Earth and Planetary Science Letters 312 (2011) 371–377
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Earth and Planetary Science Letters
journal homepage: www.elsevier.com/locate/epsl
Laboratory studies of perchlorate phase transitions: Support for metastable aqueous
perchlorate solutions on Mars
R.V. Gough a,⁎, V.F. Chevrier b, K.J. Baustian a, M.E. Wise a, M.A. Tolbert a
a
b
Department of Chemistry and Biochemistry and Cooperative Institute for Research in Environmental Sciences (CIRES), 216 UCB, University of Colorado, Boulder, CO 80309, USA
W. M. Keck Laboratory for Space and Planetary Simulation, Arkansas Center for Space and Planetary Science, University of Arkansas, Fayetteville, AR, USA
a r t i c l e
i n f o
Article history:
Received 28 March 2011
Received in revised form 12 October 2011
Accepted 13 October 2011
Available online 22 November 2011
Editor: T. Spohn
Keywords:
Mars
water
perchlorate
deliquescence
efflorescence
Phoenix
a b s t r a c t
Perchlorate salts, recently discovered on Mars, are known to readily absorb water vapor from the atmosphere
and deliquesce into the aqueous phase at room temperature. Here we study the deliquescence (crystalline
solid to liquid transition) and efflorescence (liquid to crystalline solid transition) of perchlorate salts at low
temperatures relevant to Mars. A Raman microscope and environmental cell were used to determine the deliquescence relative humidity (DRH) and efflorescence relative humidity (ERH) of NaClO4 and Mg(ClO4)2 as a
function of temperature and hydration state. We find that the deliquescence of anhydrous NaClO4 is only
slightly dependent on temperature and occurs at ~ 38% RH. The DRH of NaClO4·H2O increases with decreasing
temperature from 51% at 273 K to 64% at 228 K. The DRH of Mg(ClO4)2·6H2O also increases with decreasing
temperature from 42% at 273 K to 64% at 223 K. The efflorescence of both NaClO4 and Mg(ClO4)2 salt solutions occurs at a lower RH than deliquescence due to the kinetic inhibition of crystallization. For all temperatures studied, the ERH values of NaClO4 and Mg(ClO4)2 are 13% and 19%, respectively. These results indicate
perchlorate salts can exist as metastable, supersaturated solutions over a wide range of RH and temperature
conditions. Summer diurnal temperature and relative humidity cycles at low latitudes on Mars could allow
the surface salts to be aqueous for several hours per day.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
The recent discovery of ~0.5% perchlorate (ClO4−) in the regolith of
the Martian northern plains by the Wet Chemistry Laboratory (WCL)
on board the Phoenix lander (Hecht et al., 2009) has sparked interest
due to the unique physical and chemical properties of the salt. The eutectic temperature (TE) of perchlorate solutions, defined as the lowest
temperature at which aqueous solutions are stable, is 236 K for sodium
perchlorate and 206 K for magnesium perchlorate, the lowest of known
salts on Mars (Chevrier et al., 2009; Marion et al., 2010; Pestova et al.,
2005). Previous analyses of perchlorate phase transitions on Mars and
the stability of potential perchlorate salt solutions have been based on
equilibrium thermodynamics (Chevrier et al., 2009; Marion et al.,
2010), usually through melting of ice-salt mixtures. Alternatively, perchlorate salts are deliquescent, forming a liquid solution by absorption
of atmospheric water vapor by the crystalline salt. It has been suggested
that deliquescence of perchlorate salts on Mars was observed by the
Phoenix lander (Renno et al., 2009; Smith et al., 2009). This phase transition occurs when the relative humidity (RH) is equal to or greater than
the deliquescence relative humidity (DRH) of the salt. Values of DRH for
inorganic salts are below 100% RH. For example, at 293 K the DRH of
⁎ Corresponding author. Tel.: + 1 303 492 1433; fax: + 1 303 492 1149.
E-mail address: [email protected] (R.V. Gough).
0012-821X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.epsl.2011.10.026
NaCl is 75% (Cohen et al., 1987) and the DRH of Na2SO4 is 84% (Tang
and Munkelwitz, 1994).
Studies of Mg(ClO4)2 deliquescence have been performed only at
room temperature. DRH values of 50% were measured by both Besley
and Bottomley (1969) and Pestova et al. (2005), although both studies were performed indirectly, by measuring the vapor pressure of a
saturated salt solution rather than by deliquescing salts with water
vapor. Experimental studies of NaClO4 deliquescence conducted at
298 K have measured DRH values of 43% RH (Zhao et al., 2005) and
46% RH (Zhang et al., 2005). Because the DRH of salts often varies
with temperature (Greenspan, 1977; Tang and Munkelwitz, 1994),
it is important to study perchlorate deliquescence at a range of temperatures relevant to the Martian surface. Recently, Zorzano et al.
(2009) reported that under Mars-like conditions (T = 225 K) NaClO4
deliquesced at DRH ~ 37% although this study reported only one
data point.
Moreover, neither the deliquescence of NaClO4 hydrates nor of Mg
(ClO4)2 has been experimentally studied at low temperatures. As the
WCL results suggest the Mg 2 +/Na + ratio at the Phoenix landing site
is ~3/1 (Hecht et al., 2009) or perhaps as high as ~ 5/1 (Kounaves et
al., 2010), it is particularly important to understand the low temperature deliquescence of Mg(ClO4)2·nH2O.
The reverse process, crystallization of a salt solution when the RH
of the system is lowered, is called efflorescence. The efflorescence relative humidity (ERH) is generally lower than the DRH for a given salt
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at constant temperature. This hysteresis effect is widely known and
exists for most inorganic salts (Martin, 2000). Although deliquescence occurs as soon as the free energy of the crystalline and aqueous
phases are equal (ie: as soon as the transition is thermodynamically
favorable), salt crystallization is kinetically hindered, allowing metastable supersaturated salt solutions to exist for significant time periods at RH b DRH. Such metastable solutions could be a source of
liquid water on Mars, and therefore require laboratory study to determine conditions of formation and metastability.
There have been few experimental studies of the efflorescence of
aqueous perchlorate solutions and they have all been performed at
298 K. ERH values of 11% (Zhang and Chan, 2003) and 20% (Zhang
et al., 2005) have been measured for NaClO4, There has been only
one study of the efflorescence of aqueous Mg(ClO4)2 solutions reporting an ERH range of 10–18% at room temperature (Zhang and Chan,
2003). No experimental work has been done to determine the ERH
of any perchlorate salt at lower temperatures, and there is a scarcity
of data regarding the temperature dependence of efflorescence of
salts in general.
As both the DRH and TE of salts can decrease with decreasing hydration state (Cohen et al., 1987), it is essential to understand how
the number of waters of hydration can affect the deliquescence of perchlorate salts. Therefore, we studied the deliquescence and efflorescence of selected hydration states of sodium and magnesium
perchlorate salts using Raman microscopy to experimentally determine the DRH and ERH under a range of low temperatures relevant
to Mars (223–273 K).
2. Experimental methods
2.1. Selected samples
Under Martian surface conditions, the dominant Mg(ClO4)2 phase
is predicted to be the hexahydrate (Chevrier et al., 2009; Robertson
and Bish, 2010) as temperatures higher than 400 K are needed for dehydration of Mg(ClO4)2·6H2O to occur (Dobrynina et al., 1980).
Therefore, the hexahydrate was the Mg(ClO4)2 salt studied in this
work (Mg(ClO4)2·6H2O, purchased from Sigma Aldrich, 99% pure).
Thermodynamic calculations suggest mono- and dihydrated salts
are the NaClO4 phases most relevant to Mars (Chevrier et al., 2009).
However, the synthesis of NaClO4·2H2O presents an experimental
challenge because it is stable only below 258 K. Therefore, we chose
to study NaClO4·H2O (purchased from Aldrich, 99% pure) and anhydrous NaClO4 (purchased from Sigma Aldrich, >98% pure) salts.
2.2. Environmental cell and Raman microscope
The experimental system used here to study perchlorate deliquescence and efflorescence has been previously described in detail by
Baustian et al. (2010) and is shown schematically in Fig. S1. Briefly,
a Nicolet Almega XR Dispersive Raman spectrometer was outfitted
with a Linkam THMS600 environmental cell (Fig. S2), a Linkam automated temperature controller, and a Buck Research chilled-mirror
hygrometer. Changes in phase or hydration state can be monitored visually with the optical microscope or spectrally using Raman
spectroscopy.
A salt sample was deposited onto a hydrophobic quartz disk and
placed directly on a silver block which can be cooled to a desired temperature with a continuous flow of liquid nitrogen. The quartz disk
and perchlorate sample were maintained at a constant temperature
throughout all experiments. A platinum resistance sensor within the
silver block monitors the sample temperature to ±0.1 K. The quartz
disk and silver block are placed inside the environmental cell, which
is mounted on a high precision motorized microscope stage that sits
within the microscope.
Highly pure N2 is continuously flowed through the cell. The N2
may be humidified by running a separate stream of N2 through a
glass frit submerged in deionized H2O. Before any water vapor has
been added, the cell is very dry (RH b 1%) and error introduced by
background water vapor is very small. Variable RH values inside the
cell are created by changing the humidified flow relative to the dry
N2 flow. A diaphragm pump pulling at a rate of 1 L/min is attached
to the outlet of the hygrometer ensuring a constant airflow through
the cell regardless of any variability in flow rate through the H2O
vapor bubbler. During an experiment, H2O vapor is increased or decreased stepwise (at intervals less than or equal to 1% RH). RH was
varied slowly (1%/min) and then held at each value for several minutes until the vapor flow through the cell is constant and the
Raman spectra do not change. Therefore we are confident in each individual ERH and DRH value to within ~ 1% RH. Error in the experimental data due to reproducibility of results is slightly larger.
The humidity is monitored using a CR-1A chilled-mirror hygrometer attached to the cell outlet. The flow rates through the cell to the
hygrometer are such that a given volume of air travels from cell to hygrometer in ~1 s, which is a much shorter time scale than the humidity is varied (~1% RH per minute). Therefore, we are confident that
the humidity inside the cell is being accurately measured in real
time. The hygrometer can measure frost points as low as 120 K with
an accuracy of ±0.15 K. Frost point measurements from the hygrometer and sample temperature measurements from the platinum resistance sensor allow us to determine the RH with respect to the sample.
Although the instrumental error of the hygrometer is ±0.15% RH,
any error present in the measurement of the sample temperature is
likely to be the largest source of error in the calculated relative humidity. To quantify this error, we perform a temperature calibration
as described in detail by Baustian et al. (2010). This type of calibration
is repeated routinely and is used to determine an experimental temperature error of up to ±0.5 K. This measurement error translates
into an error of up to ±2% RH for the range of conditions present in
our experiments.
The liquid N2 lines have been insulated and the cell walls are at
room temperature, ensuring that the sample is the coldest point
within the cell. The RH will therefore always be highest with respect
to the sample, minimizing adsorption of H2O onto other surfaces. In
this paper, all reported RH values represent the relative humidity
with respect to liquid water at the temperature of the salt sample.
The Raman spectrometer has two lasers (532 and 780 nm) that
can be used to probe particles. For all experiments performed in this
study, the 532 nm laser was used to collect spectral information
(resolution = 2 cm − 1). The Raman vibrational spectra obtained
allow for molecular identification of individual particles as small as
1 μm in diameter. In this study, Raman spectra were used to detect
perchlorate phase changes (deliquescence and efflorescence), quantify the corresponding DRH and ERH, and determine the hydration
state of crystalline salts. An optical microscope (Olympus BX51)
with 10, 20, 50 and 100× magnification capabilities could also be
used to directly observe phase transitions.
3. Determination of DRH and ERH
Fig. 1 shows the spectral and morphological changes of an anhydrous NaClO4 particle as varying RH causes deliquescence (A, B, C)
and efflorescence (D, E, F) to occur. The temperature was held constant at 243 K throughout this experiment. The optical observations
of a single particle of anhydrous NaClO4 (inset, 50× magnification)
show how the crystalline particle present under “dry” conditions
(2% RH) becomes a spherical droplet at 44% RH indicating deliquescence into a saturated aqueous salt solution. As the RH is lowered,
the spherical droplet crystallizes into a solid particle with an asymmetric shape at 12% RH.
R.V. Gough et al. / Earth and Planetary Science Letters 312 (2011) 371–377
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Fig. 1. Raman spectra and optical microscope images illustrating the deliquescence (A, B, C) and efflorescence (D, E, F) of an anhydrous NaClO4 particle at 243 K. Each image is 40 μm
wide. As the RH in the environmental cell increases or decreases, spectral changes appear in the O–H stretching vibration region (broad peak at ~ 3500 cm− 1) and the ClO4− symmetric vibration region (~ 950 cm− 1). When deliquescence occurs at 44% RH (C), the broad O–H stretch appears and the ClO4− symmetric vibration shifts to smaller wave numbers,
both indicative of the formation of an aqueous solution. When efflorescence occurs at 12% RH, the broad O–H stretching vibration completely disappears and the ClO4− symmetric
vibration shifts to smaller wave numbers, both indicative of the crystallization of aqueous solution.
When deliquescence occurs, changes to the Raman spectra occur
in two primary regions: 3200–3600 cm − 1 (O–H stretch of
condensed-phase H2O) and 920–960 cm − 1 (υ1 ClO4− symmetric
stretch). The broad shape of the O–H stretch is due to intermolecular
hydrogen bonding and confirms H2O is in the liquid phase. The spectra of anhydrous NaClO4 at 2% RH (Fig. 1A) and 40% RH (Fig. 1B) have
no feature in this region. When the RH reaches 44.0% and above, this
peak appears and grows confirming liquid H2O is dissolving the salt
particle. Additionally, the υ1 ClO4− symmetric stretch shifts from 952
to 936 cm − 1 during deliquescence. Previous spectral studies of perchlorate deliquescence report a similar shift and have concluded
this spectral change is due to transformation of crystalline ClO4−
(952 cm − 1) to free ClO4− ions in solution (936 cm − 1, Miller and
Macklin, 1985). In this study, the RH at which the crystalline perchlorate peak at 952 cm − 1 fully disappears was defined as the DRH of
NaClO4, as all ClO4− exists as free ions in the aqueous phase at this
point. In the experiment shown here, the DRH was recorded to be
44%.
As the RH is lowered (Fig. 1D–F), the intensity of the O–H stretch
decreases as H2O is lost from the liquid particle. This peak disappears
entirely at 12% RH (Fig. 1F) representing the complete loss of H2O.
The shift in the υ1 ClO4− symmetric stretch corresponding to the crystallization of solid salt from aqueous solution (936 to 952 cm − 1) occurs suddenly and at a RH consistent with the disappearance of the
O–H stretch. In the experiment shown here, the ERH was recorded
to be 12%.
Similar spectral methods were used to determine the DRH and
ERH of the other perchlorate salts studied (NaClO4·H2O and Mg
(ClO4)2·6H2O). A more detailed description of the spectral changes
that occurred during the phase transitions of these salts is contained
in the supplementary material.
4. Results
4.1. Magnesium perchlorate deliquescence and efflorescence
The results of Mg(ClO4)2·6H2O deliquescence and efflorescence
are plotted on a stability diagram (T vs. RH) of the Mg(ClO4)2–H2O
system (Fig. 2) enabling a comparison to theoretical phase transitions. A model of diurnal T and RH variation on the surface of Mars
is shown in red and will be discussed in Section 6. The stability diagram is modified from Chevrier et al. (2009) by converting salt concentration to RH using the Pitzer model and parameters.
The measured DRH of Mg(ClO4)2·6H2O was 42.0 ± 2.3% at 273 K
and increased with decreasing temperature to 55.1 ± 1.6% at 223 K.
At all temperatures studied, deliquescence occurred close to the thermodynamic Mg(ClO4)2·6H2O–liquid phase transition. Efflorescence
of aqueous Mg(ClO4)2 (open symbols) was found to have little dependence on temperature and occurred at ERH = 19.3 ± 2.9%. The error
bars for each DRH and ERH data point in this study are the standard
deviation of multiple (≥3) measurements performed at a given temperature. This ERH is near the upper end of the range of 10–18% RH
found by Zhang and Chan (2003) at 298 K. The ERH was significantly
lower than the DRH at every temperature; therefore, it appears the
hysteresis effect that often occurs during salt efflorescence also occurs
for Mg(ClO4)2·6H2O. These results suggest that a supersaturated Mg
Fig. 2. Experimentally determined DRH (solid symbols) and ERH (open symbols)
values for Mg(ClO4)2·6H2O plotted on a stability diagram of the Mg(ClO4)2/H2O system. The thermodynamically predicted phase and hydrate transitions are depicted
with solid lines. The gray line represents the transition between the pentahydrate
and hexahydrate hydration states. We find Mg(ClO4)2·6H2O deliquesces at the
expected RH for all temperatures studied. However, aqueous Mg(ClO4)2 solutions effloresced at 19.3(± 2.3)% RH, exhibiting the hysteresis effect commonly observed during
salt crystallization. The red line represents a diurnal summer RH and T trajectory predicted to occur at the Viking Lander 1 site (Savijarvi, 1995).
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(ClO4)2 solution is metastable at RH values lower than the DRH but
above 19.3% RH.
Although Raman spectroscopy allows us to differentiate between
anhydrous salt, hydrated salt and aqueous solution, this technique
does not enable us to determine the number of water molecules per
formula unit in the crystalline structure (e.g.: Mg(ClO4)2·5H2O vs.
Mg(ClO4)2·6H2O). However, as efflorescence always occurs in the
hexahydrate stability region (see Fig. 2), crystallization of a Mg
(ClO4)2 solution likely results in the hexahydrate. To confirm this, a
crystalline salt particle that had already experienced one deliquescence/efflorescence cycle was deliquesced again. Deliquescence of
this sample occurred on the hexahydrate solid/liquid transition line,
supporting the conclusion that the efflorescence product is Mg
(ClO4)2·6H2O.
4.2. Sodium perchlorate deliquescence and efflorescence
Experimental DRH and ERH data for the two NaClO4 salts studied
(anhydrous and monohydrate) are plotted on the T vs. RH stability diagram of the NaClO4–H2O system (Fig. 3). As in Fig. 2, the stability diagram in Fig. 3 is modified from Chevrier et al. (2009) by converting
NaClO4 salt concentration to RH. Metastable transitions are included
as well (dashed lines). The measured DRH of anhydrous NaClO4 is
35.4 ± 2.3% RH at 273 K and appears to increase only slightly with decreasing temperature. The data point at 223 K does not follow this
trend, although the error bars are larger at low temperatures due to
the experimental difficulty of measuring RH under these conditions.
The DRH of anhydrous NaClO4 averaged over all temperatures is
38.0 ± 2.9%. This value is very similar to the DRH for anhydrous
NaClO4 of ~ 37% at 225 K measured by Zorzano et al. (2009) who performed experiments under low Martian pressures (7 mbar). This
close similarity suggests that the total pressure of the system does
not appear to affect the humidity at the deliquescence phase transition, at least within the error of our measurements.
The DRH of NaClO4·H2O is higher and has a larger temperature dependency, increasing from 50.9 ± 1.6% at 273 K to 64.1% ± 4.0% RH at
228 K. Attempts to deliquesce NaClO4·H2O at 223 K resulted in the
formation of ice, which is predicted by the stability diagram. This
temperature is below the metastable eutectic temperature for the
monohydrated salt (230 K) and deliquescence is not expected to
occur.
The DRH values of both NaClO4 salts studied can be predicted by
the thermodynamic stability diagram shown in Fig. 3 although both
transitions are metastable. Deliquescence of anhydrous NaClO4 and
NaClO4·H2O occurred on the NaClO4–liquid and NaClO4·H2O–liquid
metastable equilibrium lines, respectively. If the system had been
strictly controlled by thermodynamics, the following sequence of
events should have occurred when the RH was increased from 0%
RH above an anhydrous NaClO4 particle at T = 243 K (for example):
First, the monohydrate should form when RH > 14.8%. Then, the dihydrate should form when RH > 54.0%. Finally, deliquescence should
occur when RH > 66% (on the solid blue line).
However, salt hydration is kinetically inhibited due to the difficulty of diffusing water molecules into the crystalline structure and then
modifying this structure to accommodate the new water molecules.
Previous laboratory studies of other salts have reported that hydration can take hundreds of hours at room temperature and could be orders of magnitude slower at lower temperatures (Vaniman and
Chipera, 2006). Therefore, over the time scale of our experiments
(~1 h), the RH reaches the dashed red line before the anhydrous
NaClO4 has begun to hydrate. When the thermodynamicallypredicted hydration processes are bypassed, metastable deliquescence can occur at a much lower RH forming supersaturated liquid
solutions.
In this specific example at 243 K, metastable deliquescence occurs
at RH ~ 42% although thermodynamically predicted deliquescence
should not occur until 66%. In the case of NaClO4·H2O, deliquescence
at T ≤ 253 K is also a metastable transition, as further hydration of the
monohydrate to NaClO4·2H2O is predicted to occur when RH is
increased.
A significant hysteresis effect was observed during all NaClO4 efflorescence experiments. Efflorescence of NaClO4 solutions occurred
at 12.6 ± 2.0% RH whether the solution was produced by deliquescence of anhydrous NaClO4 or NaClO4·H2O. In all cases, Raman spectroscopy confirmed that anhydrous NaClO4 was the efflorescence
product. These results are consistent with previous studies of anhydrous NaClO4 precipitation at ERH = 11% at room temperature
(Zhang and Chan, 2003). As the ERH data exists primarily in the stability region of the anhydrous salt (see Fig. 3), this anhydrous phase
is expected to be formed. Subsequent re-deliquescence of this salt occurred on or near the anhydrous NaClO4–liquid equilibrium line. This
significant hysteresis effect suggests that a supersaturated salt solution is metastable at RH values between the relevant DRH for a
given hydration state and 12.6% RH.
4.3. Long timescale experiments
Fig. 3. Experimentally determined DRH and ERH values (symbols) for NaClO4 and
NaClO4·H2O overlaid on a stability diagram of the NaClO4/H2O system. Thermodynamically predicted stable phase transitions are depicted with solid lines (blue = NaClO4·2H2O, green = NaClO4·H2O, orange = anhydrous NaClO4), and dashed colored lines
represent metastable phase transitions of these salts. Transitions between hydration
states are depicted by gray lines. Stability regions of the NaClO4 hydration states are labeled, along with the eutectic temperature (TE) of each hydrate. Anhydrous and monohydrated NaClO4 each deliquesced close to the predicted RH values. However, all
aqueous NaClO4 solutions effloresced at an RH of 13(± 4)%, exhibiting the hysteresis
effect common during salt crystallization. The red line represents a diurnal summer
RH and T trajectory predicted to occur at the Viking Lander 1 site (Savijarvi, 1995).
The experiments discussed thus far were performed over timescales of ~ 1 h. However, longer time scale experiments were also performed in order to place an upper limit on the rates of two kinetically
hindered processes: efflorescence and salt hydration. It is critical to
understand hydration kinetics because hydration state can affect
both DRH and TE. First, in order to constrain the time needed to hydrate NaClO4, a sample of anhydrous NaClO4 was maintained at
RH = 25% and T = 263 K for 5 h. These conditions lie in the stability
region of NaClO4·H2O in Fig. 3; however, during and after this
5 hour time period Raman spectroscopy was used to confirm that
no crystalline H2O was present in the sample (i.e.: there was no
sharp peak at 3550 cm − 1). Also, subsequent deliquescence of this
sample occurred at 39% RH, meaning the anhydrous salt had not
been hydrated and suggesting hydration may not readily occur on diurnal timescales.
Next, in order to constrain the time over which supersaturated salt
solutions can remain liquid, a modified efflorescence experiment was
performed. Instead of slowly lowering the RH continuously until
R.V. Gough et al. / Earth and Planetary Science Letters 312 (2011) 371–377
efflorescence occurred, the conditions around the aqueous salt were
held constant at RH = 25% and T = 263 K. At these conditions, the
RH is below the DRH of NaClO4 (36.8% RH) but above the ERH
(12.6%), and therefore the liquid solution is supersaturated. These
conditions were maintained for 5 h, during which no crystallization
occurred. When the RH was eventually lowered to the ERH, efflorescence occurred at the expected RH value (~13% RH). This experiment
suggests the rate of humidity decrease does not affect the ERH and
also that metastable, supersaturated salt solutions can exist for at
least several hours.
5. Stability of liquid brine solutions
These results broaden in several ways the range of temperature
and RH conditions for which aqueous perchlorate liquid solutions
were thought to exist. First, we have shown that supersaturated
NaClO4 and Mg(ClO4)2 solutions can readily exist due to the path dependence of phase transitions. We have found that aqueous perchlorate solutions can exist down to humidity values as low as 13% RH for
NaClO4 and 19% RH for Mg(ClO4)2. Additionally, we have shown that
the hydration state of NaClO4 has implications for both the DRH of
solid salts and the freezing temperature of liquid brine. Three major
and related conclusions are: (1) all NaClO4 solutions effloresce into
the anhydrous phase, (2) hydration of anhydrous NaClO4 is very
slow and (3) anhydrous NaClO4 deliquesces at lower RH values than
hydrated phases. The implications of these combined results suggest
a cycle in which hydrated forms of NaClO4 might rarely or never
exist, and therefore stable transitions predicted by thermodynamics
may never occur. In such a system, anhydrous NaClO4 could deliquesce at low DRH values (~ 40%) when humidity increases, efflorescence into anhydrous salt once RH drops below 13%, and then redeliquesce at low DRH values (~40% RH) when humidity increases
again.
A related effect is the decrease in TE of NaClO4·H2O or anhydrous
NaClO4 relative to the dihydrate. Although the experimental data
shown in Fig. 2 suggests the previously reported TE value of Mg
(ClO4)2 of 206 K (Chevrier et al., 2009) is likely correct, the TE value
of 236 K reported for NaClO4 in that study considered only thermodynamically stable transitions. If metastable transitions occur (such as
the deliquescence of NaClO4·H2O or anhydrous NaClO4), then the relevant TE values are 230 K and 180 K, which represent a 6 K or 56 K
temperature decrease, respectively, relative to the thermodynamically stable dihydrate. Although we were not able to experimentally
probe the anhydrous eutectic point, anhydrous NaClO4 was observed
to deliquesce at 223 K, a value below the TE of either NaClO4·2H2O or
NaClO4·H2O, implying that a low metastable TE is likely.
The path dependence of phase transitions due to kinetic effects
can result in metastable aqueous perchlorate solutions existing at T
and RH conditions below previously published limits based solely
on equilibrium thermodynamics. Consideration of these kinetic effects is therefore critical for correctly predicting the perchlorate
phases present for any environmental conditions or cycle.
6. Implications for aqueous solutions stability on the surface
of Mars
Since the initial discovery of perchlorate on Mars (Hecht et al.,
2009; Renno et al., 2009), several studies have assessed the stability
of potential perchlorate salt solutions on the Martian surface. However, these analyses have considered only equilibrium thermodynamics
(Chevrier et al., 2009; Marion et al., 2010; Mohlmann, 2011; Mohlmann and Thomsen, 2011). This is problematic for two reasons.
First, in these studies, both deliquescence and efflorescence are predicted to occur under the RH and temperature conditions at which
the phase change is first energetically favorable. Although deliquescence may be correctly predicted using such theory; we show here
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that supersaturated aqueous perchlorate solutions can exist at
RH b DRH. This metastability will affect the persistence of aqueous
salt solutions on Mars. Second, previous studies assume intermediate
hydration states are readily reached during humidity fluctuations.
However, it has been shown here and elsewhere (Espinosa et al.,
2008; Vaniman and Chipera, 2006) that hydration of salts is a slow
process limited by the diffusion of water into the solid. Therefore, hydration of perchlorate salts may not occur during a diurnal H2O cycle
on Mars. Here, we consider kinetics as well as thermodynamics when
estimating the perchlorate phases that could exist on the Martian
surface.
The Phoenix landing site is the only location on Mars where perchlorate salts have been detected, although Phoenix has been the
only lander equipped to detect this anion. The perchlorate source is
thought to be atmospheric (Catling et al., 2010) and so the distribution of the salt is likely global. Additionally, Navarro-González et al.
(2010) reinterpreted the Viking GC-MS results to suggest perchlorate
was present at both Viking landing sites at concentrations ≤0.1%.
Although the TECP instrument onboard the Phoenix lander included an RH probe, the humidity data has not yet been fully published or
archived at this time. Therefore we instead consider RH and T conditions modeled at the Viking 1 lander (VL1) site (22.48°N, 49.97°W)
during the northern summer (Savijarvi (1995)). The results of this
1-D model are qualitatively similar to the initial results from the
Phoenix TECP humidity probe (Zent et al., 2010) in that the surface
RH varies diurnally from very low values (b1% RH) during the afternoon to 100% RH during late evening and early morning. At both locations, the saturation with respect to water is due mainly to the large
temperature decrease which occurs in the evening. The diurnal
cycle at the Viking lander site is presented as an example, and RH
and T conditions at other latitudes or seasons could be plotted on
these stability diagrams in a similar fashion in order to determine
the likelihood of an aqueous phase.
We plot the Savijarvi (1995) modeled T and RH diurnal cycle on
the stability diagrams of Mg(ClO4)2 and NaClO4 (red lines in Figs. 2
and 3 respectively). The values of T represent the modeled ground
temperature (Savijarvi, 1995) which closely matches remotely sensed
ground temperatures (Kieffer et al., 1976) at this site. The RH is the
modeled humidity at 1.6 m (Savijarvi, 1995). It can be seen in Fig. 2
that Mg(ClO4)2·6H2O in the Martian soil is not likely to be deliquesced at any point during this particular diurnal cycle. However,
any small increase in T during the morning hours may cause the trajectory to pass through the region of liquid stability. On a warmer day,
the eutectic temperature could be reached and an aqueous solution
could exist.
The same modeled diurnal cycle is plotted on the NaClO4 phase diagram (Fig. 3). If only stable equilibria are considered, this diurnal
cycle should result in the stepwise hydration of anhydrous NaClO4
during the afternoon and evening, followed by formation of H2O ice
just after 9 PM once the RH reaches 69%. No liquid phase should
occur. However, here we have shown that hydration of anhydrous
NaClO4 is slow and metastable deliquescence will preferentially
occur. During this diurnal cycle, the transition between anhydrous
NaClO4 and NaClO4·H2O is crossed at 6:30 PM, and the transition to
NaClO4·2H2O is crossed ~ 1.5 h later. The metastable anhydrous deliquescence line (dashed orange line) is reached at ~ 8:30 PM. Therefore the salt has ~ 2 h to hydrate from anhydrous NaClO4 to
NaClO4·2H2O prior to reaching the metastable deliquescence line.
Our experimental results suggest this length of time is not sufficient
for hydration to occur, especially since the cold surface temperatures
will further slow hydration processes. Therefore, at ~ 8:30 PM and
~45% RH the anhydrous NaClO4 salt will likely deliquesce directly
into aqueous solution.
The brine formed in the evening may be short-lived, as the formation of ice is predicted to occur before 9 PM. Here we assume that the
freezing of perchlorate solutions occurs at thermodynamic
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R.V. Gough et al. / Earth and Planetary Science Letters 312 (2011) 371–377
equilibrium; however, just as supersaturated salt solutions can readily exist, supercooled aqueous solutions are common as well (Martin,
2000). In both cases, the thermodynamically stable phase is not readily formed due to the kinetic limitations inherent to nucleation of either salt or ice crystals, respectively. Therefore, the conditions for
which aqueous solution may exist could be extended towards higher
RH values and thus later into the evening.
In the morning when the surface temperature on Mars rises, ice is
predicted to melt before 8 AM on this particular day. Aqueous solutions can exist again until just after 9 AM when RH is lowered to
13% and efflorescence of NaClO4 occurs. Over the course of this Martian day, NaClO4 can exist in the aqueous phase for a fraction of an
hour in the evening and 1+ hours in the morning. Fig. 4 shows this
modeled T and RH diurnal cycle as a function of local Mars time at
the Viking lander 1 site, and the shaded regions represent the time
periods during which aqueous NaClO4 solutions are expected to
exist in either stable or metastable states). The ice or crystalline
NaClO4 phases present at other times of day are labeled in Fig. 4 as
well.
Images taken by the Robotic Arm Camera on board the Phoenix
lander may show salt-containing spheroids on a strut of the lander
leg behaving as liquids, possibly as a result of deliquescence (Renno
et al., 2009). The observed merging and growth of these spheroids is
thought to require at least an intermittent aqueous phase, suggesting
that perchlorate deliquescence could be occurring at the high-latitude
Phoenix landing site during the summer, as well as at the Viking lander site discussed above. Our measurements of the temperature and
RH conditions under which aqueous perchlorate solutions can form
and exist may have implications for the observations of Renno et al.
(2009). Assuming the temperatures and RH values estimated in
Renno et al. (2009), our laboratory studies show that perchlorate
salts may exist in the aqueous phase during some diurnal cycles. In
general, different seasons and geographical locations are expected to
yield different results regarding the duration of aqueous perchlorate
solutions and should be studied on a case-by-case basis wherever
RH and T data are available.
Understanding the phase of perchlorate salts on Mars will have
implications for the history of water and the astrobiological potential
of the Martian subsurface. It has been suggested that perchlorate at
the Phoenix landing site may be locally concentrated into patches
(Cull et al., 2010). This heterogeneity is similar to the distribution of
Fig. 4. Modeled RH (solid line) and surface temperature (dashed line) as a function of
time of day on Mars at the Viking 1 landing site (Savijarvi (1995)). We use our stability
diagram of the NaClO4/H2O system (Fig. 3) and consider the likely metastable states,
transitions and eutectic points described in the text (Section 5) to predict when aqueous NaClO4 solution can exist. In the late evening and early morning, ice will exist. In
the warm, dry afternoon, the crystalline salt (likely the anhydrous form) will be present. For two periods of time on this particular Martian day, during the late morning
and then again during the evening, either stable or metastable aqueous perchlorate solutions can exist (blue shaded regions).
soluble salts on Earth and demonstrates a history of aqueous dissolution and redistribution. Thin films of water are sufficient to transport
highly soluble perchlorate from surface to subsurface in the Antarctic
Dry Valleys and perhaps on Mars (Cull et al. (2010)). Deliquescence
provides a mechanism for the formation of these thin films of brine
in environments where large volumes of liquid are unlikely. As
there could be a correlation between the length of time during a
day or year that perchlorate is deliquesced and the mobility of the
resulting brine, the current salt distribution in the Martian subsurface
may provide clues to the extent of past aqueous activity.
Terrestrial cyanobacteria can obtain liquid water via deliquescence of halite (NaCl) crusts (Davila et al., 2008). It is therefore exciting to consider the possibility of Martian microorganisms utilizing
perchlorate salt deliquescence as a mechanism for obtaining liquid
water, a substance required for life. However, it is unlikely these
aqueous perchlorate solutions are astrobiologically relevant. The saturated solutions that result from NaCl deliquescence have a water activity of 0.75 and a concentration of ~5 M. The much higher
concentration of saturated perchlorate solutions (9 to 25 M depending on T and the phase dissolved (Chevrier et al., 2009)) and much
lower water activities (0.13 to 0.56, depending on T, phase dissolved,
supersaturation) may create an uninhabitable environment. Life is
known to exist only at water activities between 0.60 and 1.0 (pure
H2O) (Brown, 1990); below this range there is insufficient water
available to cells. Additionally, the extremely high salinity may weaken electrostatic interactions between macromolecules and disrupt
cellular structures (Hallsworth et al., 2007).
7. Conclusions
We have experimentally determined DRH values for anhydrous
NaClO4, NaClO4·H2O and Mg(ClO4)2·6H2O from 223–273 K using
Raman spectroscopy. Measured DRH values for all salts were consistent with thermodynamic predictions and generally increase with decreasing temperature. Anhydrous NaClO4 deliquesced at lower
humidity values than the monohydrate and has a TE value 50 K
lower than NaClO4·H2O and 56 K lower than NaClO4·2H2O.
Efflorescence of aqueous NaClO4 and Mg(ClO4)2 solutions occurred below the RH of deliquescence. ERH values are independent
of T and are 12.6 ± 2.0% RH for NaClO4 and 19.3 ± 2.9% RH for Mg
(ClO4)2. This hysteresis between deliquescence and efflorescence results in metastable, supersaturated salt solutions. The supersaturated
liquid phase can remain liquid for more than 5 h. The results also
place kinetic limits on salt hydration, as anhydrous sodium perchlorate is not hydrated even after 5 h under conditions which should
form NaClO4·H2O. As anhydrous NaClO4 is always the efflorescence
product, the phase transitions and eutectic temperature of the anhydrous sodium perchlorate salt may be important to consider.
Deliquescence of perchlorate salts is a potential mechanism for
brine formation on Mars. By confirming the importance of kinetics,
metastability and path dependence, our results expand the range of
RH and temperature conditions, and therefore the periods of time,
during which liquid perchlorate brines can exist. Any NaClO4 salts at
the Viking lander 1 site could be in the liquid form for ~2 h/day during the summer. The duration of these brines will vary with the location on the planet, season, and soil depth. Additionally, competition
with other H2O sinks such as adsorption by mineral surfaces or
other salts has not yet been investigated and but could affect perchlorate liquid stability.
Acknowledgments
This work was supported by NASA Mars Data Analysis Program
grant #NNX10AN81G and NASA Mars Fundamental Research Program grant #NNX09AN19G.
R.V. Gough et al. / Earth and Planetary Science Letters 312 (2011) 371–377
Appendix A. Supplementary data
Supplementary data to this article can be found online at doi:10.
1016/j.epsl.2011.10.026.
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