Tiny salt cracks could shelter liquid water on Mars

Brief pockets of brine might survive into warmer daylight inside Martian crystals, a mechanism that could help identify places worth examining for microbial habitats.

Present-day Mars creates a difficult timing problem for liquid water.¹² The paper reported that relative humidity rises during cold nights, when temperatures are generally poor for active metabolism, while warmer daytime conditions bring extreme dryness that would inhibit biological activity.¹

A possible route through that mismatch begins when a water-attracting salt absorbs vapor from the atmosphere.¹² A microscopic layer of salty liquid forms on the crystal, enters a crack during the cool night, and may remain inside when daylight warms and expands the mineral enough to narrow the opening.¹³ The trapped brine would then be partly separated from the dry daytime air.¹³

Anna Bognar and Bernadett D. Pal of ELTE Eötvös Loránd University and the HUN-REN Research Centre for Astronomy and Earth Sciences, with Akos Kereszturi of HUN-REN and the European Astrobiology Institute, evaluated that proposed cycle in a paper listed in Icarus.¹⁴ Their study presents a conceptual feasibility analysis, meaning it asks whether the mechanism could work under selected Martian conditions without claiming that such water traps have been observed.¹⁵

How could a salt crystal capture water?

A salt crystal could capture water because changes in humidity and temperature affect both the water on its surface and the width of cracks inside it.¹ Mars becomes relatively more humid as the surface cools late at night and early in the morning, even though the total quantity of atmospheric water vapor does not peak then, the paper stated.¹

Certain hygroscopic salts, meaning salts that draw moisture from surrounding air, can undergo deliquescence when humidity crosses a salt-dependent threshold.¹⁶ During deliquescence, absorbed vapor produces a saturated solution that first appears as a very thin film on the mineral surface, according to the paper.¹ Calcium perchlorate is one candidate because laboratory measurements cited by the authors place its required relative humidity across a broad range that depends on temperature and hydration state.¹

The proposed trap adds a mechanical step to that water-uptake process.¹ Cooling contracts the crystal and can open or extend microscopic fractures; the newly formed liquid can then spread along those passages toward the interior.¹ As the crystal warms during the day, thermal expansion may partly close the fractures and slow the liquid’s return to the atmosphere.¹³

The paper assessed sodium chloride and calcium perchlorate as examples, while also discussing measured thermal changes in calcium chloride, gypsum and forms of magnesium sulfate.¹ The authors reported estimated relative volume changes of about 0.5% to 0.9% for crystalline magnesium sulfate and gypsum, about 1.6% for sodium chloride, and about 3.7% to 6.9% for calcium chloride solutions.¹ Those values belong to different materials and conditions, so they do not establish how a calcium perchlorate crystal would behave on Mars.¹

Other work supports pieces of the proposed chain without demonstrating the complete trap.⁷⁸ A NASA technical record reported that laboratory mixtures of sulfate and chlorine salts could absorb water, expand and deliquesce under Mars-analogue conditions, while earlier climate calculations found possible nighttime windows for microscopic brine formation at several landing sites.⁸⁷

Where might the cycle occur?

The cycle would require the right salt, sufficient nighttime humidity and a daily temperature swing capable of changing its fractures.¹ The paper stated that chloride-bearing and hydrated minerals have been detected at many Martian locations, although a mineral occurrence alone does not establish that deliquescence happens there today.¹

Previous atmospheric modelling by Bernadett D. Pál and Ákos Kereszturi identified possible calcium perchlorate deliquescence in restricted places and seasons.² Their model placed the strongest northern opportunities in Acidalia Planitia and Utopia Planitia during parts of the night, and it identified weaker possibilities near Argyre Planitia and Hellas Planitia in the Southern Hemisphere.²

The new paper combined mapped salt occurrences with the environmental conditions needed for water uptake.¹ Its calculations suggested that microscopic nighttime liquid could occur near several salt sites around Acidalia Planitia for a cumulative total of approximately 100 to 130 Martian days during a Martian year.¹ That figure describes calculated opportunities across the year, not continuous liquid water or a measurement made on the ground.¹

Location alone would still be insufficient. Earlier landing-site calculations reported that local thermal inertia, which describes how readily the ground heats and cools, could shift the estimated brine windows, while the concentration of calcium perchlorate in the soil would also affect formation.⁷ Mixtures introduce another complication because their water-uptake thresholds can differ from those of each pure salt, the new paper stated.¹

If the trapping mechanism holds, maps of humidity, temperature and salts could help narrow the search for places where brine persists beyond the coldest hours.¹² The earlier modelling paper proposed its probability maps as a guide for landing-site analysis and for selecting locations for further research.²

Would trapped brine make a habitat?

Trapped brine would supply only one part of a possible microbial habitat.¹ The paper proposed that a hypothetical organism inside a salt crystal might remain in contact with liquid during warmer daylight, when temperatures could be more favourable for metabolism than they are at night.¹

A small amount of overlying soil could provide another form of protection.¹⁶ The authors estimated that a depth of approximately 2 to 3 millimetres might offer partial shielding from ultraviolet radiation while allowing some visible light through, depending on the optical properties of the surrounding material.¹ They consequently identified photosynthetic organisms as one hypothetical case worth considering, without reporting evidence that any organism occupies these crystals.¹

Independent simulation experiments underline how sharply habitability can depend on the salt and depth involved.⁶ Florian Carlo Fischer and colleagues at Technische Universität Berlin reported that survival increased with regolith depth for three test organisms in chlorate-treated and salt-free samples, while survival remained lower at every tested depth in perchlorate-treated samples.⁶ The conference abstract attributed that difference under its experimental conditions to greater toxicity from perchlorate brines than from chlorate brines.⁶

The proposed Martian water trap remains a conceptual model rather than an observation of liquid or life.¹⁵ It cannot establish that fractures close tightly enough to retain brine, how long any retained liquid would last, or whether the chemistry would support metabolism.¹ Direct thermal-expansion measurements for hydrated calcium perchlorates under relevant Martian conditions remain limited, so the authors described comparisons with other salts as preliminary.¹ The model also omitted ionizing particle radiation, which the paper identified as a major limitation for shallow habitats.¹

What would have to be tested next?

The central uncertainty is whether real Martian salt cracks can retain enough liquid for long enough to matter.¹ The paper noted that a narrowed crack need not become a complete seal, leaving vapor transport and diffusion as possible routes for water loss.¹⁵

Crack behaviour also depends on properties that the simplified concept cannot settle.¹ The authors identified mineral anisotropy, meaning direction-dependent physical behaviour, along with existing microfractures and local thermal conditions as factors that could change how an opening evolves.¹³ Measurements would therefore need to follow both the motion of the fracture and the water inside it through repeated temperature cycles.¹

Tests using hydrated calcium perchlorate under Mars-relevant humidity and temperature changes would address one of the paper’s largest data gaps.¹ Experiments could determine whether nighttime liquid enters fractures, whether warming narrows those fractures, and how rapidly water escapes after closure.¹ No retention time for such an enclosed Martian brine appears in the supplied paper text.¹

Even a successful laboratory demonstration would identify a possible microenvironment, not evidence of Martian biology.¹⁹ Mars has no definite evidence of past or present life, according to the supplied overview of the planet’s water, and the proposed cracks would remain targets for investigation.⁹¹ Their value lies in connecting three conditions that rarely coincide at the surface: atmospheric moisture, protective mineral structure and warmer daytime temperatures.¹

Reporting note: This piece was prepared from the paper and public records; the authors have not been interviewed.¹

References

This article is based on 11 sources, listed in the order they are cited.

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