Constraint Analysis 001 predictedthat sulfate would poison the oceanand close the carbon pathway.
An original geochemical analysis. Model validated against published benchmarks. Code and figures released.
The acid problem is not real.
The test is a mass balance between the acid stored in Martian sulfate and the acid-neutralizing capacity of the basalt beneath it. Only Fe(III)-sulfate carries stored acidity. Calcium and magnesium sulfates are already charge-balanced by cations previously leached from basalt, so dissolving them changes nothing.
| Test | Result |
|---|---|
| SO₃ required to block carbonate, all as Fe(III)-sulfate | 42.8 wt% |
| SO₃ actually observed in Martian soil | 5 to 8 wt% |
| Shortfall of the observed inventory against the blocking threshold | ~7× |
| A regolith composed entirely of jarosite | 6.00 eq/kg vs basalt's 8.02 |
| Calcite saturation index across the dilute regime | SI +2.9 to +5.6, supersaturated |
Even a regolith made entirely of jarosite cannot acidify a Martian ocean past the buffering capacity of the basalt beneath it.
Mars looks acidic because its water-to-rock ratio has always been tiny. A thin reaction rind dissolves its soluble sulfate completely while the silicate beneath contributes almost none of its buffering capacity, and acid wins locally. Hurowitz et al. (2006) identified exactly this mechanism for observed Mars. Ocean-scale water-to-rock ratio inverts it. The mineralogy is evidence about the regime Mars has occupied, not the regime terraforming would create.
The iron problem.
Carbonate precipitation consumes divalent cations. Martian basalt supplies calcium at 1.236, magnesium at 2.248, and iron at 2.533 mol/kg. Iron is the most abundant of the three, and ancient Martian carbonate is dominantly siderite, FeCO₃. Under an oxygenated atmosphere, dissolved Fe(II) oxidizes within seconds at circumneutral pH and precipitates as ferric oxide. It is then permanently unavailable.
Iron is available. Carbon buries as siderite, the pathway ancient Mars actually used.
Iron is oxidized and lost. Only calcium and magnesium remain.
Magnesite does not nucleate at ocean temperatures. Calcium alone must carry the entire budget.
The oxygenation that makes Mars breathable destroys 42% of its carbon burial capacity.
The cost of the atmosphere you choose to build.
| CO₂ removed | Anoxic | Oxic (Ca+Mg) | Oxic (Ca only) |
|---|---|---|---|
| 10 mbar | 0.46 m | 0.89 m | 5.0 m |
| 100 mbar | 4.6 m | 8.9 m | 50 m |
| 1000 mbar | 46 m | 89 m | 502 m |
Model. Plummer & Busenberg (1982) carbonic acid and calcite constants, Millero (1995) for K_w, Davies activity coefficients, proton-promoted basalt dissolution kinetics after Gislason & Oelkers (2003). Composition: Taylor & McLennan (2009) average Martian crust, with Rocknest soil as sensitivity end-member. No fitted parameters.
Validation. pK₁ 6.352 against 6.35. pK₂ 10.329 against 10.33. Calcite-saturated water at pCO₂ 10⁻³·⁵: model gives 0.485 mmol/kg Ca and pH 8.28, against textbook 0.50 and 8.3.
Limitations, stated. Davies is reliable to ionic strength 0.5 mol/kg. Benchmarked against seawater the model overestimates pH by ~0.3 units, so all quantitative pH claims here are restricted to the dilute regime and the concentrated magnesium-sulfate brine regime requires a Pitzer treatment. This is the largest methodological gap. Batch equilibration stands in for reactive transport. Magnesite kinetics are bounded, not modeled. Redox is imposed, not computed.
Status. Preprint. Not peer reviewed. https://doi.org/10.5281/zenodo.21452137