Brine & deliquescent-salt water extraction
Recovers water from Martian brines and deliquescent perchlorate/chloride salts — sources that stay liquid (or pull water from the air) at temperatures where pure water freezes. Water is separated from the salts by distillation, membrane, or controlled crystallization, leaving a concentrated perchlorate brine that feeds remediation and the chlor-alkali plant. The defining feature: these are the most perchlorate-rich water sources on Mars, so extraction is tightly coupled to purification — the contaminant and the resource are the same molecule.
Governing equations
Deliquescence: perchlorate salts absorb atmospheric water vapor above a threshold humidity and dissolve into liquid brine — a passive water-harvesting mechanism the extraction exploits. [1]
The freezing-point depression that makes brines special: magnesium-perchlorate brine stays liquid down to about −68 °C, far below pure water — liquid water where ice could not exist. [1]
Water separation from the salt: distillation (evaporate water, condense), reverse osmosis, or crystallization. The reject is a concentrated perchlorate brine — exactly the chlor-alkali / remediation feed. [2]
Each salt deliquesces above a characteristic humidity (DRH); Mars conditions cross the DRH of perchlorates seasonally/diurnally, driving natural brine formation that extraction can collect or induce. [1]
Key constants & quantities
| Symbol | Value | Units | Conditions | Description |
|---|---|---|---|---|
| Mg-perchlorate eutectic | -68 | °C | — | Lowest temperature Mg(ClO₄)₂ brine stays liquid — why brines exist where ice cannot, a Mars-specific liquid-water regime.[1] |
| Regolith perchlorate | 0.4–0.6 | wt% ClO₄⁻ | — | Perchlorate content of regolith salts — the brine source material; concentrated relative to ice or atmospheric water.[3] |
| Brine water content | 40–70 | wt% H₂O (saturated brine) | — | Water fraction of a saturated perchlorate brine — the recoverable water per kg of brine processed.[1] |
| Separation energy | 1–10 | kWh / kg H₂O | — | Energy to separate water from brine (membrane low, distillation high) — between WAVAR and ice melting depending on method.[2] |
| Perchlorate co-product | 1 | concentrated brine → chlor-alkali | — | The reject perchlorate brine is the feed for remediation and the chlor-alkali chain — extraction is also salt mining.[4] |
Operating envelope
Mass balance
Basis: 1 kg water recovered from saturated perchlorate brine
Energy sits between WAVAR and ice melting, depending on separation method. Brine extraction earns its place by tapping liquid water where ice can't form and by co-producing the perchlorate salts the chlor-alkali chain wants — two outputs from one operation.
Variants & trade-offs
Mined-salt dissolution + separation
[2]Mine perchlorate-rich regolith/salt deposits, dissolve to brine, then separate water (distillation/RO) and recover the salt.
- Taps the most concentrated salt sources; co-produces chlor-alkali feed
- Uses mature separation technology
- Energy for dissolution + separation; handles the highest perchlorate loads (safety)
- Requires identifying salt-rich deposits
When preferred: Sites with concentrated perchlorate/chloride salt deposits.
Deliquescence harvesting (passive water magnet)
[1]Expose deliquescent salts to peak-humidity atmosphere; they pull water from the air into brine, which is then separated — a hybrid of brine and atmospheric capture.
- Passive water pickup from atmosphere — low fan energy vs WAVAR
- Self-replenishing salt sorbent
- Low rate; humidity-dependent; lower TRL
- Brine handling and perchlorate management
When preferred: Low-energy supplementary capture exploiting natural deliquescence cycles.
Recurring-brine collection (if RSL-type sources confirmed)
[1]Collect naturally-forming seasonal brines (recurring slope lineae and analogues), should in-situ exploration confirm accessible flows.
- Taps a naturally-concentrated, self-forming liquid source
- Existence/accessibility unconfirmed; planetary-protection considerations; speculative
When preferred: Only if exploration confirms accessible recurring brines — currently speculative.
Failure modes
| Mode | Cause | Detection | Mitigation |
|---|---|---|---|
| High-perchlorate handling hazard (safety-critical)[3] | Brine extraction concentrates the planet's most perchlorate-rich material; leaks or mishandling expose crew and contaminate loops with the worst-case load. | Perchlorate monitoring on all streams; containment leak detection. | Closed handling, tight coupling to remediation, segregated loops, robust containment — treat as hazardous-material processing. |
| Scaling / salt fouling of separators[2] | Hypersaline brine scales distillation surfaces and fouls RO membranes rapidly. | Heat-transfer/flux decline; scaling inspection. | Antiscalant, operate below saturation, periodic descaling/CIP, crystallizer designs that manage solids. |
| Freezing despite depression[1] | Even depressed-freezing brines can freeze if cooled below their eutectic in lines/equipment. | Line-temperature monitoring. | Heat tracing, keep brine above its eutectic, insulated/heated equipment. |
| Resource shortfall / uncertainty[1] | Brine resource turns out to be smaller, more dispersed, or less accessible than hoped (the TRL-2 risk). | Resource assessment during exploration. | Treat brine as a supplement, not a primary source, until in-situ data confirms it; lead with ice mining + WAVAR. |
Mars adjustments
The poison is the resource[1]
Impact: Perchlorate makes water toxic AND makes brines liquid below freezing AND deliquesces to pull water from air. Brine extraction embraces the very contaminant the rest of the water system fights — and feeds the recovered salt to chlor-alkali.
Mitigation: Integrate extraction, perchlorate remediation, and chlor-alkali as one tightly-coupled salt-and-water complex.
Liquid water where ice cannot exist[1]
Impact: Freezing-point depression to ~−68 °C means brines are liquid across much of the Mars surface temperature range — a water source available as a fluid, not a solid to be melted.
Mitigation: Exploit the liquid state (pumpable, no melting energy) where brines are accessible.
Honest about maturity[1]
Impact: Unlike ice mining and WAVAR, Martian brine as a mined resource is largely unproven — observed and inferred, not sampled. It is the most speculative of the three water sources.
Mitigation: Treat as a supplement contingent on exploration; do not architect the colony's water supply around it until confirmed.
Planetary protection[5]
Impact: Liquid brines (especially recurring slope lineae) are potential habitable niches — extracting from them raises planetary-protection and contamination-control considerations beyond pure engineering.
Mitigation: Follow planetary-protection protocols; prefer mined-salt dissolution over disturbing potential special regions.
Couples to chlor-alkali by construction[4]
Impact: Every kg of water from brine yields concentrated perchlorate/chloride salt — the chlor-alkali plant's feed. Water extraction and the chlorine economy share this stream.
Mitigation: Size brine extraction against both water and chlor-alkali demand; route salt product, never discard it.
Alternatives & substitutes
water-ice-mining (cleaner, higher yield)[6]
- Higher yield, lower perchlorate than brine, mature concept
- Needs accessible ice; ice water still carries perchlorate (less than brine)
When preferred: The primary bulk source wherever ice exists — brine is the niche/supplement.
atmospheric-water-capture (cleaner water)[7]
- Perchlorate-free water, site-independent
- Much lower yield and higher fan energy
When preferred: Clean-water supplement where brine's perchlorate load is undesirable.
Requires
References
- (2009). Stability of perchlorate hydrates and their liquid solutions at the Phoenix landing site, Mars. Geophysical Research Letters, 36(10), L10202. doi:10.1029/2009GL037497 — Perchlorate-brine deliquescence and stability: the physical chemistry of liquid brines forming from deliquescent salts, the basis for brine-based water extraction.
- (2012). MWH's Water Treatment: Principles and Design, 3rd Edition. Wiley. ISBN 978-0-470-40539-0. — The definitive water-treatment engineering reference: coagulation, filtration, adsorption, ion exchange, membranes, disinfection, and process-train design.
- (2009). Detection of Perchlorate and the Soluble Chemistry of Martian Soil at the Phoenix Lander Site. Science, 325(5936), 64-67. doi:10.1126/science.1172466 — First in-situ measurement of perchlorate in Mars regolith — 0.4–0.6 wt%.
- (2013). Perchlorate on Mars: a chemical hazard and a resource for humans. International Journal of Astrobiology, 12(4), 321-325. doi:10.1017/S1473550413000164 — Biological reduction of perchlorate as pre-treatment for ISRU water.
- (2014). Recurring slope lineae in equatorial regions of Mars. Nature Geoscience, 7, 53-58. doi:10.1038/ngeo2014 — RSL flow features as evidence for transient liquid brines on Mars surface.
- (2021). Availability of subsurface water-ice resources in the northern mid-latitudes of Mars. Nature Astronomy, 5, 230-236. doi:10.1038/s41550-020-01290-z — SWIM (Subsurface Water Ice Mapping) project — quantifies accessible ice at < 1 m depth in Arcadia / Utopia Planitia.
- (1998). Water Vapor Extraction from the Martian Atmosphere by Adsorption in Molecular Sieves. AIAA 98-3301, 34th AIAA/ASME/SAE/ASEE Joint Propulsion Conference. doi:10.2514/6.1998-3301 — The WAVAR concept: capturing Mars atmospheric water vapor (~210 ppm) by molecular-sieve adsorption and thermal regeneration — a water source independent of ice deposits.