first 10 minutes of leaching for the model subjected to 30 seconds of vibration. That reflects the effect of earthquakes on structures constructed on such problematic soil.
... Other types of hydrated sulfates Lichtenberg et al. 2010), especially Fe sulfates (e.g., jarosite, hydroxylated ferric sulfates, and szomolnokite) and Al sulfates (alunite), have been found by remote sensing in low abundance and/or in localized small areas. Nevertheless, in situ measurements performed at all landing sites have revealed Mg, Ca, and Fe 3+ sulfates, either in outcrops or throughout the subsurface regoliths (e.g., McSween et al. 1999;Squyres et al. 2004;Wang et al. 2006;Arvidson et al. 2010;Kounaves et al. 2010;Vaniman et al. 2013). ...
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An essential part of the Exomars 2022 payload is the Mars Multispectral Imager for Subsurface Studies (Ma_MISS) experiment hosted by the drill system. Ma_MISS is a visible and near-infrared (0.4–2.3 μ m) miniaturized spectrometer with an optical head inside the drill tip capable of observing the drill borehole with a spatial resolution of 120 μ m. Here we report on how the Ma_MISS hyperspectral information provides in situ investigation of the subsurface at very fine resolution, prior to the collection of the samples that will be manipulated and crushed for further analysis by the analytical laboratory on the rover. Ma_MISS is the instrument that will closely investigate the subsurface mineralogical characteristics in its original geologic context at depths never reached before in Mars exploration. Ma_MISS recognizes all the major spectral features of the clays, basaltic, and minor phases expected at the ExoMars landing site, Oxia Planum. The high spatial resolution on the borehole wall is such that single grains of about 100 μ m can be distinguishable in the assemblage of minerals observed by Ma_MISS. The spatial distribution of the mineralogies within the borehole walls is associated with the rocks and the processes that put these materials in place and possibly altered them with time, characterizing the habitats found in the stratigraphic record, indicating which ones are the most suitable to have held or to be holding nowadays traces of life.
... Various types of salts have been detected on Mars and in Martian meteorites. Among those are sulfates [40], nitrates [41,42], bromides [43], chlorides [6], chlorates [42], and perchlorates [6]. The latter two are of special interest for this study as natural occurrences of these two salt types are relatively rare on Earth and are restricted to hyperarid environments [44,45], while perchlorates are known to occur in higher concentrations and widely distributed only on Mars [6,46]. ...
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The availability of liquid water is a prerequisite for all lifeforms on Earth. In hyperarid subzero environments like the Dry Valleys in Antarctica or the near-subsurface of Mars liquid water might be provided temporarily by hygroscopic substances that absorb water from the atmosphere and lower the freezing point of water. To evaluate the potential of hygroscopic compounds to serve as a habitat, it is necessary to explore the microbial tolerances towards these substances and their life-limiting properties. Here we present a study investigating the tolerances of the halotolerant yeast Debaryomyces hansenii to various solutes. Growth experiments were conducted via counting colony forming units (CFUs) after inoculation of a liquid growth medium containing a specific solute concentration. The lowest water activities (aw) enabling growth were determined to be ~0.83 in glycerol and fructose-rich media. For all other solutes the growth-enabling aw was higher, due to additional stress factors such as chaotropicity and ionic strength. Additionally, we found that the solute tolerances of D. hansenii correlate with both the eutectic freezing point depressions and the deliquescence relative humidities of the respective solutes. Our findings strongly impact our understanding of the habitability of solute-rich low aw environments on Earth and beyond.
... The depleted concentrations of Fe 2+ , Mg 2+ , and Al 3+ found within these fluids are due to these components being partitioned into mineral phases, such as chlorite for Mg 2+ and nontronite for Fe 2+ . These groundwatertype fluids are different from surface fluids, because the latter would incorporate soluble salts that would influence ion concentrations, such as Mg 2+ , Ca 2+ , and SO 4 2− , which would been concentrated by surface process, such as evaporation (Hecht et al. 2009;Kounaves et al. 2010). These compounds are unlikely to have formed in the subsurface environments modeled in this study and have, therefore, not been considered in our models. ...
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Hydrothermal systems that formed as a result of impact events possess all the key requirements for life: liquid water, a supply of bio-essential elements, and potential energy sources. Therefore, they are prime locations in the search for life on other planets. Here, we apply thermochemical modeling to determine secondary mineral formation within an impact-generated hydrothermal system, using geochemical data returned for two soils on Mars found in regions that have previously experienced alteration. The computed mineral reaction pathways provide a basis for Gibbs energy calculations that enable both the identification of available geochemical energy, obtained from Fe-based redox reactions, that could be utilized by potential microbial life within these environments, and an estimate of potential cell numbers. Our results suggest that water–rock interactions occurring within impact-generated hydrothermal systems could support a range of Fe-based redox reactions. The geochemical energy produced from these reactions would be substantial and indicates that crater environments have the potential to support microbial cell numbers similar to what has been identified in terrestrial environments.
... On early Earth, prior to biological N 2 fixation, nitrates and nitrites formed by lightening may have been the source of N needed for prebiotic chemical evolution [134]. The nitrate ion was sought in the Wet Chemistry Laboratory (WCL) soil water experiment on the Phoenix mission [135,136] but was not found at a detection limit that would be equivalent to 25 mM (if at a 1:1 W/R ratio). However, nitrate has been detected by the Evolved Gas Analyzer (EGA) in certain Gale crater soils, albeit at only~5 µM equivalent (300 ppm), and up to about 3× this Life 2021, 11, 539 11 of 45 amount in some samples [132]. ...
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Citation: Clark, B.C.; Kolb, V.M.; Steele, A.; House, C.H.; Lanza, N.L.; Gasda, P.J.; VanBommel, S.J.; Newsom, H.E.; Martínez-Frías, J. Abstract: Although the habitability of early Mars is now well established, its suitability for conditions favorable to an independent origin of life (OoL) has been less certain. With continued exploration, evidence has mounted for a widespread diversity of physical and chemical conditions on Mars that mimic those variously hypothesized as settings in which life first arose on Earth. Mars has also provided water, energy sources, CHNOPS elements, critical catalytic transition metal elements, as well as B, Mg, Ca, Na and K, all of which are elements associated with life as we know it. With its highly favorable sulfur abundance and land/ocean ratio, early wet Mars remains a prime candidate for its own OoL, in many respects superior to Earth. The relatively well-preserved ancient surface of planet Mars helps inform the range of possible analogous conditions during the now-obliterated history of early Earth. Continued exploration of Mars also contributes to the understanding of the opportunities for settings enabling an OoL on exoplanets. Favoring geochemical sediment samples for eventual return to Earth will enhance assessments of the likelihood of a Martian OoL.
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Perchlorate (ClO4⁻) salts were discovered on Mars and are known to absorb water vapor from the atmosphere and deliquesce into the aqueous phase. Other species such as chlorides (Cl⁻) and chlorates (ClO3⁻) were also identified; these species can affect the deliquescence of perchlorates. Here we generate phase diagrams of perchlorate / chloride and perchlorate / chlorate binary mixtures for K, Na, Mg and Ca in the temperature range 223 – 273 K. Using a new approach based on thermodynamic modelling of evaporation, we determined the deliquescence relative humidity (the minimum relative humidity at which a salt converts into a liquid by absorbing atmospheric water vapor) and the eutonic relative humidity (the minimum relative humidity at which two salts are in equilibrium with liquid) for binary salt mixtures. Our modelling results show that the deliquescence relative humidity values of all salt mixtures is always lower than that of each individual end-member at a fixed temperature, typically a few percent lower. The closer the eutonic is to one of the end-member, the smaller the decrease in relative humidity compared to the pure pole. Thus, only eutonics which are far from both poles exhibit a significant drop in relative humidity. Moreover, the eutonic relative humidity always increases with decreasing temperature, which does not favor liquids in the dry and cold Martian environment. Finally, the increased stability of water ice at the lowest temperatures always reduces or even eliminates the stability of liquids. Therefore, the favorable temperature and relative humidity conditions under which binary salt liquid mixtures exist are generally not significantly improved compared to single salts.
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The formation and stability of brines on the surface of present-day Mars remains an important question to resolve the astrobiological potential of the red planet. Although modeling and experimental work have constrained the processes controlling the stability of single-salt brines exhibiting low freezing temperatures, such as calcium perchlorate, the Martian regolith is far more complex because multiple salts coexist in various concentrations, leading to brines whose behavior remains untested. Here we modeled the stability of complex brines of compositions determined from the Phoenix lander’s Wet Chemistry Laboratory. We find that such brines would form in equilibrium with sodium and magnesium perchlorates, chlorides, and calcium chlorate, but never calcium perchlorate, which has been widely considered as the most likely to produce brines on Mars. Furthermore, we find that only chlorate-rich brines can potentially remain liquid, for small periods of time, at temperatures compatible with those measured by the Phoenix lander. Therefore, liquid brines remain overly unstable under present-day Martian conditions and are unlikely to contribute to surface geomorphological activity, such as recurring slope lineae. In these conditions, of cold and salty brines, the present-day Martian surface remains highly unhabitable.
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Salts and basalt are widespread on the surface of Mars. Therefore, basalt-brine interactions may have significant effects on both the aqueous history of the planet, and near-surface alteration assemblages. Raman spectra were collected from McKinney Basalt samples that were immersed in eight near-saturated brines composed of Na-Cl-H2O, Na-SO4-H2O, Na-ClO4-H2O, Mg-Cl-H2O, Mg-SO4-H2O, and two salt mixtures (Mg-Cl-SO4-H2O and Na-ClO4-SO4-H2O), as well as ultra-pure water for up to one year. Secondary minerals were observed in the Raman specta, including iron oxides, hydrated sulfates, amorphous silica, phosphates, and carbonates. Detection of these secondary minerals demonstrates the utility of Raman spectroscopy to identify basalt-brine alteration assemblages on Mars. This work also demonstrates that major classes of alteration phases can be distinguished using Raman spectra with resolution similar to those expected from the Raman instruments aboard the Perseverance and Rosalind Franklin Mars rovers. In addition, observations of carbonate minerals within alteration assemblages suggest CO2 from the atmosphere readily reacted with ions released from the basalt during alteration in near-saturated brines.
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The study of space has always been a field of great interest and thus space missions are becoming more and more ambitious with time. Therefore, with the 50th anniversary of the first spacecraft to land on Mars, a review about how traditional analytical techniques have been adapted to the era of in situ space exploration is presented. From the Viking Project to the future MMX mission, the techniques used for the in situ study of the geochemistry of the Martian surface is described. These techniques have been differentiated according to the type of analysis: elemental and molecular. On the one hand, among the elemental analytical techniques the XRF, APXS, ISE and LIBS stand out. On the other hand, GCMS, TEGA, MBS, XRD, Raman and IR spectroscopy have been the molecular techniques used in the missions to Mars. Miniaturization, real-time measurements, automation, low power consumption and reliability of operation under extreme conditions are some of the major challenges that analytical chemistry has faced as a result of the technological and scientific requirements of space missions. In this way, this review gathers all the in situ analytical techniques that have reached the surface of Mars onboard landers or rovers with the aim of studying its geochemistry.
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Chemical analyses of three Martian soil samples were performed using the Wet Chemistry Laboratories on the 2007 Phoenix Mars Scout Lander. One soil sample was obtained from the top ˜2 cm (Rosy Red) and two were obtained at ˜5 cm depth from the ice table interface (Sorceress 1 and Sorceress 2). When mixed with water in a ˜1:25 soil to solution ratio (by volume), a portion of the soil components solvated. Ion concentrations were measured using an array of ion selective electrodes and solution conductivity using a conductivity cell. The measured concentrations represent the minimum leachable ions in the soil and do not take into account species remaining in the soil. Described is the data processing and analysis for determining concentrations of seven ionic species directly measured in the soil/solution mixture. There were no significant differences in concentrations, pH, or conductivity, between the three samples. Using laboratory experiments, refinement of the surface calibrations, and modeling, we have determined a pH for the soil solution of 7.7(±0.3), under prevalent conditions, carbonate buffering, and PCO2 in the cell headspace. Perchlorate was the dominant anion in solution with a concentration for Rosy Red of 2.7(±1) mM. Equilibrium modeling indicates that measured [Ca2+] at 0.56(±0.5) mM and [Mg2+] at 2.9(±1.5) mM, are consistent with carbonate equilibrium for a saturated solution. The [Na+] and [K+] were 1.4(±0.6), and 0.36(±0.3) mM, respectively. Results indicate that the leached portion of soils at the Phoenix landing site are slightly alkaline and dominated by carbonate and perchlorate. However, it should be noted that there is a 5-15 mM discrepancy between measured ions and conductivity and another species may be present.
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The Mars Exploration Rover Spirit analyzed multiple occurrences of sulfur-rich, light-toned soils along its traverse within Gusev Crater. These hydrated deposits are not readily apparent in images of undisturbed soil but are present at shallow depths and were exposed by the actions of the rover wheels. Referred to as “Paso Robles” class soils, they are dominated by ferric iron sulfates, silica, and Mg-sulfates. Ca-sulfates, Ca-phosphates, and other minor phases are also indicated in certain specific samples. The chemical compositions are highly variable over both centimeter-scale distances and between the widely separated exposures, but they clearly reflect the elemental signatures of nearby rocks. The quantity of typical basaltic soil mixed into the light-toned materials prior to excavation by the rover wheels is minimal, suggesting negligible reworking of the deposits after their initial formation. The mineralogy, geochemistry, variability, association with local compositions, and geologic setting of the deposits suggest that Paso Robles class soils likely formed as hydrothermal and fumarolic condensates derived from magma degassing and/or oxidative alteration of crustal iron sulfide deposits. Their occurrence as unconsolidated, near-surface soils permits, though does not require, an age that is significantly younger than that of the surrounding rocks.
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The Wet Chemistry Labs on Phoenix gave pH, conductivity, ions, and evidence that the salts have previously interacted with water and contain high levels of carbonates. Carbonate results show the need for more extensive laboratory work and equilibrium modeling.
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[1] Juventae Chasma contains four light-toned sulfate-bearing mounds (denoted here as A–D from west to east) inside the trough, mafic outcrops at the base of the mounds and in the wall rock, and light-toned layered deposits of opal and ferric sulfates on the plateau. Hyperspectral visible/near-infrared Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) spectra were used to identify monohydrated and polyhydrated sulfate (PHS) outcrops of layered material on the bright mounds. Most of the monohydrated sulfate signatures closely resemble those of szomolnokite (FeSO4·H2O), characterized by a water band near 2.08 μm, while some areas exhibit spectral features more similar to those of kieserite (MgSO4·H2O), with a band centered closer to 2.13 μm. The largest PHS outcrops occur on the top of mound B, and their spectral features are most consistent with ferricopiapite, melanterite, and starkeyite, but a specific mineral cannot be uniquely identified at this time. Coordinated analyses of CRISM maps, Mars Orbiter Laser Altimeter elevations, and High Resolution Imaging Science Experiment images suggest that mounds A and B may have formed together and then eroded into separate mounds, while mounds C and D likely formed separately. Mafic minerals (low-Ca pyroxene, high-Ca pyroxene, and olivine) are observed in large ∼2–10 km wide outcrops in the wall rock and in smaller outcrops ∼50–500 m across at the floor of the canyon. Most of the wall rock is covered by at least a thin layer of dust and does not exhibit strong features characteristic of these minerals. The plateau region northwest of Juventae Chasma is characterized by an abundance of light-toned layered deposits. One region contains two spectrally unique phases exhibiting a highly stratified, terraced pattern. CRISM spectra of one unit eroded into swirling patterns with arc-like ridges exhibit a narrow 2.23-μm band assigned to hydroxylated ferric sulfate. A thin layer of a fractured material bearing an opaline silica phase is observed at the contact between the older plateau unit and the younger hydroxylated ferric sulfate-bearing light-toned layered deposits. Hydrothermal processes may have produced an acidic environment that fostered formation of the hydrated silica and hydroxylated ferric sulfate units.
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The Mössbauer (MB) spectrometer on Opportunity measured the Fe oxidation state, identified Fe-bearing phases, and measured relative abundances of Fe among those phases at Meridiani Planum, Mars. Eight Fe-bearing phases were identified: jarosite (K,Na,H3O)(Fe,Al)(OH)6(SO4)2, hematite, olivine, pyroxene, magnetite, nanophase ferric oxides (npOx), an unassigned ferric phase, and metallic Fe (kamacite). Burns Formation outcrop rocks consist of hematite-rich spherules dispersed throughout S-rich rock that has nearly constant proportions of Fe3+ from jarosite, hematite, and npOx (29%, 36%, and 20% of total Fe). The high oxidation state of the S-rich rock (Fe3+/FeT ~ 0.9) implies that S is present as the sulfate anion. Jarosite is mineralogical evidence for aqueous processes under acid-sulfate conditions because it has structural hydroxide and sulfate and it forms at low pH. Hematite-rich spherules, eroded from the outcrop, and their fragments are concentrated as hematite-rich soils (lag deposits) on ripple crests (up to 68% of total Fe from hematite). Olivine, pyroxene, and magnetite are primarily associated with basaltic soils and are present as thin and locally discontinuous cover over outcrop rocks, commonly forming aeolian bedforms. Basaltic soils are more reduced (Fe3+/FeT ~ 0.2-0.4), with the fine-grained and bright aeolian deposits being the most oxidized. Average proportions of total Fe from olivine, pyroxene, npOx, magnetite, and hematite are ~33%, 38%, 18%, 6%, and 4%, respectively. The MB parameters of outcrop npOx and basaltic-soil npOx are different, but it is not possible to infer mineralogical information beyond octahedrally coordinated Fe3+. Basaltic soils at Meridiani Planum and Gusev crater have similar Fe-mineralogical compositions.
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