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Exposed water ice scarp cutting through Martian regolith
Constraint III · Oxygen

Oxygen comes from water.Mars has enough water.

§06Constraint III · Oxygen
Requirement (alveolar gas equation)
pO₂,alv = 0.21 × (1013 − 47) − 50 = 153 mbar
Target: ≥ 130 mbar minimum, ~160 mbar comfortable
Mass required
160 mbar × 3.88×10¹⁵ = 6.2×10¹⁷ kg O₂
= 1.94×10¹⁹ mol
Route: 2 H₂O → 2 H₂ + O₂
Requires 7.0×10¹⁷ kg water ≈ 4.8 m global equivalent layer.

Against 20–40 m accessible. 15–25% of the inventory. The water exists.

The CO₂ shortfall everyone leads with does not block oxygenation. It blocks pressure.
Energy floor
1.94×10¹⁹ mol × 572 kJ/mol = 1.11×10²⁵ J
At 60% electrolysis efficiency ≈ 1.9×10²⁵ J
At Type I power (10¹⁶ W): ~35 years of output
MOXIE (Perseverance) caveat

MOXIE produced 122 g total, ~12 g/hr peak on ~300 W. Extrapolating a demonstrator to 10¹⁸ kg is not informative; the energy floor is the meaningful statement.

Interactive · Iteration 01
ATMOSPHERE BUILDER
Choose the air, then read the bill.

Set a total pressure and an oxygen fraction. Everything else follows from physics: the mass of the atmosphere, the water you must split to make the oxygen, the nitrogen you must haul from Titan, and whether a human can actually breathe the result.

Drag the sliders, or pick a target atmosphere
Verdict
THIN, TOLERABLE
Alveolar pO₂
96 mbar
Target ≥ 130 · comfortable ~160
Total pressure
570 mbar
200 mbar1013 mbar · Earth
O₂ fraction
28%
15%40% · fire risk above 30%

Alveolar oxygen between 90 and 130 mbar, comparable to a high mountain town. Habitable for acclimatised people, but not a comfortable working atmosphere. Partial pressures: O₂ 160 mbar, N₂ 410 mbar.

Atmospheric mass
2.21 × 10¹⁸ kg
O₂ mass
6.19 × 10¹⁷ kg
1.94 × 10¹⁹ mol
N₂ mass
1.59 × 10¹⁸ kg
Water to split for the oxygen
4.8 m
6.97 × 10¹⁷ kg · global equivalent layer
24% of the low estimate20 to 40 m accessible
Nitrogen as a fraction of Titan
19%
1.59 × 10¹⁸ kg of Titan's 8.6 × 10¹⁸ kg N₂
Hauled across eight astronomical unitsTitan's entire atmosphere
Energy floor, oxygen only
1.94 × 10¹⁹ mol × 572 kJ/mol = 1.11 × 10²⁵ J
At 60% electrolysis efficiency ≈ 1.84 × 10²⁵ J
At Type I power (10¹⁶ W): 58 years of output

Oxygen is the cheap half. The nitrogen bar above is the one that sets the schedule, and none of it is manufactured on Mars.

All Constraints
Next · Constraint IV →Carbon Disposal