Deliquescing salts taking up moisture from the Mars atmosphere

Mars is rich in perchlorates - a discovery made by Phoenix, and later confirmed by Curiosity and by analysis of Martian meteorites on Earth. It now seems that perchlorates probably occur over much of the surface of Mars. This is of especial interest since perchlorates deliquesce more easily than chlorides and at a lower temperature, so they could, potentially, take up water from the atmosphere more readily.

It is not yet clear how they formed. Sulfates, chlorides and nitrates can be made in sufficient quantities by atmospheric processes, but this mechanism doesn't seem sufficient to explain the observed abundances of perchlorates on Mars.

Though there is little by way of water vapour in the Mars atmosphere, which is also a near vacuum - still it reaches 100% humidity at night due to the low nighttime temperatures. This effect creates the Martian morning frosts, which were observed by Viking in the extremely dry equatorial regions of Mars.



The discovery of perchlorates raises the possibility of thin layers of salty brines that could form a short way below the surface by taking moisture from the atmosphere when the atmosphere is cooler. It is now thought that these could occur almost anywhere on Mars if the right mixtures of salts exist on the surface, even possibly in the hyper-arid equatorial regions. In the process of deliquescence, the humidity is taken directly from the atmosphere. It does not require the presence of ice on or near the surface.

Some microbes on the Earth are able to survive in dry habitats without any ice or water, using only liquid obtained by deliquescence. For instance this happens in salt pillars in the hyper arid core of the Atacama desert. They can do this at a remarkably low relative humidity, presumably making use of deliquescence of the salts.

Perchlorates are poisonous to many lifeforms. However, perchlorates are less hazardous at the low temperatures on Mars, and some Haloarchaea are able to tolerate them in these conditions, and some of them can use them as a source of energy as well.

These layers are predicted to lie a few cms below the surface, and are likely to be thin films or droplets or patches of liquid brine. So, they probably won't be detected from orbit, at least not directly. Confirmation may have to wait until we can send landers to suitable locations with the capabilities to detect these layers. Some of the layers may form in equatorial regions, and analysis of results from Curiosity in early 2015 has returned indirect evidence for presence of subsurface deliquescing brines in Gale Crater.

Whether any of these layers are habitable for life will depend on the temperatures and the water activity (how salty the brines are), which in turn depends on conditions and the composition of salts, whether they are mixed with soil, atmospheric conditions, and even the detailed structure of the microhabitats.

Eutectic and eutonic mixtures, e.g. of chlorides and perchlorates deliquesce at a lower relative humidity, and remain liquid at a lower temperature than either separately
The possibility of liquid brines forming on Mars is improved hugely by the process of eutectic mixtures. The name comes from the Greek "ευ" (eu = easy) and "Τήξις" (tecsis = melting). If you have a mixture of two salts, for example, a mixture of chloride with perchlorate, then the mixture stays liquid at a lower temperature than each of the salts separately. The melting temperature is the "eutectic point". This phenomenon is related to the way that Antifreeze works, and the reason why salt keeps roads free from ice. See also Freezing-point depression.

Its the same with humidity, in which case it is called a eutonic mixture, or a eutonic solution (when it has taken up enough water vapour to become liquid), and the relative humidity at which this happens is the eutonic point. A mix of salts is able to take up water from drier air (lower relative humidity) than either of the salts separately, which again improves the possibility of liquid brines forming by deliquescence.

How a mix of salts can take up water from drier air than either individually
Some background is needed for these studies.

The Deliquescing Relative Humidity for a mixture of salts is the humidity needed for the entire mixture to become liquid. This varies depending on the proportion of each salt in the mixture.

The relative proportions of two salts needed to remain liquid with the lowest level of humidity is known as the eutonic point.

Any mixture of two salts, even if the proportions are well away from the eutonic point, can still take up some water vapour at this lowest level of humidity. It will continue to do this until one of the salts is entirely used up to create this optimal mixture. If there is an excess of the other salt, it remains out of solution in the solid phase.

This diagram shows how it works - for a fictitious mixture A and B.



Here DRH = Deliquescing Relative Humidity, ERH = Eutonic Relative Humidity.

E(A+B) is the optimal or Eutonic mixture. And L here refers to the liquid phase. So, to the left we have a mixture of A with E(A+B) and, once it reaches the eutonic point, only part of it is liquid, and some of the salt A will remain in its solid phase. To the right, similarly, some of the salt B remains in its solid phase above the eutonic point.

So as the humidity is increased, for a given A / B mixture, first the lower horizontal line is reached, at which point some of the mixture of salts becomes liquid. This is known as the "eutonic relative humidity" - the point at which any mixture will start to take up some water vapour.

As humidity is raised further, more and more of the mixture becomes liquid. Eventually the upper, curved line is reached - and at that point, the entire mixture will be in its liquid phase.

Similarly if the axis is temperature - then as the temperature is raised, first part of the mixture will go liquid, at a temperature corresponding to the optimal mixture of the salts, and then when the upper curved line is reached, the entire mixture will be liquid.

Effect of this
Because of this eutonic mixture effect, if you add a tiny amount of perchlorates to the less deliquescent chlorides, this is enough to reduce the minimum relative humidity needed to deliquesce to the eutonic relative humidity for the mixture. This is not only lower than the deliquescence relative humidity of the chlorides, it is also lower than the deliquescence relative humidity for the perchlorates as well.

You can also get similar eutonic mixtures of three or more different types of salts, which typically have even lower ERH than any of the mixtures of two salts. Salts on Mars could have a mixture of perchlorates, chlorates, sulfates, and chlorides and perhaps nitrates also if present, along with cations of sodium, potassium, calcium, and magnesium. So there are many possibilities to consider here.

In this way, it doesn't matter much what the actual percentages of the two salts are, so long as there is some of both in the mixture.

After salt mixtures take up water, they retain it after supercooling, and reduced humidity
In addition to this, once the salt mixtures take up water, they lose it less readily, so they can stay liquid even when the humidity is then reduced again below the eutonic point (delayed efflorescence). Similarly for eutectic freezing, they can be supercooled below the temperature where they would normally freeze, and may remain liquid for some time below the eutonic point.

You get a eutectic also for freezing of a single salt, with molar concentrations. If you have a mixture of salt and water then different mixtures will freeze at different temperatures. The eutectic is the optimal mix of water and salt with the lowest freezing temperature. As you freeze a mixture, then no matter what the original concentration, some of it will remain liquid down to the freezing point of the eutectic mixture.

However, as you freeze further below that temperature, you may find that the salt continues to remain liquid. The reason for this is that for a salt to come out of solution through nucleation, it has to form a new interface between the crystal surface and the liquid, which requires energy. Once the nucleation starts, then crystallization is rapid, but the nucleation can be delayed often for many hours.

For instance, MgSO4 has a eutectic of -3.6 °C but through supercooling can remain liquid for an extra -15.5 °C below that. Here is a table of some salts likely to be found on Mars, showing the eutectic temperature for each one (with the molar concentration for the optimal eutectic concentration in brackets) and the amount of supercooling below that temperature that they found with experiments (adapted from table 2 of - omitted some of the columns).

As the salt / liquid solution cools in Mars simulation conditions, then the results can be complicated, because for instance MgSO4 releases heat in an exothermic reaction when it crystallizes. This keeps it liquid for longer than you'd expect. In their experiments, it remained liquid for twelve hours as it gradually cooled below the eutectic temperature before eventually it froze at 15.5 degrees below the eutectic temperature. In simulated Mars conditions you also have to take account of the effect of soil mixed in with the salts. Surprisingly, using Mars analogue soil, this does not reduce the supercooling and can in some cases permit more supercooling.

With some of the salt solutions, depending on chemical composition, then the supercooling produces a glassy state instead of crystallization, and this could help to protect supercooled microbes from damage.

Effects of micropores in salt pillars
In experimental studies of salt pillars in the Atacama desert, microbes are able to access liquid at extremely low relative humidities due to micropores in the salt structures. They do this through spontaneous capillary condensation, at relative humidities far lower than the deliquescence point of NaCl of 75%. Micro-environmental data measured simultaneously outside and inside halite pinnacles in the Yungay region (table 2 from )

The researchers, Wierzchos et al., did detailed studies with scanning electron microscopes. At 75% relative humidity then brine was abundant inside the salt pillars. As the humidity was reduced, even at 30% RH, the cyanobacteria aggregates shrunk due to water loss, but still there were small pockets of brine in the salt pillars.

""Endolithic communities inside halite pinnacles in the Atacama Desert take advantage of the moist conditions that are created by the halite substrate in the absence of rain, fog or dew. The tendency of the halite to condense and retain liquid water is enhanced by the presence of a nano-porous phase with a smooth surface skin, which covers large crystals and fills the larger pore spaces inside the pinnacles... Endolithic microbial communities were observed as intimately associated with this hypothetical nano-porous phase. While halite endoliths must still be adapted to stress conditions inside the pinnacles (i.e. low water activity due to high salinity), these observations show that hygroscopic salts such as halite become oasis for life in extremely dry environments, when all other survival strategies fail. Our findings have implications for the habitability of extremely dry environments, as they suggest that salts with properties similar to halite could be the preferred habitat for life close to the dry limit on Earth and elsewhere. It is particularly tempting to speculate that the chloride-bearing evaporites recently identified on Mars may have been the last, and therefore most recently inhabited, substrate as this planet transitioned from relatively wet to extremely dry conditions""

Microbes also inhabit Gypsum deposits (CaSO4.2H2O), however Gypsum doesn't deliquesce. Researchers found that the regions of the desert that had microbial colonies within the gypsum correlated with regions with over 60% relative humidity for a significant part of the year. They also found that the microbes imbibed water whenever the humidity increased above 60% and gradually became desiccated when it was below that figure.

Implications of these effects
The combination of all these effects means that mixtures of salts, including perchlorates in the mixture, can be liquid at lower temperatures than any of the salts separately, and also take up water from the atmosphere at lower relative humidity, and once liquid, can remain liquid for longer than you would predict if you didn't take account of these effects. And if there are micropores in the salt deposits, any life within them could also take advantage of an internal relative humidity higher than the external humidity of the atmosphere.

On Mars the relative humidity of the atmosphere goes through extremes. It reaches 100% humidity every night in the extreme cold, even in equatorial regions. In the daytime the relative humidity becomes much less, approaching 0%, and any exposed salts would lose their liquid.

The surface temperatures of the top few cms also change enormously from day to night (more stable but lower temperatures are encountered deeper below the surface) and over the entire surface of Mars, temperatures are tens of degrees below freezing every night.

But because of these other effects these liquid layers, may resist efflorescence and remain liquid longer than you'd expect as the air dries out in the daytime, and also stay liquid longer than you'd expect through supercooling as the temperatures plummet at night.

The result is that you could have layers of liquid, on Mars, quite some way below the surface 1 or 2 cms where liquid water in its pure state can form.

So this discovery of perchlorates on Mars has major implications for presence of liquid, and so habitability.

Challenges for life in these liquid layers of deliquescing salts
Given the presence of salts, and including perchlorates, widespread over Mars, it would seem that these liquid layers must surely exist, though not yet directly confirmed by observation.

However some of these liquid layers may be too cold for life (some are liquid at temperatures as low as -90C or lower), or too salty (not enough "water activity). The main focus of research here for habitability is to find out whether there are mixtures of salts that can deliquesce on Mars at the right temperature range and with sufficient water activity for life to be able to take advantage of the liquid. The consensus so far is that though many of these would be too cold, or too salty for life, it seems possible that some of these, in optimal conditions, with the right mixture of salts and at the right depth below the surface, may also be habitable for suitable haloarchaea. The lifeforms would need to be perchlorate tolerant, and ideally, able to use it as a source of energy as well.

The conditions for these liquid layers to form may include regions where there is no ice present on the surface such as the arid equatorial regions of Mars.

Temporary liquid brines forming every night at depths down to 15 cm below the surface of equatorial sand dunes
Researchers using data from Curiosity in April 2015 found indirect evidence that liquid brines form through deliquescence of perchlorates in equatorial regions, at various times, both at the surface, and down to depths up to 15 cms below the surface. When it leaves sandy areas, the humidity increases, suggesting that the sand takes up water vapour.

At night, the water activity is high enough for life, but it is too cold, and in the day time it is warm enough but too dry. The authors concluded that the conditions in the Curiosity region were probably beyond the habitability range for replication and metabolism of known terrestrial micro-organisms.