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Investigating the hysteretic behavior of Mars-relevant chlorides
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Investigating the hysteretic behavior of Mars-relevant chlorides ⁎
K.M. Primma, , D.E. Stillmana, T.I. Michaelsb a b
Dept. of Space Studies, Southwest Research Institute, 1050 Walnut St. #300, Boulder, CO 80302, USA SETI Institute, 189 Bernardo Ave Suite 200, Mountain View, CA 94043, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Brines Mars Chlorides Metastability Hysteresis
Liquid solution stability has been a highly studied topic in the Martian community since the detection of perchlorate (ClO4−) at the Phoenix landing site and the global detection of chloride (Cl−) by THEMIS (Thermal Emission Imagining System, onboard Mars Odyssey). Understanding how brines form and react to changing environmental conditions helps identify potentially habitable environments on Mars, both at present and in the past. Here we measure the extent of metastability of magnesium chloride (MgCl2) and sodium chloride (NaCl) brines when freezing. We find that the metastable eutectic temperature of MgCl2 depends on the maximum temperature (Tmax) reached before freezing. If Tmax < −15 °C, the metastable eutectic temperature (mTeu) is only 3 °C below the stable eutectic temperature, and if Tmax > −15 °C, mTeu is 15 °C below the stable eutectic temperature (Teu). We speculate that this metastable behavior follows the phase diagram for the transition into the 8 hydrate for MgCl2, thus, yielding a different freezing temperature, the peritectic for MgCl2·8H2O. However, mTeu for NaCl is independent of Tmax and was constantly at 3 °C below Teu with no peritectic (consistent with the phase diagram). We also found that MgCl2 brine can exist for at least 60 h at 5 °C below its Teu. Applying our findings, we determined the potential time evolution of brines at Palikir crater, using a time-series of modeled temperature profiles. Surficial layers melt more frequently, but layers at 2–3 cm depth are able to warm above Tmax > −15 °C and maintain brine for longer than surficial layers. The evaporation rate of brine buried by 2–3 cm of regolith is greatly reduced due to the generally cold temperatures, solute concentration, and by the regolith overburden. We also found that at Palikir crater, only the deep subsurface (~9.5 cm depth) has water activities (~0.75) high enough to support life. Overall, the metastable properties of brines can drastically affect their formation and longevity on Mars, and should be considered in future models.
1. Introduction In addition to perchlorate (ClO4−), chloride (Cl−) salts have also been found in several places on the surface and subsurface of Mars (e.g., Hecht et al., 2009; Osterloo et al., 2010, 2008). Perchlorate and chloride salts have remarkable properties, such as: (1) brines that do not fully freeze except at very cold temperatures (~205 K); (2) they can absorb atmospheric water vapor to form a brine (a process known as deliquescence) (e.g., Gough et al., 2011; Primm et al., 2017); and (3) ClO4− and Cl− brines can maintain a metastable liquid phase down to even colder temperatures (~123 K for Mg(ClO4)2; Toner et al., 2014a, 2014b) and exist over a larger relative humidity (RH) range (Primm et al., 2017). According to a binary mixture phase diagram, a stable brine occurs when a salt-H2O mixture has a salt mass percentage within a specific temperature-dependent range and, if the mixture is in contact with gas, when the relative humidity is also within a specific temperature-dependent range. More quantitatively, stable brines exist
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(often in combination with salt hydrate or ice) when: (1) the salt mass percentage is less than the salt mass percentage of the lowest peritectic (point at which a liquid phase and solid phase form a new solid phase), (2) the temperature is greater than the eutectic temperature (Teu), and (3) when the RH is greater than the deliquescence relative humidity (DRH; the RH at which deliquescence occurs). Thus, Cl− and ClO4− brines are much more stable than pure liquid water and have been suggested to play a role in processes on contemporary Mars that might involve water: cryosuction (Sizemore et al., 2015), Recurring Slope Lineae-RSL, (e.g., Chevrier and Rivera-Valentin, 2012; Heinz et al., 2016; Bhardwaj et al., 2019), gullies (Goldspiel and Squyres, 2011), slope streaks (Bhardwaj et al., 2019, 2017; Mushkin et al., 2010), dark dune spots (Kereszturi and Rivera-Valentin, 2012), and subglacial brines (Orosei et al., 2018). There has been extensive work on the deliquescence, efflorescence (recrystallization from a brine via the loss of liquid water to the environment as water vapor), and freezing of Cl− and ClO4− brines (e.g.,
Corresponding author. E-mail address: [email protected] (K.M. Primm).
https://doi.org/10.1016/j.icarus.2019.06.003 Received 8 March 2019; Received in revised form 28 May 2019; Accepted 4 June 2019 0019-1035/ © 2019 Elsevier Inc. All rights reserved.
Please cite this article as: K.M. Primm, D.E. Stillman and T.I. Michaels, Icarus, https://doi.org/10.1016/j.icarus.2019.06.003
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2. Experimental methods
Gough et al., 2011; Nuding et al., 2014; Primm et al., 2017). These studies show that if Cl− and ClO4− salts are exposed to certain water vapor concentrations (e.g., relative humidities), they would deliquesce into liquid solutions on present-day Mars. Because of metastability and hysteretic behavior, these brines could exist outside of their thermodynamically-predicted stability range. In this paper, we use metastability to define when the phase of the salt-H2O mixture is outside the bounds of the thermodynamically-predicted phase. Hysteresis occurs when the history of the salt-H2O mixture determines when a phase transition will occur. Thus, once a brine is formed it is metastable and hysteretic, as it requires (1) a lower RH to transform it into a brine-salt hydrate mixture, (2) a higher RH to transform it into a brine-ice mixture, and (3) the temperature of the brine must be decreased below the eutectic temperature before freezing into an ice and salt hydrate mixture. The latter condition is the focus of this paper. Although the exact salts on the surface of Mars have not been confirmed, magnesium chloride (MgCl2) and sodium chloride (NaCl) have been model-predicted from Phoenix Wet Chemistry Laboratory data to be present the surface of Mars (Marion et al., 2010; Toner et al., 2014b). Previous measurements found that solutions of MgCl2 (Teu = −33 °C) were metastable at temperatures between 9 and 14 °C below the eutectic temperature, and for NaCl, 6.6–8.3 °C below its eutectic temperature (Teu = −21.3 °C) when the sample temperature was decreased (Toner et al., 2014a, 2014b). Primm et al. (2017) observed similar behavior with MgCl2, observing no freezing even at temperatures 20 °C below the stable eutectic temperature. Even so, Toner et al. (2014a, 2014b) and Primm et al. (2017) did not study the effects of the duration spent at temperatures below the eutectic. Toner et al. (2014a, 2014b) controlled the temperature rate by changing the sample size from a salt-water mixture of 20 g to 50 g. In contrast, the samples in Primm et al. (2017) were much smaller (sub-mm particles) and used a temperature controller to change the temperature. Their experimental time scales were on the order of minutes to an hour. The experimental studies in this paper aim to represent bulk subsurface ice. Bulk ice describes multiple grains (sub-mm) of ice in a sample where liquid solutions can exist between the individual ice grains. The spatial distribution of liquid solution between these ice grains is termed a liquid vein network (LVN) and can be easily detected by measuring the electrical properties of the ice (Grimm et al., 2008; Stillman et al., 2013, 2010). Here we examine the metastability of bulk solutions of MgCl2 and NaCl over different starting concentrations, varying the time spent below the eutectic temperature, and varying the starting temperature to see if any of these factors change the metastability properties of these salts.
2.1. Sample preparation and sample holder Solutions of MgCl2 (5, 7, 10, 20, 30, 63 and 100 mM) and NaCl (10, 30, 60 and 100 mM) were prepared in high purity water. The salts used were MgCl2·6H2O and anhydrous NaCl, purchased from Sigma-Aldrich (309303 and 793566, respectively). Each sample solution was poured into a three electrode sample holder where the sample is fixed at a height of 5 mm (relative to the 31 mm diameter Teflon cup it is enclosed within). The sample holder with the sample solution was then loaded into an insulated box within an ultra-low freezer (So-Low C85-9) where we can control the temperature with a Lakeshore 331 temperature controller. The full experimental set-up is described in more detail in Stillman et al. (2010) and Grimm et al. (2008). 2.2. Electrical property measurements At every temperature, the electrical properties (complex impedance) of the sample are measured using a Solartron 1260A Impedance Analyzer and the Solartron 1296A Dielectric Interface over a frequency range of 100 mHz–1 MHz. The complex impedance was converted into complex dielectric permittivity using the electrode geometry. The data were then fit using Cole-Cole parameters and the Direct-Current (DC or zero-frequency) conductivity (e.g., Stillman et al., 2013). To verify the literature value of the eutectic temperature, the sample was initially frozen onto the electrodes quickly by decreasing the temperature directly to near −85 °C. The temperature was then raised by intervals between 0.25 and 5 °C until −0.5 °C was reached. The DC conductivity of brine is much higher than that of ice, salt hydrate, mineral grains, or adsorbed water. Thus, when brine forms a connected path through the sample, the value of the DC conductivity greatly increases. For example, a few degrees below the eutectic temperature the change in DC conductivity with increasing temperature is large producing a steep slope, as premelted (quasi-liquid films that exist below the melting temperature) eutectic brines increase the conductivity. At temperatures warmer than the eutectic, the DC conductivity slowly increases with temperature yielding a much smaller slopes as the brine becomes more conductive with increasing temperature and more of the sample melts. Thus, the eutectic temperature can be identified at the cusp or change in slope in plots of DC conductivity versus temperature (Grimm et al., 2008; Stillman et al., 2010; Stillman and Grimm, 2011a). The temperature was also held at that warmest temperature (−0.5 °C) for ~8 h to allow recrystallization of the ice to occur in order to obtain a more uniform grain size distribution, allowing more-repeatable measurements. To determine the extent of metastability of the sample, the temperature was then lowered by 0.25–10 °C increments until −85 °C was reached. Such warming, recrystallization, and cooling cycles were
Fig. 1. Example of 12 experimental temperature profiles where the maximum temperature was varied. The temperature reaches the warmest temperature, then is cooled, and the next experiment is then warmed up to a different starting temperature and follows a similar cooling temperature profile. 2
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temperature of MgCl2 are consistent with previous studies, the temperature was then decreased to determine the eutectic temperature when freezing. Fig. 3 shows a 7 mM MgCl2 sample in which the warming and freezing experiments exhibit vastly different eutectic temperatures. The sample did not freeze until −48 °C, expressing a large (16 °C) region where the LVN are metastable. This is also consistent with Toner et al. (2014a, 2014b) where they found that metastable liquid existed between 9 and 14 °C below the eutectic temperature. To test how stable these metastable LVN in mixtures of MgCl2 and ice are, the sample was warmed to −0.5 °C and then measured at numerous temperatures above the eutectic, then held at −37 °C (5 °C below the eutectic temperature) for > 60 h. Fig. 4a demonstrates that the DC conductivity of the sample at −37 °C varies little and still possesses LVNs. Fig. 4b shows how the conductivity at 0.1 Hz changed throughout the 60 h. Note, typically the DC conductivity value is fit via the Cole-Cole model, so we assume the lowest frequency (0.1 Hz) is a good approximation to the DC conductivity value as it is accurate to a few percent. Although this solution is technically metastable, it is quite stable. Next, we tested whether different concentrations and maximum warming temperatures affected the metastability of brine. Fig. 5 shows that metastability does vary between samples, but it varies independently of concentration. Our measurements are consistent with Toner et al. (2014a, 2014b), but are much less metastable than Primm et al. (2017). We hypothesize that the discrepancy in our measurements vs. Primm et al. (2017) is due to the size of the samples. Primm et al. (2017) studied single-particle samples (diameter of 20 μm; volume ≈ 3 × 10−8 cm3), while our measurements are for a bulk sample volume of approximately 3 cm3. To test our hypothesis about how metastability varies with maximum warming temperature, we froze each sample to −55 °C and then raised the temperature to the maximum temperature for at least an hour, before refreezing the sample (see the temperature profile in Fig. 1 for more information). Fig. 5 displays the observed metastable eutectic temperature versus the maximum temperature reached before refreezing occurred. These change-in-maximum-temperature experiments were performed on 7 mM (blue) and 100 mM (cyan) solutions of MgCl2. In Fig. 5, when the maximum temperature was below −15 °C the extent of metastability was only ~3 °C below the eutectic. However, for the warmer temperatures above −15 °C, the LVN were metastable for 15 °C below the eutectic. Thus, metastability and hysteresis greatly increase for MgCl2 samples that have been raised to temperatures > −15 °C. Furthermore, there is also considerable variation in the metastable eutectic temperature when freezing from a temperature above −15 °C. We hypothesize this variation is natural and just a product of measuring a stochastic process of ice nucleation. The lines in this study (black dashed lines) were calculated by taking the average of either all the points below or above −15 °C, and the error bars are the standard deviation of the points.
repeated multiple times. To further investigate the metastability of the sample solution, the maximum temperature the sample was allowed to warm to was varied. That is, after the previous freezing experiment the sample would not be warmed all the way up to −0.5 °C, but instead only to various colder temperatures above the eutectic temperature (e.g., −5, −10, −15 °C). Fig. 1 shows a typical temperature profile for this change-in-maximumtemperature procedure. 2.3. MarsFlo modeling To model a representative subsurface temperature environment, we use MarsFlo (Grimm et al., 2019, 2017, 2014; Grimm and Painter, 2009; Painter, 2011) integrated with the Mars Atmospheric Regional Modeling System (MRAMS; Michaels and Rafkin, 2009). MarsFlo replaces the standard subsurface module in MRAMS that computes subsurface volatile and heat distribution. The MarsFlo component proficiently simulates three-phase (liquid, ice, vapor) H2O migration and thermal evolution in porous media, while the MRAMS component provides realistic surface boundary conditions (e.g., surface energy balance, atmospheric pressure) as functions of time-of-day, season, and latitude. The MarsFlo run in this work was a 1-D run using a grid of 90 points nonlinearly-spaced between the surface (1 mm cell thickness) and 50 m depth (~4 m cell thickness). Palikir crater (41.6°S, 202.3°E) was chosen because it has some of the largest confirmed RSL in the southern midlatitudes (McEwen et al., 2013; Stillman and Grimm, 2018) that actively lengthen from Ls ~200–315° (Ls, or solar longitude, where Ls = 270° is southern summer solstice) for west- and northwest-facing slopes. Active gully formation has also have been observed at this site at around Ls ~180° (Dundas et al., 2012, 2015, 2019). Two main zones of representative thermophysical properties were used: an uppermost 6 cm of fines (sand; 45% porosity), over fractured bedrock (15% porosity). The run was initialized with an isothermal profile (210 K) and run for two Mars-years, with only the second year of output being used for analysis. The realistic surface forcing used was for a 30-degree westfacing slope (albedo of 0.12) at Palikir crater, present-day Mars. 3. Results 3.1. Verification of stable eutectic temperature The formation of LVN is identified by analyzing how DC conductivity varies with temperature, as the formation of LVNs significantly increases the conductivity of the sample. Thus, we can measure the eutectic and metastable eutectic temperatures (mTeu) by identifying the temperature at which these changes in DC conductivity occur. Fig. 2a shows eight different experiments where four MgCl2-H2O mixtures and four NaCl-H2O mixtures (Fig. 2b) were warmed until melting (eutectic temperature) was observed. Our warming measurements for varying MgCl2 concentrations yield a eutectic temperature of −32.2 ± 0.2 °C, which is slightly lower than the literature value of −33.0 °C (Toner et al., 2014a). The eutectic temperature found here for NaCl (Fig. 2b) is consistent with that in the literature, 21.3 °C (Toner et al., 2014a). The average value and error of our eutectic temperature values come from the standard mean of the experimental eutectic temperatures and the standard deviation of the measured values. Fig. 2 also confirms that the eutectic temperature is independent of salt concentration. The DC conductivity of the mixtures increases with increasing salt concentration because at higher concentrations a greater percentage of the sample is brine at any given temperature.
3.2.2. NaCl The eutectic temperature of NaCl (−21.3 °C) is much higher than that of MgCl2 (−33 °C), and freezing measurements indicated a small difference in the metastable eutectic temperature of only ~3 °C (Fig. 6). This smaller metastability range was expected because Toner et al. (2014a, 2014b) only found metastability 6.6–8.3 °C below the eutectic temperature, compared to 14 °C for MgCl2. We speculate that the timescales and sample sizes for our experiment versus the Toner et al. (2014a, 2014b) experiments could explain this discrepancy between our metastability values. We use much slower timescales (approximately an hour per each temperature reached) and a smaller sample size (~5 g), while Toner et al. (2014a, 2014b) changed the temperature much more quickly and did not let the temperature come to an equilibrium with the sample with a sample size of ~20–50 g. We would expect that a sample that has more time to come into equilibrium with a
3.2. Metastability 3.2.1. MgCl2 After verifying that our experimental results for the eutectic 3
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Fig. 2. Direct Current (DC) conductivity vs temperature of eight different concentrations of MgCl2 (a) and five different concentrations of NaCl (b). For these experiments, the sample was initially quickly frozen at between −70 to −85 °C and then was warmed until the eutectic temperature was observed (cusp or no change in slope of DC conductivity). All plots are for warming experiments. DC conductivity of the mixture increases with increasing salt concentration. This increase in conductivity shows that there is brine present, which is significantly more conductive than water ice alone. Fig. 3. A warming experiment followed by a freezing experiment, showing the hysteretic behavior of MgCl2 (7 mM). The blue box on the left represents stable liquid solutions and the red box in the middle represents metastable liquid solutions, followed by the grey box representing ice. This plot illustrates that the sample melts at the eutectic temperature, but then freezes at 15 °C below the eutectic temperature. This behavior is seen throughout several experiments and concentrations. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
of NaCl (i.e., 30 and 60 mM). NaCl exhibits no significant change in metastability with changing concentration and maximum temperature, resulting in a constant metastable eutectic that is 3° below the stable eutectic of −21.3 °C. Note that the 10 and 100 mM samples were not measured while freezing.
temperature change would exhibit less metastability, however, it is unclear as to if a smaller bulk sample would change metastability (as opposed to single particle samples in Primm et al., 2017). As with our MgCl2 experiments, the maximum temperature of our NaCl experiments was varied (Fig. 7). Fig. 7 shows the results from varying the maximum temperature reached for each experiment for 2 different concentrations 4
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Fig. 4. a) Plot of DC conductivity (S/m) vs temperature (°C) of MgCl2, showing no jump in DC conductivity, indicating that no freezing occurred. Fig. 3 (cyan trace) shows the typical drop in DC conductivity for a freezing transition (~DC conductivity drop to 10−8 S/m) b) Plot of conductivity at 0.1 Hz vs. elapsed time (hours) We assume the lowest frequency (0.1 Hz) is a good approximation to the DC conductivity value as it is accurate to a few percent. This plot displays the last point in (a) throughout time. The conductivity throughout the 60 h does not dramatically change slope, and the sample is thus not changing phase. The colors represent the time elapsed. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
≤−15 °C, MgCl2 transitions to the 12 hydrate which has a higher eutectic (~−33 °C). In the case of NaCl, there is no hydration transition within the temperature range studied (Toner et al., 2014a, 2014b). However, we cannot confirm this because our measurements cannot determine the specific salt hydration state(s) present. Additionally, when we re-warm the ice/salt-hydrate mixture that froze at the mTeut it does not melt until it reaches the normal 12 hydrate eutectic temperature. For the above hypothesis to be correct, the frozen 8 hydrate would have to change to 12 hydrate once the sample is heated. Understanding the metastability of these salty ices is highly relevant to the subsurface ice at the Phoenix landing site (Mellon et al., 2009), where we could have LVN within the ice or in the near-subsurface (e.g., Fischer et al., 2016; Stillman and Grimm, 2011a, 2011b). If we know the temperature profile of the subsurface where the ice is and the identity of the salt(s) incorporated into the ice, we can map out how long the LVN can exist using our results. Fig. 8 shows a modeled temperature profile at Palikir Crater on Mars for four different sols at different depths. The top left (a) plot shows that liquid solutions can exist longer (1.5 Mars-hours more) when the temperature is higher, however, the top right (b) plot shows that liquid solutions cannot exist as long, and because they do not exceed −15 °C, their metastability only adds 14 Mars-minutes. Additionally, (c) and (d) show that below the surface (where the temperature profile is not as sharp long as the temperature is higher than the stable eutectic) you can have brines for a long time (~3 Mars-hours). Fig. 9 shows how MgCl2 brine duration varies seasonally depending on the (a) stable eutectic temperature or the (b) metastable eutectic
4. Discussion The shallow subsurface environmental conditions (e.g., pressure and temperature) on contemporary Mars only rarely support the existence of pure liquid water. However, salts can depress the freezing point of water, allowing brines to exist in colder conditions. Understanding the extent of liquid solution stability can help determine if liquid brine exists near the surface on present-day (and past) Mars. Current models of the subsurface of Mars do not consider metastability regions of these salty solutions (e.g. Cl− or ClO4−), considering only stable liquid solutions (e.g., Chevrier and Rivera-Valentin, 2012; Martín-Torres et al., 2015; Pál and Kereszturi, 2017). If we were to incorporate the metastability regions of liquid solutions, it would provide for a more accurate representation of the presence or absence of liquid solutions. Here we demonstrated that the temperature at which the LVN within the ice freezes depends on how warm the ice was before the temperature is decreased for MgCl2 mixtures. However, no change in metastability for the NaCl mixture was observed with variable maximum temperatures. We hypothesize that this change in metastable eutectic temperature for MgCl2 could be due to changes in the hydration state of the salt. Theoretical models of MgCl2 show that three hydration states exist (6, 8, and 12) (Primm et al., 2017 Fig. 4). The transition point between the 8 and 12 hydrate is near −15 °C. It is possible that when the sample is warmed up to greater than −15 °C, the brine does not freeze at its typically eutectic temperature but instead freezes at when MgCl2 transitions to the 8 hydrate which has a lower metastable eutectic (~−42 °C), but if the sample is warmed up to
Fig. 5. Freezing temperature vs. starting temperature (°C) of various MgCl2 concentrations, where the maximum temperature is the highest temperature that experiment reached before cooling again. The red box under the eutectic temperature is the region where Toner et al. (2014a, 2014b) saw metastable liquid solutions of MgCl2, and the dashed green line is the lowest freezing temperature at which Primm et al. (2017) observed liquid solutions. These experiments were performed with 7 mM and 100 mM concentrations of MgCl2. When the samples were warmed to temperatures above −15 °C, metastable liquid solutions existed 15 °C below the eutectic (black dashed line). However, when the samples were only warmed to temperatures below −15 °C, metastable liquid solutions existed only 3 °C below the eutectic temperature (top black dashed line). The lines in this study were calculated by taking the average of either all the points below or above −15 °C, and the error bars are the standard deviation of the points. Note that there are also error bars on the traces, but the majority of them are smaller than the trace. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 5
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Fig. 6. A warming experiment followed by a freezing experiment, showing the hysteretic behavior of NaCl (30 mM). The blue box on the left represents stable liquid solutions, the red box in the middle represents metastable liquid solutions, followed by the grey box representing ice. This plot shows that the sample melts at the eutectic temperature, but then freezes at 3 °C below the eutectic temperature. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
humidity) and temperatures of these solutions are lower than what is required for life. However, if the water activity of these solutions becomes > 0.61 (Grant, 2004), then life is said to be able to survive and grow. The water activities of NaCl solutions can be as much as 0.83 (Rummel et al., 2014; Wise et al., 2012) compared to MgCl2 with a range between 0.33 (experiments from Primm et al., 2017) and 0.73 (model from Primm et al., 2017), but the temperatures of these solutions can be as low as −48 °C. This is less than the temperature at which metabolic activity is seen, −33 °C (Wierzchos et al., 2006). Fig. 8(d) reports the maximum water activity, Aw,max,l (maximum value for water activity when the temperature was above the eutectic), for a particular sol. Because life requires a water activity of > 0.61, only the deeper subsurface (i.e., 9.5 cm) which has a Aw,max,l = 0.75 could support life. Although understanding when/where liquid solutions are possible on Mars (e.g., stable and metastable brines) is helpful in understanding Mars' habitability, the temperature of these liquid solutions must also be considered. Nevertheless, we do not know if or how organisms might have evolved on different planets.
Fig. 7. Freezing temperature vs. maximum temperature (°C) of 30 and 60 mM NaCl concentrations, where the maximum temperature is the highest temperature each experiment reached before cooling again. The red box under the eutectic temperature is the region where Toner et al. (2014a, 2014b) observed metastable liquid solutions of NaCl. No change in metastability was observed when changing the maximum temperature of these 30 and 60 mM NaCl samples. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
5. Conclusions Here we have used electrical property measurements to study the metastability of LVN within bulk ice mixed with varying concentrations of MgCl2 and NaCl. Two previous studies have looked at the metastability of MgCl2 below the eutectic temperature (bulk, 20–50 g samples) or at the metastability of ice-saturated conditions (sub-mm particles), but here we validated the chief findings of those studies using a different technique. Both previous studies always warmed the sample up to room temperature (20–25 °C) before the start of the next experiment, but there are temperature profiles on Mars that do not reach these warm temperatures daily. Because of this, we decided to test the extent of metastability when the sample was not warmed up to well above the eutectic. We find that the extent of metastability for MgCl2 depends on the highest temperature it reached before the temperature decreased, while no change was seen for NaCl when the warmest starting temperature was changed. We varied the warmest temperature the MgCl2 sample reached from −30 to −0.5 °C and decreased the temperature to determine at what temperature the sample would freeze. When the sample was warmed to below −15 °C the sample froze only 3 °C below the eutectic temperature, but when the sample was warmed to above −15 °C the sample did not freeze until 15 °C below the eutectic temperature. This held true for two different starting concentrations, 7 mM and 100 mM, showing that the starting concentration does not affect the metastability conditions. Furthermore, we examined the effect that time might have on the metastable solution. It is often assumed that these metastable solutions are not allowed to reach equilibrium, and after time, the sample should
temperature that corresponds with the maximum temperature reached. The brine duration was calculated by starting the brine formation at the stable eutectic, and when the temperature starts to decrease after the maximum, brine continues to exist at one of the different metastable/ stable eutectic temperatures determined. We find that when mTeu = −48 °C, brines can exist up to 2 Mars-hours longer than if you were to use the stable eutectic temperature to predict brine formation. Surficial layers melt more frequently, but layers at 2–3 cm depth are also able to reach warm temperatures (i.e., Tmax > −15 °C) and maintain brine for longer (around Ls = 270°) than surficial layers (Fig. 9). While not modeled, the evaporation rate of brine at 2–3 cm depth is greatly reduced due to the cold temperatures, solute concentration, and by the regolith overburden (Bryson et al., 2008; Chevrier and Altheide, 2008). Furthermore, brine duration using the metastable eutectic temperature demonstrated that brines could exist 2–3 cm below the surface for more than half a sol. Not only do these findings help understand the metastability of liquid solutions, they can also provide insight into the possibility of organisms living within these LVN. Because life as we know it on Earth requires liquid water and the detection of liquid water on Mars has yet to be confirmed, life on Mars appears unlikely. Although several studies tested the likelihood of liquid solutions on Mars with only high humidities and salts present, the water activities (partial pressure of water in a substance divided by the saturated vapor pressure of pure liquid water at the same temperature; generally can be equated to relative 6
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Fig. 8. Modeled temperature profile at Palikir Crater using MarsFlo. This plot shows the region of stable (dark blue) and metastable liquid solutions (light blue) from our results for MgCl2 (dashed magenta line), depending on the maximum temperature reached. Each plot has the maximum water activity while liquid solutions exist for that sol, Aw,max,l. (a) shows a surface temperature time-series of a sol at Ls = 270° where the temperature reaches > −15 °C. When Tmax > −15 °C, metastable solutions of MgCl2 can exist down to −48 °C (mTeu), and for this particular sol, liquid solutions can exist for 1.5 Mars-hours longer than previous predictions. (b) shows a colder surface temperature time-series at Ls = 192° where T < −15 °C and the metastable eutectic (mTeu = −35 °C) only extends the liquid solution duration by approximately 14 Mars-minutes. (c) shows a temperature time-series at 3 cm depth and Ls = 274°, where mTeu = −48 °C because Tmax > −15 °C, and thus metastable liquid solutions can exist for at least 3 Mars-hours longer. (d) shows a temperature time-series at 9.5 cm depth and Ls = 291°, where mTeu = −35 °C because Tmax < −15 °C, and thus metastable liquid solutions can exist 2 Mars-hours longer. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
metastability of brines 3 °C below the stable eutectic no matter the maximum temperature. These findings call attention to a key issue with current models that model the possibility of brines on Mars – that they do not consider metastability. Current models only consider when/where stable liquid solutions can exist using thermodynamics, but ignore any hysteretic effects. Thus, to fully understand when and where brines could exist on Mars today, metastability must be considered. These findings should be included in models to more accurately map out the potential for liquid water on the surface and subsurface of Mars today.
eventually freeze. However, we did not see this. A 7 mM MgCl2 solution was held at 5 °C below the eutectic temperature for 60 h and the sample did not freeze (Fig. 4). In order to predict the places and seasons in which liquid solutions are possible on Mars today, we need consider all the different conditions in which these solutions can exist. This study expands on the many conditions when metastable liquids can exist and adds another layer of complexity in predicting their existence. Not only is there hysteretic behavior of the eutectic temperature between freezing and warming, but it is also important to consider how warm the sample was before freezing. If there are MgCl2 salts in the area of interest, the maximum temperature reached for that sol is important in determining how long brines can exist, while an area with NaCl salts can only extend the
Fig. 9. Plot of MgCl2 brine duration vs solar longitude (Ls), as a function of subsurface depth at Palikir crater, Mars (Ls values > 360° indicates the start of the second Mars-year). (a) Brine durations predicted by the temperature being above the stable eutectic temperature, and (b) brine durations predicted by taking into account metastability (i.e., dependent on the maximum temperature reached and the corresponding metastable eutectic temperature).
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Investigating the hysteretic behavior of Mars-relevant chlorides ⁎
K.M. Primma, , D.E. Stillmana, T.I. Michaelsb a b
Dept. of Space Studies, Southwest Research Institute, 1050 Walnut St. #300, Boulder, CO 80302, USA SETI Institute, 189 Bernardo Ave Suite 200, Mountain View, CA 94043, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Brines Mars Chlorides Metastability Hysteresis
Liquid solution stability has been a highly studied topic in the Martian community since the detection of perchlorate (ClO4−) at the Phoenix landing site and the global detection of chloride (Cl−) by THEMIS (Thermal Emission Imagining System, onboard Mars Odyssey). Understanding how brines form and react to changing environmental conditions helps identify potentially habitable environments on Mars, both at present and in the past. Here we measure the extent of metastability of magnesium chloride (MgCl2) and sodium chloride (NaCl) brines when freezing. We find that the metastable eutectic temperature of MgCl2 depends on the maximum temperature (Tmax) reached before freezing. If Tmax < −15 °C, the metastable eutectic temperature (mTeu) is only 3 °C below the stable eutectic temperature, and if Tmax > −15 °C, mTeu is 15 °C below the stable eutectic temperature (Teu). We speculate that this metastable behavior follows the phase diagram for the transition into the 8 hydrate for MgCl2, thus, yielding a different freezing temperature, the peritectic for MgCl2·8H2O. However, mTeu for NaCl is independent of Tmax and was constantly at 3 °C below Teu with no peritectic (consistent with the phase diagram). We also found that MgCl2 brine can exist for at least 60 h at 5 °C below its Teu. Applying our findings, we determined the potential time evolution of brines at Palikir crater, using a time-series of modeled temperature profiles. Surficial layers melt more frequently, but layers at 2–3 cm depth are able to warm above Tmax > −15 °C and maintain brine for longer than surficial layers. The evaporation rate of brine buried by 2–3 cm of regolith is greatly reduced due to the generally cold temperatures, solute concentration, and by the regolith overburden. We also found that at Palikir crater, only the deep subsurface (~9.5 cm depth) has water activities (~0.75) high enough to support life. Overall, the metastable properties of brines can drastically affect their formation and longevity on Mars, and should be considered in future models.
1. Introduction In addition to perchlorate (ClO4−), chloride (Cl−) salts have also been found in several places on the surface and subsurface of Mars (e.g., Hecht et al., 2009; Osterloo et al., 2010, 2008). Perchlorate and chloride salts have remarkable properties, such as: (1) brines that do not fully freeze except at very cold temperatures (~205 K); (2) they can absorb atmospheric water vapor to form a brine (a process known as deliquescence) (e.g., Gough et al., 2011; Primm et al., 2017); and (3) ClO4− and Cl− brines can maintain a metastable liquid phase down to even colder temperatures (~123 K for Mg(ClO4)2; Toner et al., 2014a, 2014b) and exist over a larger relative humidity (RH) range (Primm et al., 2017). According to a binary mixture phase diagram, a stable brine occurs when a salt-H2O mixture has a salt mass percentage within a specific temperature-dependent range and, if the mixture is in contact with gas, when the relative humidity is also within a specific temperature-dependent range. More quantitatively, stable brines exist
⁎
(often in combination with salt hydrate or ice) when: (1) the salt mass percentage is less than the salt mass percentage of the lowest peritectic (point at which a liquid phase and solid phase form a new solid phase), (2) the temperature is greater than the eutectic temperature (Teu), and (3) when the RH is greater than the deliquescence relative humidity (DRH; the RH at which deliquescence occurs). Thus, Cl− and ClO4− brines are much more stable than pure liquid water and have been suggested to play a role in processes on contemporary Mars that might involve water: cryosuction (Sizemore et al., 2015), Recurring Slope Lineae-RSL, (e.g., Chevrier and Rivera-Valentin, 2012; Heinz et al., 2016; Bhardwaj et al., 2019), gullies (Goldspiel and Squyres, 2011), slope streaks (Bhardwaj et al., 2019, 2017; Mushkin et al., 2010), dark dune spots (Kereszturi and Rivera-Valentin, 2012), and subglacial brines (Orosei et al., 2018). There has been extensive work on the deliquescence, efflorescence (recrystallization from a brine via the loss of liquid water to the environment as water vapor), and freezing of Cl− and ClO4− brines (e.g.,
Corresponding author. E-mail address: [email protected] (K.M. Primm).
https://doi.org/10.1016/j.icarus.2019.06.003 Received 8 March 2019; Received in revised form 28 May 2019; Accepted 4 June 2019 0019-1035/ © 2019 Elsevier Inc. All rights reserved.
Please cite this article as: K.M. Primm, D.E. Stillman and T.I. Michaels, Icarus, https://doi.org/10.1016/j.icarus.2019.06.003
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2. Experimental methods
Gough et al., 2011; Nuding et al., 2014; Primm et al., 2017). These studies show that if Cl− and ClO4− salts are exposed to certain water vapor concentrations (e.g., relative humidities), they would deliquesce into liquid solutions on present-day Mars. Because of metastability and hysteretic behavior, these brines could exist outside of their thermodynamically-predicted stability range. In this paper, we use metastability to define when the phase of the salt-H2O mixture is outside the bounds of the thermodynamically-predicted phase. Hysteresis occurs when the history of the salt-H2O mixture determines when a phase transition will occur. Thus, once a brine is formed it is metastable and hysteretic, as it requires (1) a lower RH to transform it into a brine-salt hydrate mixture, (2) a higher RH to transform it into a brine-ice mixture, and (3) the temperature of the brine must be decreased below the eutectic temperature before freezing into an ice and salt hydrate mixture. The latter condition is the focus of this paper. Although the exact salts on the surface of Mars have not been confirmed, magnesium chloride (MgCl2) and sodium chloride (NaCl) have been model-predicted from Phoenix Wet Chemistry Laboratory data to be present the surface of Mars (Marion et al., 2010; Toner et al., 2014b). Previous measurements found that solutions of MgCl2 (Teu = −33 °C) were metastable at temperatures between 9 and 14 °C below the eutectic temperature, and for NaCl, 6.6–8.3 °C below its eutectic temperature (Teu = −21.3 °C) when the sample temperature was decreased (Toner et al., 2014a, 2014b). Primm et al. (2017) observed similar behavior with MgCl2, observing no freezing even at temperatures 20 °C below the stable eutectic temperature. Even so, Toner et al. (2014a, 2014b) and Primm et al. (2017) did not study the effects of the duration spent at temperatures below the eutectic. Toner et al. (2014a, 2014b) controlled the temperature rate by changing the sample size from a salt-water mixture of 20 g to 50 g. In contrast, the samples in Primm et al. (2017) were much smaller (sub-mm particles) and used a temperature controller to change the temperature. Their experimental time scales were on the order of minutes to an hour. The experimental studies in this paper aim to represent bulk subsurface ice. Bulk ice describes multiple grains (sub-mm) of ice in a sample where liquid solutions can exist between the individual ice grains. The spatial distribution of liquid solution between these ice grains is termed a liquid vein network (LVN) and can be easily detected by measuring the electrical properties of the ice (Grimm et al., 2008; Stillman et al., 2013, 2010). Here we examine the metastability of bulk solutions of MgCl2 and NaCl over different starting concentrations, varying the time spent below the eutectic temperature, and varying the starting temperature to see if any of these factors change the metastability properties of these salts.
2.1. Sample preparation and sample holder Solutions of MgCl2 (5, 7, 10, 20, 30, 63 and 100 mM) and NaCl (10, 30, 60 and 100 mM) were prepared in high purity water. The salts used were MgCl2·6H2O and anhydrous NaCl, purchased from Sigma-Aldrich (309303 and 793566, respectively). Each sample solution was poured into a three electrode sample holder where the sample is fixed at a height of 5 mm (relative to the 31 mm diameter Teflon cup it is enclosed within). The sample holder with the sample solution was then loaded into an insulated box within an ultra-low freezer (So-Low C85-9) where we can control the temperature with a Lakeshore 331 temperature controller. The full experimental set-up is described in more detail in Stillman et al. (2010) and Grimm et al. (2008). 2.2. Electrical property measurements At every temperature, the electrical properties (complex impedance) of the sample are measured using a Solartron 1260A Impedance Analyzer and the Solartron 1296A Dielectric Interface over a frequency range of 100 mHz–1 MHz. The complex impedance was converted into complex dielectric permittivity using the electrode geometry. The data were then fit using Cole-Cole parameters and the Direct-Current (DC or zero-frequency) conductivity (e.g., Stillman et al., 2013). To verify the literature value of the eutectic temperature, the sample was initially frozen onto the electrodes quickly by decreasing the temperature directly to near −85 °C. The temperature was then raised by intervals between 0.25 and 5 °C until −0.5 °C was reached. The DC conductivity of brine is much higher than that of ice, salt hydrate, mineral grains, or adsorbed water. Thus, when brine forms a connected path through the sample, the value of the DC conductivity greatly increases. For example, a few degrees below the eutectic temperature the change in DC conductivity with increasing temperature is large producing a steep slope, as premelted (quasi-liquid films that exist below the melting temperature) eutectic brines increase the conductivity. At temperatures warmer than the eutectic, the DC conductivity slowly increases with temperature yielding a much smaller slopes as the brine becomes more conductive with increasing temperature and more of the sample melts. Thus, the eutectic temperature can be identified at the cusp or change in slope in plots of DC conductivity versus temperature (Grimm et al., 2008; Stillman et al., 2010; Stillman and Grimm, 2011a). The temperature was also held at that warmest temperature (−0.5 °C) for ~8 h to allow recrystallization of the ice to occur in order to obtain a more uniform grain size distribution, allowing more-repeatable measurements. To determine the extent of metastability of the sample, the temperature was then lowered by 0.25–10 °C increments until −85 °C was reached. Such warming, recrystallization, and cooling cycles were
Fig. 1. Example of 12 experimental temperature profiles where the maximum temperature was varied. The temperature reaches the warmest temperature, then is cooled, and the next experiment is then warmed up to a different starting temperature and follows a similar cooling temperature profile. 2
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temperature of MgCl2 are consistent with previous studies, the temperature was then decreased to determine the eutectic temperature when freezing. Fig. 3 shows a 7 mM MgCl2 sample in which the warming and freezing experiments exhibit vastly different eutectic temperatures. The sample did not freeze until −48 °C, expressing a large (16 °C) region where the LVN are metastable. This is also consistent with Toner et al. (2014a, 2014b) where they found that metastable liquid existed between 9 and 14 °C below the eutectic temperature. To test how stable these metastable LVN in mixtures of MgCl2 and ice are, the sample was warmed to −0.5 °C and then measured at numerous temperatures above the eutectic, then held at −37 °C (5 °C below the eutectic temperature) for > 60 h. Fig. 4a demonstrates that the DC conductivity of the sample at −37 °C varies little and still possesses LVNs. Fig. 4b shows how the conductivity at 0.1 Hz changed throughout the 60 h. Note, typically the DC conductivity value is fit via the Cole-Cole model, so we assume the lowest frequency (0.1 Hz) is a good approximation to the DC conductivity value as it is accurate to a few percent. Although this solution is technically metastable, it is quite stable. Next, we tested whether different concentrations and maximum warming temperatures affected the metastability of brine. Fig. 5 shows that metastability does vary between samples, but it varies independently of concentration. Our measurements are consistent with Toner et al. (2014a, 2014b), but are much less metastable than Primm et al. (2017). We hypothesize that the discrepancy in our measurements vs. Primm et al. (2017) is due to the size of the samples. Primm et al. (2017) studied single-particle samples (diameter of 20 μm; volume ≈ 3 × 10−8 cm3), while our measurements are for a bulk sample volume of approximately 3 cm3. To test our hypothesis about how metastability varies with maximum warming temperature, we froze each sample to −55 °C and then raised the temperature to the maximum temperature for at least an hour, before refreezing the sample (see the temperature profile in Fig. 1 for more information). Fig. 5 displays the observed metastable eutectic temperature versus the maximum temperature reached before refreezing occurred. These change-in-maximum-temperature experiments were performed on 7 mM (blue) and 100 mM (cyan) solutions of MgCl2. In Fig. 5, when the maximum temperature was below −15 °C the extent of metastability was only ~3 °C below the eutectic. However, for the warmer temperatures above −15 °C, the LVN were metastable for 15 °C below the eutectic. Thus, metastability and hysteresis greatly increase for MgCl2 samples that have been raised to temperatures > −15 °C. Furthermore, there is also considerable variation in the metastable eutectic temperature when freezing from a temperature above −15 °C. We hypothesize this variation is natural and just a product of measuring a stochastic process of ice nucleation. The lines in this study (black dashed lines) were calculated by taking the average of either all the points below or above −15 °C, and the error bars are the standard deviation of the points.
repeated multiple times. To further investigate the metastability of the sample solution, the maximum temperature the sample was allowed to warm to was varied. That is, after the previous freezing experiment the sample would not be warmed all the way up to −0.5 °C, but instead only to various colder temperatures above the eutectic temperature (e.g., −5, −10, −15 °C). Fig. 1 shows a typical temperature profile for this change-in-maximumtemperature procedure. 2.3. MarsFlo modeling To model a representative subsurface temperature environment, we use MarsFlo (Grimm et al., 2019, 2017, 2014; Grimm and Painter, 2009; Painter, 2011) integrated with the Mars Atmospheric Regional Modeling System (MRAMS; Michaels and Rafkin, 2009). MarsFlo replaces the standard subsurface module in MRAMS that computes subsurface volatile and heat distribution. The MarsFlo component proficiently simulates three-phase (liquid, ice, vapor) H2O migration and thermal evolution in porous media, while the MRAMS component provides realistic surface boundary conditions (e.g., surface energy balance, atmospheric pressure) as functions of time-of-day, season, and latitude. The MarsFlo run in this work was a 1-D run using a grid of 90 points nonlinearly-spaced between the surface (1 mm cell thickness) and 50 m depth (~4 m cell thickness). Palikir crater (41.6°S, 202.3°E) was chosen because it has some of the largest confirmed RSL in the southern midlatitudes (McEwen et al., 2013; Stillman and Grimm, 2018) that actively lengthen from Ls ~200–315° (Ls, or solar longitude, where Ls = 270° is southern summer solstice) for west- and northwest-facing slopes. Active gully formation has also have been observed at this site at around Ls ~180° (Dundas et al., 2012, 2015, 2019). Two main zones of representative thermophysical properties were used: an uppermost 6 cm of fines (sand; 45% porosity), over fractured bedrock (15% porosity). The run was initialized with an isothermal profile (210 K) and run for two Mars-years, with only the second year of output being used for analysis. The realistic surface forcing used was for a 30-degree westfacing slope (albedo of 0.12) at Palikir crater, present-day Mars. 3. Results 3.1. Verification of stable eutectic temperature The formation of LVN is identified by analyzing how DC conductivity varies with temperature, as the formation of LVNs significantly increases the conductivity of the sample. Thus, we can measure the eutectic and metastable eutectic temperatures (mTeu) by identifying the temperature at which these changes in DC conductivity occur. Fig. 2a shows eight different experiments where four MgCl2-H2O mixtures and four NaCl-H2O mixtures (Fig. 2b) were warmed until melting (eutectic temperature) was observed. Our warming measurements for varying MgCl2 concentrations yield a eutectic temperature of −32.2 ± 0.2 °C, which is slightly lower than the literature value of −33.0 °C (Toner et al., 2014a). The eutectic temperature found here for NaCl (Fig. 2b) is consistent with that in the literature, 21.3 °C (Toner et al., 2014a). The average value and error of our eutectic temperature values come from the standard mean of the experimental eutectic temperatures and the standard deviation of the measured values. Fig. 2 also confirms that the eutectic temperature is independent of salt concentration. The DC conductivity of the mixtures increases with increasing salt concentration because at higher concentrations a greater percentage of the sample is brine at any given temperature.
3.2.2. NaCl The eutectic temperature of NaCl (−21.3 °C) is much higher than that of MgCl2 (−33 °C), and freezing measurements indicated a small difference in the metastable eutectic temperature of only ~3 °C (Fig. 6). This smaller metastability range was expected because Toner et al. (2014a, 2014b) only found metastability 6.6–8.3 °C below the eutectic temperature, compared to 14 °C for MgCl2. We speculate that the timescales and sample sizes for our experiment versus the Toner et al. (2014a, 2014b) experiments could explain this discrepancy between our metastability values. We use much slower timescales (approximately an hour per each temperature reached) and a smaller sample size (~5 g), while Toner et al. (2014a, 2014b) changed the temperature much more quickly and did not let the temperature come to an equilibrium with the sample with a sample size of ~20–50 g. We would expect that a sample that has more time to come into equilibrium with a
3.2. Metastability 3.2.1. MgCl2 After verifying that our experimental results for the eutectic 3
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Fig. 2. Direct Current (DC) conductivity vs temperature of eight different concentrations of MgCl2 (a) and five different concentrations of NaCl (b). For these experiments, the sample was initially quickly frozen at between −70 to −85 °C and then was warmed until the eutectic temperature was observed (cusp or no change in slope of DC conductivity). All plots are for warming experiments. DC conductivity of the mixture increases with increasing salt concentration. This increase in conductivity shows that there is brine present, which is significantly more conductive than water ice alone. Fig. 3. A warming experiment followed by a freezing experiment, showing the hysteretic behavior of MgCl2 (7 mM). The blue box on the left represents stable liquid solutions and the red box in the middle represents metastable liquid solutions, followed by the grey box representing ice. This plot illustrates that the sample melts at the eutectic temperature, but then freezes at 15 °C below the eutectic temperature. This behavior is seen throughout several experiments and concentrations. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
of NaCl (i.e., 30 and 60 mM). NaCl exhibits no significant change in metastability with changing concentration and maximum temperature, resulting in a constant metastable eutectic that is 3° below the stable eutectic of −21.3 °C. Note that the 10 and 100 mM samples were not measured while freezing.
temperature change would exhibit less metastability, however, it is unclear as to if a smaller bulk sample would change metastability (as opposed to single particle samples in Primm et al., 2017). As with our MgCl2 experiments, the maximum temperature of our NaCl experiments was varied (Fig. 7). Fig. 7 shows the results from varying the maximum temperature reached for each experiment for 2 different concentrations 4
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Fig. 4. a) Plot of DC conductivity (S/m) vs temperature (°C) of MgCl2, showing no jump in DC conductivity, indicating that no freezing occurred. Fig. 3 (cyan trace) shows the typical drop in DC conductivity for a freezing transition (~DC conductivity drop to 10−8 S/m) b) Plot of conductivity at 0.1 Hz vs. elapsed time (hours) We assume the lowest frequency (0.1 Hz) is a good approximation to the DC conductivity value as it is accurate to a few percent. This plot displays the last point in (a) throughout time. The conductivity throughout the 60 h does not dramatically change slope, and the sample is thus not changing phase. The colors represent the time elapsed. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
≤−15 °C, MgCl2 transitions to the 12 hydrate which has a higher eutectic (~−33 °C). In the case of NaCl, there is no hydration transition within the temperature range studied (Toner et al., 2014a, 2014b). However, we cannot confirm this because our measurements cannot determine the specific salt hydration state(s) present. Additionally, when we re-warm the ice/salt-hydrate mixture that froze at the mTeut it does not melt until it reaches the normal 12 hydrate eutectic temperature. For the above hypothesis to be correct, the frozen 8 hydrate would have to change to 12 hydrate once the sample is heated. Understanding the metastability of these salty ices is highly relevant to the subsurface ice at the Phoenix landing site (Mellon et al., 2009), where we could have LVN within the ice or in the near-subsurface (e.g., Fischer et al., 2016; Stillman and Grimm, 2011a, 2011b). If we know the temperature profile of the subsurface where the ice is and the identity of the salt(s) incorporated into the ice, we can map out how long the LVN can exist using our results. Fig. 8 shows a modeled temperature profile at Palikir Crater on Mars for four different sols at different depths. The top left (a) plot shows that liquid solutions can exist longer (1.5 Mars-hours more) when the temperature is higher, however, the top right (b) plot shows that liquid solutions cannot exist as long, and because they do not exceed −15 °C, their metastability only adds 14 Mars-minutes. Additionally, (c) and (d) show that below the surface (where the temperature profile is not as sharp long as the temperature is higher than the stable eutectic) you can have brines for a long time (~3 Mars-hours). Fig. 9 shows how MgCl2 brine duration varies seasonally depending on the (a) stable eutectic temperature or the (b) metastable eutectic
4. Discussion The shallow subsurface environmental conditions (e.g., pressure and temperature) on contemporary Mars only rarely support the existence of pure liquid water. However, salts can depress the freezing point of water, allowing brines to exist in colder conditions. Understanding the extent of liquid solution stability can help determine if liquid brine exists near the surface on present-day (and past) Mars. Current models of the subsurface of Mars do not consider metastability regions of these salty solutions (e.g. Cl− or ClO4−), considering only stable liquid solutions (e.g., Chevrier and Rivera-Valentin, 2012; Martín-Torres et al., 2015; Pál and Kereszturi, 2017). If we were to incorporate the metastability regions of liquid solutions, it would provide for a more accurate representation of the presence or absence of liquid solutions. Here we demonstrated that the temperature at which the LVN within the ice freezes depends on how warm the ice was before the temperature is decreased for MgCl2 mixtures. However, no change in metastability for the NaCl mixture was observed with variable maximum temperatures. We hypothesize that this change in metastable eutectic temperature for MgCl2 could be due to changes in the hydration state of the salt. Theoretical models of MgCl2 show that three hydration states exist (6, 8, and 12) (Primm et al., 2017 Fig. 4). The transition point between the 8 and 12 hydrate is near −15 °C. It is possible that when the sample is warmed up to greater than −15 °C, the brine does not freeze at its typically eutectic temperature but instead freezes at when MgCl2 transitions to the 8 hydrate which has a lower metastable eutectic (~−42 °C), but if the sample is warmed up to
Fig. 5. Freezing temperature vs. starting temperature (°C) of various MgCl2 concentrations, where the maximum temperature is the highest temperature that experiment reached before cooling again. The red box under the eutectic temperature is the region where Toner et al. (2014a, 2014b) saw metastable liquid solutions of MgCl2, and the dashed green line is the lowest freezing temperature at which Primm et al. (2017) observed liquid solutions. These experiments were performed with 7 mM and 100 mM concentrations of MgCl2. When the samples were warmed to temperatures above −15 °C, metastable liquid solutions existed 15 °C below the eutectic (black dashed line). However, when the samples were only warmed to temperatures below −15 °C, metastable liquid solutions existed only 3 °C below the eutectic temperature (top black dashed line). The lines in this study were calculated by taking the average of either all the points below or above −15 °C, and the error bars are the standard deviation of the points. Note that there are also error bars on the traces, but the majority of them are smaller than the trace. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 5
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Fig. 6. A warming experiment followed by a freezing experiment, showing the hysteretic behavior of NaCl (30 mM). The blue box on the left represents stable liquid solutions, the red box in the middle represents metastable liquid solutions, followed by the grey box representing ice. This plot shows that the sample melts at the eutectic temperature, but then freezes at 3 °C below the eutectic temperature. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
humidity) and temperatures of these solutions are lower than what is required for life. However, if the water activity of these solutions becomes > 0.61 (Grant, 2004), then life is said to be able to survive and grow. The water activities of NaCl solutions can be as much as 0.83 (Rummel et al., 2014; Wise et al., 2012) compared to MgCl2 with a range between 0.33 (experiments from Primm et al., 2017) and 0.73 (model from Primm et al., 2017), but the temperatures of these solutions can be as low as −48 °C. This is less than the temperature at which metabolic activity is seen, −33 °C (Wierzchos et al., 2006). Fig. 8(d) reports the maximum water activity, Aw,max,l (maximum value for water activity when the temperature was above the eutectic), for a particular sol. Because life requires a water activity of > 0.61, only the deeper subsurface (i.e., 9.5 cm) which has a Aw,max,l = 0.75 could support life. Although understanding when/where liquid solutions are possible on Mars (e.g., stable and metastable brines) is helpful in understanding Mars' habitability, the temperature of these liquid solutions must also be considered. Nevertheless, we do not know if or how organisms might have evolved on different planets.
Fig. 7. Freezing temperature vs. maximum temperature (°C) of 30 and 60 mM NaCl concentrations, where the maximum temperature is the highest temperature each experiment reached before cooling again. The red box under the eutectic temperature is the region where Toner et al. (2014a, 2014b) observed metastable liquid solutions of NaCl. No change in metastability was observed when changing the maximum temperature of these 30 and 60 mM NaCl samples. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
5. Conclusions Here we have used electrical property measurements to study the metastability of LVN within bulk ice mixed with varying concentrations of MgCl2 and NaCl. Two previous studies have looked at the metastability of MgCl2 below the eutectic temperature (bulk, 20–50 g samples) or at the metastability of ice-saturated conditions (sub-mm particles), but here we validated the chief findings of those studies using a different technique. Both previous studies always warmed the sample up to room temperature (20–25 °C) before the start of the next experiment, but there are temperature profiles on Mars that do not reach these warm temperatures daily. Because of this, we decided to test the extent of metastability when the sample was not warmed up to well above the eutectic. We find that the extent of metastability for MgCl2 depends on the highest temperature it reached before the temperature decreased, while no change was seen for NaCl when the warmest starting temperature was changed. We varied the warmest temperature the MgCl2 sample reached from −30 to −0.5 °C and decreased the temperature to determine at what temperature the sample would freeze. When the sample was warmed to below −15 °C the sample froze only 3 °C below the eutectic temperature, but when the sample was warmed to above −15 °C the sample did not freeze until 15 °C below the eutectic temperature. This held true for two different starting concentrations, 7 mM and 100 mM, showing that the starting concentration does not affect the metastability conditions. Furthermore, we examined the effect that time might have on the metastable solution. It is often assumed that these metastable solutions are not allowed to reach equilibrium, and after time, the sample should
temperature that corresponds with the maximum temperature reached. The brine duration was calculated by starting the brine formation at the stable eutectic, and when the temperature starts to decrease after the maximum, brine continues to exist at one of the different metastable/ stable eutectic temperatures determined. We find that when mTeu = −48 °C, brines can exist up to 2 Mars-hours longer than if you were to use the stable eutectic temperature to predict brine formation. Surficial layers melt more frequently, but layers at 2–3 cm depth are also able to reach warm temperatures (i.e., Tmax > −15 °C) and maintain brine for longer (around Ls = 270°) than surficial layers (Fig. 9). While not modeled, the evaporation rate of brine at 2–3 cm depth is greatly reduced due to the cold temperatures, solute concentration, and by the regolith overburden (Bryson et al., 2008; Chevrier and Altheide, 2008). Furthermore, brine duration using the metastable eutectic temperature demonstrated that brines could exist 2–3 cm below the surface for more than half a sol. Not only do these findings help understand the metastability of liquid solutions, they can also provide insight into the possibility of organisms living within these LVN. Because life as we know it on Earth requires liquid water and the detection of liquid water on Mars has yet to be confirmed, life on Mars appears unlikely. Although several studies tested the likelihood of liquid solutions on Mars with only high humidities and salts present, the water activities (partial pressure of water in a substance divided by the saturated vapor pressure of pure liquid water at the same temperature; generally can be equated to relative 6
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Fig. 8. Modeled temperature profile at Palikir Crater using MarsFlo. This plot shows the region of stable (dark blue) and metastable liquid solutions (light blue) from our results for MgCl2 (dashed magenta line), depending on the maximum temperature reached. Each plot has the maximum water activity while liquid solutions exist for that sol, Aw,max,l. (a) shows a surface temperature time-series of a sol at Ls = 270° where the temperature reaches > −15 °C. When Tmax > −15 °C, metastable solutions of MgCl2 can exist down to −48 °C (mTeu), and for this particular sol, liquid solutions can exist for 1.5 Mars-hours longer than previous predictions. (b) shows a colder surface temperature time-series at Ls = 192° where T < −15 °C and the metastable eutectic (mTeu = −35 °C) only extends the liquid solution duration by approximately 14 Mars-minutes. (c) shows a temperature time-series at 3 cm depth and Ls = 274°, where mTeu = −48 °C because Tmax > −15 °C, and thus metastable liquid solutions can exist for at least 3 Mars-hours longer. (d) shows a temperature time-series at 9.5 cm depth and Ls = 291°, where mTeu = −35 °C because Tmax < −15 °C, and thus metastable liquid solutions can exist 2 Mars-hours longer. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
metastability of brines 3 °C below the stable eutectic no matter the maximum temperature. These findings call attention to a key issue with current models that model the possibility of brines on Mars – that they do not consider metastability. Current models only consider when/where stable liquid solutions can exist using thermodynamics, but ignore any hysteretic effects. Thus, to fully understand when and where brines could exist on Mars today, metastability must be considered. These findings should be included in models to more accurately map out the potential for liquid water on the surface and subsurface of Mars today.
eventually freeze. However, we did not see this. A 7 mM MgCl2 solution was held at 5 °C below the eutectic temperature for 60 h and the sample did not freeze (Fig. 4). In order to predict the places and seasons in which liquid solutions are possible on Mars today, we need consider all the different conditions in which these solutions can exist. This study expands on the many conditions when metastable liquids can exist and adds another layer of complexity in predicting their existence. Not only is there hysteretic behavior of the eutectic temperature between freezing and warming, but it is also important to consider how warm the sample was before freezing. If there are MgCl2 salts in the area of interest, the maximum temperature reached for that sol is important in determining how long brines can exist, while an area with NaCl salts can only extend the
Fig. 9. Plot of MgCl2 brine duration vs solar longitude (Ls), as a function of subsurface depth at Palikir crater, Mars (Ls values > 360° indicates the start of the second Mars-year). (a) Brine durations predicted by the temperature being above the stable eutectic temperature, and (b) brine durations predicted by taking into account metastability (i.e., dependent on the maximum temperature reached and the corresponding metastable eutectic temperature).
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Acknowledgements
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