Abstract
The possible presence of brines on Mars adds an intriguing dimension to the exploration of Martian environments. Their potential involvement in the formation of recurring slope lineae has sparked debates on the existence of liquid water versus alternative dry processes. In situ instrumentation on rovers and landers has been instrumental in providing valuable data for comprehending the dynamics of brines. Laboratory experiments and thermodynamic simulations conducted under Martian conditions offer insights into the formation and persistence of brines, shedding light on the planet’s current hydrological processes. Despite these findings, the prevailing surface conditions on Mars, characterized by a combination of low temperature, pressure, and water vapor pressure, generally hinder the stability of most brines. In such environments, only a few select salts, notably calcium perchlorate, could play a pivotal role in potentially forming brines through deliquescence or melting. These environmental factors emerge as critical contributors influencing the stability of brines, but such limitations generally restrict the locations, timescales, and amounts of brine formed. However, the exploration of brines extends beyond geochemical considerations, serving as a lens through which we can examine potential habitability and gain a broader understanding of the Martian climate. Therefore, observing brines on Mars would offer valuable insights into the dynamic interplay of various factors that influence their stability, contributing to our overall comprehension of Mars’ unique environmental conditions.
Keywords: Mars, water, brines
The astrobiological potential of Mars has always been intimately connected to the existence of liquid water. NASA even made it the main objective for missions to the red planet under the motto “Follow the water” (1). Despite the massive amount of mineralogical, geochemical, and geomorphological evidence for ancient liquid water in the early history of Mars, and the similar amount of data showing that water ice is still abundant today, finding liquid water on the surface of Mars has proven particularly difficult. The low temperatures and pressure on the surface limit water thermodynamically to ice deposits, as demonstrated by the extensive permafrost and large ice caps in the polar regions, and water vapor. Liquid water in its pure state is simply not stable on the surface. However, small but significant observations by orbiters and landers have suggested that intermittent liquid water might be possible. Therefore, recent research has extensively focused on the possibility of brines (aqueous solutions containing one or several dissolved ionic salts) as an alternative to pure water, as they exhibit enhanced stability with respect to the Martian surface environment. Moreover, brines are very important for various geomorphological and geochemical processes, such as ground ice processes and ice distribution, chemical weathering, duricrust formation, and even organics preservation or degradation. This perspective paper starts with a short review on possible liquid water-related features on Mars. It then presents brine properties and their behavior under martian environmental conditions and how different aspects of brines affect their potential habitability. Finally, this paper discusses the challenges associated with the in situ detection of brines and the identification of regions where brines could be present.
Observational Evidence for Present-Day Liquid Water on Mars
Although the first observations of water on Mars date back 60 y, those were initially confined to atmospheric water vapor (2, 3). Later observations confirmed the existence of water ice in the polar caps (4, 5). The Mars Odyssey neutron spectrometer and gamma-ray spectrometer provided the first comprehensive observation of surface and shallow subsurface water on a global scale (6). Notably, regions above 60° latitude exhibit high water ice abundances in the top meter of the regolith, with concentrations exceeding 25 wt% across most areas and nearing 100 wt% at the poles (7). This widespread ice presence is further corroborated by the SHARAD (Mars SHAllow RADar) and MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) radar sounding instruments, which identified ice-rich features in individual surface formations (8, 9). While glaciers persist on Mars, they are not as prevalent as in the past. There is still evidence of their existence though, whether they are buried beneath the surface with only the lobate debris aprons visible (10) or covered by rocks on the floors of numerous channels within the fretted terrains (remnant terrains standing above the plains with relatively few craters) surrounding Arabia Terra and other regions (11, 12).
The Mars Reconnaissance Orbiter’s HiRISE (High Resolution Imaging Science Experiment) camera made significant contributions by revealing eight eroding slopes displaying exposed water ice sheets (13, 14). Ground ice, observed at the Phoenix landing site (15) and within recently excavated craters via HiRISE (16, 17), further emphasizes the diverse manifestations of water on Mars. Distinct stripes and polygonal features, indicative of thermal cycles, have been identified in permafrost regions (18, 19), including at the Phoenix landing site (20).
Despite most of these features being subsurface or surface-level, the Martian atmosphere also harbors water. Multiple missions have analyzed the atmospheric column abundance, revealing variations in water content depending on location and season, with an average of 10 to 20 precipitable µm [thickness of the water layer resulting from the condensation of the atmospheric column (21)], but reaching up to 80 precipitable µm near the northern polar cap (22). In addition to these water vapor mixing ratio variations, Mars’ atmospheric relative humidity fluctuates throughout the day, reaching up to 100% in the early morning when temperatures are at their lowest (23–27). However, despite abundant evidence of ancient liquid water on Mars, instances of liquid water on present-day Mars are scarce and heavily disputed.
Phoenix Lander Results.
Analysis of Phoenix Mars lander data revealed potential but debated liquid water formation in the shallow subsurface during the mission (28). The lander’s robotic arm camera captured images of tiny globules on its struts, consistent with hypothesized liquid water behavior in Martian soil (28). Abrupt temperature and humidity changes aligned with the appearance and disappearance of liquid water droplets (Fig. 1A). Thermodynamic calculations indicated that, under specific conditions, the shallow subsurface could support thin films of liquid water due to soil salts acting as antifreeze agents below the freezing point (0 °C). Subsequent observations by the Phoenix’s surface stereo imager of perchlorate salt patches in the regolith’s shallow subsurface suggested that salts were recently remobilized by thin films of liquid water (29).
Fig. 1.
Two of the most-discussed observations of potential liquid water activity on the surface of Mars. (A) Images of droplets on the Phoenix Lander struts (each colored arrow indicates a different droplet) that have been observed to change in size and darkness over the course of the mission. These droplets were interpreted as grains of salts experiencing deliquescence and efflorescence. Figure adapted from ref. 28. (B) Sequence of images from the Mars Reconnaissance Orbiter HiRISE camera showing recurring slope lineae (white arrows) on the slopes in a crater in Terra Cimmeria (Latitude: −32°, Longitude: 140.8°). The images are taken during the southern summer of Mars year 30 and show the evolution of dark streaks on the surface. Images credit: NASA/JPL-Caltech/UArizona. Uploaded by Surajt88 at en.wikipedia, Public domain, via Wikimedia Commons.
The second line of evidence emerged from the Thermal and Electrical Conductive Probe (TECP) onboard Phoenix, designed to penetrate the top few centimeters of the Martian regolith. Variations in permittivity, directly correlated with temperature, were attributed to the melting of salty ice during daylight hours (30). Analysis of recorded eutectic temperatures (lowest temperature at which a brine is thermodynamically stable in the liquid phase, ~−34 °C) suggested the presence of either NaClO4 (eutectic temperature at −34 °C) or MgCl2 (eutectic temperature at −33 °C) brine, likely as disconnected films or droplets, consistent with the negative electrical conductivity results.
Mars Science Laboratory (MSL) Results.
The MSL Curiosity rover monitored diurnal and seasonal trends in relative humidity and temperature at 1.6 m above the ground and at the surface of Gale Crater using the Rover Environmental Monitoring Station (REMS) and the Dynamic Albedo of Neutrons (DAN) instruments. Analysis of these data revealed temperatures at which calcium perchlorate brines could remain stable at 0 and 5 cm depths during winter nights and throughout the rest of the year for shorter durations (31). However, at depths of 15 cm and beyond, brines of calcium perchlorate, and potentially other salt hydrates, could remain permanently stable due to consistent temperature and relative humidity. DAN data, along with rover traverse data, suggested that 1.2 wt% of the water signal is composed of perennially stable subsurface calcium perchlorate hydrates (31). However, a more recent study using recalculated relative humidity data with respect to liquid instead of ice showed that the conditions for brine formation were in fact far more exceptional and could only occur for specific terrains exhibiting low thermal inertia (32).
Recurring Slope Lineae (RSL).
RSL are narrow (0.5 to 5 m), dark linear features that appear on steep slopes and lengthen during the warm season in equatorial and mid-latitude regions. Debate has surrounded the formation of RSL (Fig. 1B) on Martian crater and valley walls since they were first observed (33, 34). Multiple studies proposed liquid water as the cause, supported by several lines of evidence. First, RSL displays seasonal changes, appearing in warmer seasons and fading in colder periods, resembling liquid water behavior (34, 35). Observations from Mars Reconnaissance Orbiter’s Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) initially indicated spectral features consistent with hydrated salts in RSL locations, suggesting a potential source for brine formation (36, 37). Laboratory and terrestrial field studies also showed a striking resemblance between RSL and wetted surface flow features (38–40).
However, a more recent body of literature has contested the “wet” hypothesis. Dry processes, like granular flows or movement of dust and sand without liquid water involvement, have been proposed (41–45). Moreover, the detection of hydrated salts in RSL locations doesn’t explicitly confirm liquid water involvement, and interpretations of other spectroscopic datasets (46), including a reevaluation of the CRISM data (45, 47) even contradict the presence of salts in RSL locations. Seasonal temperature variations on Mars contribute to RSL appearance and disappearance without liquid water, yet salts may remain a key factor, influencing regolith cohesion (44, 48).
Liquid Brine Formation and Stability
Instability of Liquid Water in the Martian Environment.
Mars possesses a thin atmosphere, essentially composed of carbon dioxide, and with a mean surface pressure of around 6 to 7 mbar. Mars is also a cold world, with average surface temperatures around −50 °C at the equator. However, the presence of a thin atmosphere results in large surface temperature variations reaching 80° between day and night. Therefore, although the average temperatures are low, peak temperatures can easily reach above the melting temperature of water. The belief that the surface of Mars is close to the triple point of water was an original argument for the presence of liquid on the surface (49). However, this is missing an important point, because the pressure is essentially the pressure of CO2. The partial pressure of water vapor is 2 to 3 orders of magnitude lower than the saturation pressure at the triple point temperature, which is 6.10 mbar. As such, Mars is an extremely cold and dry desert in which liquid water would very rapidly evaporate (50, 51). Moreover, such low total pressure also implies a very low boiling point, equal to or close to the triple point. In other words, even if pure liquid water was somehow introduced on the surface, it would almost instantly freeze, evaporate, or boil anywhere on the planet (50). Finally, it is very difficult for ice to melt because of the latent heat of sublimation and high thermal inertia, both preventing melting temperatures from being reached.
Effect of Temperature and Melting.
While pure water is not stable on the martian surface, this is not the case for some brines. The initial indications of brines potentially existing on Mars stemmed from the principle that the presence of dissolved salt in water lowers the freezing point of the mixture down to a minimum temperature referred to as eutectic [Fig. 2, (52)]. This is the reason why salts are sprinkled over iced roads in the winter. The temperature of the eutectic depends on the nature of the salt. Mars is a planet extremely rich in salts, which include sulfates, chlorides, and some more exotic salts such as perchlorates, chlorates, and nitrates (53, 54). Perchlorates were discovered by the Phoenix mission in 2008 in the regolith and they appear ubiquitous on the planet (55–57). Perchlorates exhibit remarkably low eutectic temperatures, with calcium perchlorate holding the record at −75 °C, well within the range of martian surface temperatures. Perchlorates can also easily undergo supercooling and remain liquid below their eutectic (58). In these conditions, liquid brines could be possible (59, 60).
Fig. 2.
Eutectic temperatures versus water activities for several salts relevant to Mars. Each symbol color corresponds to a different anion. The dotted line represents the “ice line” or the thermodynamic liquidus boundary between water ice and liquid brine. The gray square at low water activity and temperature corresponds to the zone where salts can be metastable on Mars, e.g., the liquids have a low enough evaporation rate to remain liquid on Mars for a few hours (61). Above this range, boiling and freezing prevent salts from even forming on the surface. The small blue square in the lowest corner corresponds to the limits for a liquid brine to be thermodynamically stable on the surface with respect to freezing and evaporation, where both the temperature and water relative humidity at the martian surface are simultaneously above the eutectic values. Figure adapted from ref. 61.
Atmospheric Water Vapor and Deliquescence.
The drawback to the melting process as a source of brine is that it requires the presence of water ice in contact with the salts. Therefore, more recently, research focused on deliquescence as a process to form liquid brines on Mars (62–64). Deliquescence occurs when the relative humidity (which is the pressure of water normalized to the saturation vapor pressure at the same temperature) above a salt reaches a certain threshold. When the deliquescence relative humidity is reached, the salt absorbs water vapor from the gas phase to form a liquid. For almost all salts relevant to Mars, the deliquescence relative humidity increases with decreasing temperature and higher hydrates. This provides a second process by which liquid brines could form on Mars, and one that does not depend on the presence of ice deposits but on the atmosphere. Moreover, the cycle of temperature and humidity suggests that brines could form diurnally or seasonally. When the relative humidity drops, the brine would then evaporate, and the salt effloresce from the liquid phase. It is also noted that efflorescence always occurs at lower humidity than deliquescence and this hysteresis provides an extended region of metastability for liquid brines (62).
The Temperature and Relative Humidity Conundrum.
Having two processes by which brines can form implies the existence of two thermodynamic parameters controlling their stability. The first is the temperature and the second is the relative humidity (which depends on the atmospheric water vapor pressure and the temperature). Previous meteorological measurements by landers and rovers have systematically shown an inverse correlation between temperature and relative humidity (Fig. 3). In other words, Mars is either warm and dry or cold and humid and even though the temperatures can be high enough for most martian brines to be liquid, the associated dry environment prevents them from forming or staying in the liquid state. This is typically the case in equatorial regions (50, 61). Alternatively, regions at high latitudes reach high humidity values but are too cold and below the eutectic temperature of most salts. In those conditions, only a handful of salts (calcium perchlorate, calcium chloride, calcium chlorate, and magnesium perchlorate, Fig. 2) have the correct thermodynamic properties to form metastable liquid brines on the surface of Mars (61). Of those, only the calcium perchlorate’s eutectic brine can remain thermodynamically stable against all three processes (freezing, evaporation, and boiling, Fig. 2) simultaneously (61, 65). However, this stability occurs only in some regions on the surface and during specific times of the day and year (Fig. 4). No brine has been shown to be permanently stable on the surface over the course of a year (61). So even though brines can be stable, it is geographically limited and intermittent.
Fig. 3.
Temperature as a function of water relative humidity (equivalent to water activity at equilibrium) as measured by the Phoenix Lander (Orange: Thermal and Electrical Conductivity Probe TECP, blue: Measurements performed two meters above ground using the Meteorological MET station), and overlaid on the phase diagram for calcium perchlorate Ca(ClO4)2. The gray zone indicates stable liquid phase. Thick black lines indicate liquidus boundaries, black dotted lines indicate metastable liquidus lines (with respect to other hydrates). Finally, the green box at high relative humidity and temperature indicates the limits for life as we know it: Water activity of 0.6 and temperature of −18 °C (66).
Fig. 4.
Map of thermodynamically stable brine continuous duration for a water activity of 0.52 (Ca-perchlorate), projected on a Mars Orbiter Laser Altimeter shaded relief map, with NASA landers and rovers indicated in red. This map indicates how long a brine remains stable against freezing (temperature), evaporation (atmospheric relative humidity), and boiling (total pressure). This map shows that even the salt with the lowest eutectic temperature is never fully stable over the entire martian surface for an entire day. In most stable locations, the continuous stability reaches around 6 h, possibly up to 12 in the northern high latitudes. Figure adapted from ref. 61.
Stability of Liquid Brines in the Subsurface.
The previous situation at the surface appears slightly different in the shallow subsurface (61). A few meters deep, the seasonal temperature cycle is dampened so the temperature remains quite uniform and varies almost only with latitude. In those conditions, the low temperatures only allow the few salts mentioned above to be present in the liquid phase. Additionally, the existence of a thick layer of regolith above significantly reduces the evaporation rates, due to the slow diffusion of water vapor in a porous medium. In those conditions, the metastable brines could remain liquid for extended periods of time, however, it is difficult to evaluate whether they could be fully stable since they are not in direct contact with the atmospheric water vapor, but rather affected by diffusion through the porous regolith.
Mixtures of Brines.
Another way to increase the stability of brines on the surface (or shallow subsurface) is to have mixtures of salts. A binary mixture of two salts exhibits a lower eutectic temperature than each individual salt. Similarly, experiments demonstrated that a binary mixture of salts exhibited a lower deliquescence relative humidity than each individual salt, down to a minimum value for a fixed composition called the eutonic relative humidity (67). Eutonic relative humidities have been measured and modeled at various temperatures for binary, and even ternary, mixtures involving perchlorates (as the single salt with the lowest eutectic temperature) and other associated ions like chloride or chlorate (67–69). Results showed that even though the resulting lower values extended the stability range of liquid brines on Mars, that change was relatively modest (68). Even binary or ternary (with chloride or chlorate) mixtures with calcium perchlorate only provided an incremental change because the calcium perchlorate’s deliquescence relative humidity is significantly lower than the other salts so the resulting eutonic occurs very close in composition and relative humidity to pure perchlorate. Only salts with comparable eutectic temperatures or deliquescence relative humidity would exhibit a large change when mixed (such as sodium perchlorate and sodium chlorate). Some studies went as far as to model the behavior of complex multicomponent solutions such as those measured by the Phoenix lander using the Wet Chemistry Laboratory (WCL) suite (Fig. 5) (70). But there again, the resulting increase in brine stability was relatively limited. All those studies have nonetheless shown that the best liquid brine candidates are probably perchlorate–chlorate–chloride mixtures of magnesium or calcium as dominant cations (69).
Fig. 5.
Stability of a complex brine of ionic composition as measured by the Phoenix WCL in the system Cl−/ClO3−/ClO4−/SO42−for Na+/K+/Mg2+/Ca2+ (blue region of the diagram). The black line indicates the liquidus between brine and water ice, while the dark blue line indicates the liquidus with the various salts (and is therefore the deliquescence relative humidity). Although the complex brine has a eutectic temperature slightly lower than Ca-perchlorate, around −80 °C, the stability field for the complex brine is relatively similar. The Phoenix meteorological station data presented by Stillman and Grimm (30) are also plotted on this diagram (blue dots on Fig. 3). The colored dots represent various phases of water as determined or suggested by Stillman and Grimm (30) based on the TECP electrical measurements: adsorbed water (red), melted liquid (blue), and undefined between both (green). Figure adapted from ref. 70.
Brine Abundance.
The last important point to consider is the amount of brine formed. The lower the temperatures or relative humidity are, the more specific the composition must be (e.g., restricted to mixtures of salts with the lowest eutectic temperatures, and with abundances closer and closer to the mixture’s eutectic composition) and therefore, the smaller the amount of brine in a natural martian environment (71). In other words, even in a complex salt mixture, only the fraction of salts with the lowest eutectic temperatures will contribute to the formation of the brine. Simplifying to a single salt, calcium perchlorate is the best candidate for brine formation on Mars today, based on its lowest eutectic temperature. But the martian regolith typically contains 1% or less of perchlorate (57), of which only a fraction will be calcium perchlorate. Therefore, even if converted into a brine, this represents only a small fraction of the salts available. Even if there are locations with large ice deposits, the limiting factor is the amount of salt available and capable of forming a brine. In the case of deliquescence, the main limiting factor is the amount of water available in the atmosphere, which quantifies in the tens of precipitable microns according to observations and climate models (72). Even if all the water in an atmospheric column was absorbed for deliquescence, this would still result in microscopic amounts of brine in the regolith, regardless of the amount of salt available. Tens of microns of precipitable water would result in tens of microns of brine. The idea that brines could flow on the surface to form geomorphological features such as RSLs or gullies is not feasible in the present conditions (73). However, they could contribute to initializing a dry flow through lubrication or phase change (48).
Habitability of Brines on Mars.
When considering brines as potential habitable environments on Mars, all the above observations and studies point to the fact that if brines form, they would be present in very small amounts (akin to droplets or very thin films on grains), extremely concentrated, and at very low temperatures. Besides temperature, water activity is another important parameter for estimating the habitability of brines. Water activity is a thermodynamic parameter representing the interactions of water with other solutes. Regardless of the salt, the higher the concentration, the lower the water activity of the brine. At the same time, it is the low water activity that allows the freezing temperature of the brine to drop (the freezing temperature depends only on the water activity, so two brines with the same water activity but different salts will freeze at the same temperature). For example, a calcium perchlorate brine at the eutectic temperature (−75 °C) has a water activity around 0.52. There is an abundant wealth of data on extremophiles on Earth and their capabilities at resisting the harsh martian environment. However, the need for accessible water remains a fundamental requirement. Even the most resilient organisms require water activities above 0.6 (74) and temperatures above ~−20 °C (65, 75). Unfortunately for any putative life, no brine anywhere on Mars can provide these conditions simultaneously, another consequence of the anticorrelation between temperature and relative humidity (65) (Fig. 3). Thus, Mars brines might form but remain highly unhabitable by terrestrial standards.
Finally, two brines of identical water activity and temperature may not be similarly habitable. Other complex parameters have to be taken into account, such as ionic strength (76). Another important parameter is the chao- or kosmotropicity of the salts in the liquid. These parameters refer to the effect a substance has on the structure of proteins (or other macromolecules) in solution. Chaotropic salts favor unfolding and denaturation of proteins while kosmotropic salts maintain their structure (77). This is not a straightforward problem: Too much chaotropicity and the proteins lose their structure and function, but some chaotropicity is beneficial, particularly in cold environment to maintain some protein flexibility. Unfortunately, because this property refers to very complex molecules, it is very difficult to quantify, let alone measure, and therefore apply to natural environments (77, 78).
Challenges in Detecting and Studying Brines on Mars
Past and Present In Situ Detection Methods.
Despite these drawbacks and limitations, there is always the possibility that martian life adapted to those brines and some terrestrial organisms could survive in them, which is a consideration for planetary protection because in that case life on Mars might exist today. Hence, detecting brines in situ remains a major objective in the exploration of the red planet. Unfortunately, detecting brines in such small amounts presents a significant challenge for instrument development. Typical remote sensing methods such as infrared reflectance or Raman spectroscopy exhibit subtle differences in spectra between the liquid, ice, and structural water in hydrated minerals (62, 79). However, the serious limitation with those techniques is the low abundances of brines compared to other forms of water. This is especially true considering that environments where brines could be present are also likely to have other forms of water (ice, adsorbed, hydration, etc.). Alternatively, methods that involve the dielectric constant might be more suitable for characterizing liquid brines (30). In situ electrical conductivity measurements by the Phoenix lander were not conclusive regarding brines, mostly because at such small abundances, brines would not be directly conductive. That said, the dielectric values extracted from the data suggested possible liquids or adsorbed films (30). Dielectric constants extracted from MARSIS radar measurements onboard Mars Express have also been used to show the possible presence of a subsurface brine aquifer close to the South Pole (80), although this interpretation remains highly debated (81) not only regarding the radar data analysis but also the geochemical implications (a lake of almost pure perchlorate at saturation seems highly unlikely). Another approach is to use Mars as a natural lab and directly test the formation of brines. This is the objective of the Habitability, Brine Irradiation and Temperature instrument, which would bring salts to the martian surface and attempt to harvest atmospheric water vapor through deliquescence under ambient martian conditions (82). While this would not prove that Mars has endogenous brines, this would demonstrate that brines can form provided the right salts are present on the surface. Characterizing the presence and nature of brines will require knowledge not only of the environment (temperature, pressure, relative humidity) but also of the salt composition in the regolith, which controls eutectic temperatures and eutonic relative humidity. This could be achieved using a similar WCL instrument akin to the one carried by Phoenix. Mounted on a rover, such instrument could provide important information on the variability of salt composition, and thus brine formation, in the regolith.
Optimum Regions for Future Exploration.
Before sending any instrument to Mars to detect brines, it is important to determine the optimum regions for their future detection. Previous study groups tried to use a multidisciplinary approach to determine the best regions in terms of habitability potential and with the highest risk from a perspective of planetary protection (66, 83). However, the resulting “special regions” were rather broad (66). The recent body of work on brines’ stability on Mars suggests that these special regions are probably rather small on the spatial and temporal scale. As such, RSL represents a possible target for future exploration. Despite the recent literature suggesting they are dry flows, they do not rule out the potential involvement of brines in their formation, especially as an initiation mechanism (44, 48). Even minute amounts of brine could create movement on a slope and initiate a flow, especially through phase transitions between liquid and solid and associated volume or morphological changes. Therefore, despite the controversy on their formation mechanism, RSLs represent one of the best potential targets in the search for liquid brines. However, there are other potential candidates. For example, small pockets of brines could be trapped in ice deposits (84). The polar caps have huge deposits of ice, but the very low temperature in these regions probably prevents any liquid, even calcium perchlorate brines. Alternatively, ice deposits have been recently identified in lower latitude regions (14), where the warmer temperatures might allow seasonal melting or even permanent liquid at depths of a few meters within the ice (as long as the average temperature is above the eutectic).
Due to their atmospheric origin, perchlorates should be relatively homogeneously distributed in the regolith (56), although local processes related to brine activity could have concentrated them (29, 85). Localized chloride deposits could also be interesting targets. Ca-chloride is a good potential candidate for metastable brines, and chlorides would naturally be rich in perchlorates and chlorates. Sulfates are very abundant on Mars, but their eutectic temperatures and eutonic relative humidities are usually quite high, except for ferric sulfate which can remain in the liquid state even below its eutectic temperature through supercooling (50, 86). However, the geochemical conditions required for ferric sulfate to concentrate [extremely acidic environments to prevent the formation of iron (oxy)hydroxides] would make it a rare occurrence on the surface. It is important to remember that of all the landers and rovers on Mars, Phoenix was the best equipped to detect brines and landed in a region with very high potential but was unfortunately unsuccessful at finding direct evidence for their presence. There is a chance a lander with a much-improved instrument for liquid detection would successfully detect brines at the Phoenix landing site.
The Need for More Laboratory Measurements under Martian Conditions.
There is also much-needed laboratory work to be done with respect to thermodynamics and kinetics in the martian environment (e.g., at low temperatures). The Pitzer model is widely used to simulate highly concentrated brines, but the necessary parameters have only recently been measured at low temperatures for some key chlorate or perchlorate species (87, 88) and only a small fraction of binary mixtures have been studied at Mars-relevant temperatures (67). Most measurements of deliquescence have been made at temperatures relatively high for the martian environment and therefore with much higher water vapor pressures. Some studies have suggested that deliquescence might be kinetically hindered at lower temperatures (89, 90). Considering that brines are never continuously stable, deliquescence and efflorescence kinetics might be of prime importance to determine whether salts have enough time to form a liquid in the short window when conditions are amenable. Efflorescence occurs at lower relative humidity than deliquescence because the nucleation of salt crystals requires supersaturation especially at low temperatures. Therefore, a full understanding of the liquefaction cycle would require a proper theoretical analysis of these two processes.
Although not directly liquid per se, adsorption might be important for liquid formation. A substantial body of research suggests that adsorption is likely significant on Mars, particularly due to lower temperatures. The impact of adsorption on deliquescence in the Martian regolith remains uncertain, but analog and lab experiments provide insights. In the Atacama Desert, a clay layer beneath the surface maintains soil moisture year-round by retaining adsorbed water vapor (91). This adsorbed water vapor could then desorb and diffuse through the soil, potentially becoming available for salts to trigger deliquescence. Deliquescence experiments in a Mars-like environment indicated that an adsorbing regolith does not outcompete salt-related water vapor sinks in a hyperarid setting, allowing deliquescence to occur at a normal rate (92). Additionally, laboratory results demonstrated that regolith can inhibit ice and salt crystallization, enabling brines to persist in drier and colder conditions for extended periods (93). On a larger scale, adsorption might function as a water molecule pump from the atmosphere, potentially enhancing deliquescence by increasing water content in the regolith. Alternatively at the grain size scale, direct adsorption or desorption on the salt grains might affect the deliquescence of the salt. Finally, there is still a crucial need for longer-timescale experiments simulating the diurnal or seasonal cycle of water vapor in the regolith, providing a more integrated view of these various processes and their interplay. The effect of the regolith itself remains poorly understood, that is by favoring nucleation, promoting adsorption, the effects of combined heat and mass transfer, etc.
Conclusions.
In conclusion, Mars is located at the extreme limits of brine stability. Only a combination of the most favorable environmental conditions and lowest eutectic temperature salt candidate(s) allows for brines to be at least temporarily stable on the surface. Moreover, the various limiting factors, namely salts abundances, water vapor pressure, and ice location strongly limit the abundances of brines on the surface or shallow subsurface. These combined factors explain why brines remain so elusive and undetected to this day, despite a lot of effort in modeling, experiments, and spacecraft data analysis. Future generations of instruments might finally give an answer to this important question. However, even a positive detection of liquid brine would not necessarily mean a significant change in our understanding of the habitability of Mars, at least not without a full knowledge of the brine composition, and how the unique martian environment affects the stability and transformations of these liquids.
Acknowledgments
We acknowledge funding from the NASA Habitable Worlds program grant # 80NSSC20K0227. The authors would like to thank Dr. David Catling and an anonymous reviewer for their comments which helped improve the manuscript.
Author contributions
V.F.C. and R.A.S. wrote the paper.
Competing interests
The authors declare no competing interest.
Footnotes
This article is a PNAS Direct Submission. A.P. is a guest editor invited by the Editorial Board.
Data, Materials, and Software Availability
There are no data underlying this work.
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