Atmospheric CO₂ capture
Extracts and concentrates CO₂ from the Mars atmosphere as Sabatier feedstock. Mars CO₂ is abundant (95.3 % of atmosphere) but dilute (600 Pa total pressure → ~570 Pa CO₂ partial pressure). Cryogenic freezing during the −90 °C Mars night, pressure-swing adsorption on zeolite, and direct mechanical compression are the three competing architectures. Energy-per-kilogram differs by an order of magnitude between the cheapest (radiative) and most flight-mature (compression).
Governing equations
Mars atmospheric CO₂ partial pressure at typical surface conditions. Bookmark the number: this is what the inlet sees. [1]
Ideal isothermal compression work per mole of gas. Mars surface inlet → 30 bar Sabatier feed is a 5170× ratio; ideal work is ~21 kJ/mol = 480 kJ/kg CO₂. [2]
Direct deposition of CO₂ to solid (dry ice) — bypasses the liquid phase entirely below the triple point (5.18 bar, 216.55 K). On Mars, deposition occurs at ~150 K at typical low pressures. [3]
Carnot coefficient of performance for active cryocooling. Pumping heat from 150 K to a 300 K Mars-day sink gives COP ≈ 1.0; using a passively-radiated 200 K cold plate gives COP ≈ 2.0 — substantial energy savings. [4]
Key constants & quantities
| Symbol | Value | Units | Conditions | Description |
|---|---|---|---|---|
| x_CO₂ | 95.32 ±0.5% | vol% | Gale Crater, multi-season average | CO₂ mole fraction in Mars atmosphere from Curiosity SAM measurements. Remainder: N₂ 2.7 %, Ar 1.6 %, O₂ 0.13 %, CO 0.08 %, water 0.03 %.[5] |
| p_Mars | 610 ±20 % seasonal | Pa | — | Reference Mars surface pressure. Varies seasonally by ~25 % from CO₂ polar cap condensation/sublimation cycle.[1] |
| T_deposition | 150 | K (at Mars ambient pressure) | — | Temperature at which gaseous CO₂ deposits directly to solid at ~600 Pa. The polar caps reach this in winter — natural cryotrap.[3] |
| ΔH_sublimation | 25.2 | kJ/mol | — | CO₂ sublimation enthalpy at 195 K. Energy required to convert solid CO₂ back to gas during the capture-cycle desorb phase.[3] |
| W_compression,real | 800–2000 ±25 % | kJ / kg CO₂ | — | Real multi-stage compression energy from 600 Pa to 30 bar Sabatier feed, including intercooling losses, blower inefficiency, motor losses. ~3× ideal isothermal.[6] |
| W_cryo,radiative | 50 ±30 % | kJ / kg CO₂ | — | Energy for cryogenic capture via radiative cooling during Mars night. Most of the heat is rejected for free; only blower work + sublimation energy charged.[7] |
Operating envelope
Mass balance
Basis: 1 kg CO₂ delivered to Sabatier at 30 bar
Inputs
| Mars atmosphere processed | 22 | kg (95% CO₂ + 5% inerts) | [5] |
| Electrical energy (cryogenic radiative) | 0.014 | kWh | [7] |
| Electrical energy (mechanical compression) | 0.45 | kWh | [6] |
- Mars atmosphere processed: Inert N₂, Ar, O₂, CO removed by selective adsorption or cryogenic separation; vented back to atmosphere.
- Electrical energy (cryogenic radiative): Blower work to move 22 kg of atmosphere through cold trap during night cycle.
- Electrical energy (mechanical compression): Multi-stage isentropic compression — order of magnitude more energy-intensive than radiative.
Outputs
| Pure CO₂ at delivery pressure | 1 | kg | [6] |
| Inert gas vent (N₂, Ar, O₂) | 1.05 | kg | [5] |
- Pure CO₂ at delivery pressure: > 99.5 % purity after one separation stage; > 99.99 % after two.
- Inert gas vent (N₂, Ar, O₂): Vented back to atmosphere. Could be captured separately for ECLSS buffer gas (N₂) or breathing oxidant.
Radiative cryotrap reduces this to ~0.014 kWh/kg by exploiting Mars-night cold-side temperature. For a 4-crew base producing 20 kg CH₄/sol, the compression route demands ~25 kWh/sol — significant but tractable on a 10 kWe nuclear baseload.
Variants & trade-offs
Mechanical compression (Hartvigsen-class)
[6]Multi-stage scroll or piston compressor pulls CO₂ from Mars atmosphere through dust filters and chemical drying, delivers directly to Sabatier at 1–30 bar. The MOXIE / industrial-prototype approach.
- Compression stages
- 2–5
- Inlet flow
- 10–100 kg/sol atmosphere processed
- Highest TRL — direct flight heritage from MOXIE front-end
- Continuous operation, day or night
- Tight control over outlet purity and pressure
- Highest energy demand of the three variants
- Compressor wear is the primary failure mode
- Vibration sensitivity — careful mounting needed
Cryogenic radiative freeze (Zubrin Mars Direct architecture)
[7]During Mars night (−90 °C ambient), a radiator panel passively cools a cold-plate to < 150 K. CO₂ deposits as dry ice on the plate, leaving N₂ and Ar gaseous. By dawn, the cold-plate is heated to 200 K, dry ice sublimes, and CO₂ exits at 6 bar — high enough for Sabatier without further compression.
- Cold-plate T (capture)
- 120–150 K
- Cycle period (one Mars sol)
- 12–14 h (capture phase)
- Lowest energy demand by an order of magnitude — radiative cooling is free
- Mechanically simple — minimal moving parts
- Self-purifies: N₂, Ar, O₂ remain gaseous at capture T
- Mars Direct flagship architecture; Zubrin's mass budget rests on it
- Diurnal duty cycle — only collects during cold half of sol
- Dust storms suppress radiative cooling for weeks
- Cold-plate fouling by N₂/Ar deposition at very low temperatures
- Lower flight TRL than mechanical
Pressure-swing adsorption (PSA, zeolite)
[9]Zeolite molecular sieve selectively adsorbs CO₂ at low pressure, releases at lower pressure (vacuum desorption) or higher temperature. Two-bed alternating architecture provides continuous output, similar to CDRA design.
- Adsorption T
- 200–280 K
- Desorption pressure
- 10–100 Pa (vacuum swing)
- Continuous operation possible with bed alternation
- Direct heritage from ISS life-support hardware
- High purity output (> 99.9 %)
- Vacuum desorption needs a vacuum pump on Mars (atmosphere is the "vacuum" but only ~600 Pa — borderline)
- Bed thermal swing demands sustained heater power
- Throughput per sorbent mass lower than mechanical compression
Failure modes
| Mode | Cause | Detection | Mitigation |
|---|---|---|---|
| Compressor seal wear (mechanical variant)[6] | Mars dust ingress to compressor cylinders abrades seals; thousands of hours of duty cycling degrades elastomers. | Outlet pressure droop at constant flow; vibration signature change. | HEPA + electrostatic dust pre-cleaner; oil-free compressor design (avoids dust-oil agglomeration); programmed seal replacement every 8000 h. |
| Dust storm radiator fouling (cryotrap variant)[7] | Sustained dust storms deposit fines on radiator panels, reducing emissivity and lifting cold-side T above the CO₂ deposition threshold. | Cold-plate T fails to reach setpoint during night cycle; sublimation rate drops. | Vertical-facing radiator orientation; electrostatic dust shedding; mechanical wiper; buffer storage to bridge multi-sol storms. |
| Zeolite water co-adsorption (PSA variant)[9] | Trace water vapor in Mars atmosphere (~0.03 %) co-adsorbs onto CO₂-selective sites; reduces sorbent capacity over thousands of cycles. | CO₂ throughput declines at fixed cycle period; outlet humidity climbs. | Upstream silica-gel pre-dryer; periodic high-T bake-out (200 °C); programmed sorbent replacement. |
| Inert gas accumulation in cryotrap[7] | At cold-plate T < 130 K, N₂ and Ar begin to deposit alongside CO₂, contaminating the product stream. | Outlet GC shows rising N₂/Ar fraction. | Tight cold-plate T control (130–145 K window); short collection cycle to limit inert build-up; vent flush between cycles. |
| Cold-end heater failure (cryotrap)[7] | Resistance heater that drives sublimation phase fails open-circuit. | Output flow drops to zero during desorb phase; cold-plate T stays cold. | Redundant heater elements; back-up sublimation via natural daytime warming with vent valve closed. |
| Intake clogging by dust (all variants)[6] | Mars dust storms (regional or global) accumulate fine particulates on intake filters faster than nominal. | Pressure drop across intake filter climbs; throughput falls. | Multi-stage dust filter with field-replaceable cassettes; self-cleaning intake with reverse-flow purge; redundant intake paths. |
Mars adjustments
Atmospheric pressure 0.6 % of Earth[7]
Impact: The Sabatier reactor needs 1–30 bar feed; Mars atmosphere is 600 Pa. Compression ratio of 50–5000× sets the energy floor for any non-cryogenic architecture.
Mitigation: Cryogenic radiative capture sidesteps compression entirely — CO₂ deposits as solid at ~150 K, sublimes to 6 bar at 220 K. The compression "ratio" is replaced by a T cycle that radiative cooling delivers for free.
Night-side cold trap availability[7]
Impact: Mars night-side surface drops to −90 °C (183 K) typically, −125 °C (148 K) during polar winter. A radiator pointing at the cold sky reaches even lower temperatures — natural cryotrap.
Mitigation: Cryogenic capture only works during the cold half of each sol; system sized to collect a full sol's production during ~12 hours. Storage buffer between capture and Sabatier feed bridges the gap.
Dust contamination at the intake[8]
Impact: Mars dust is fine (1–3 µm modal), perchlorate-rich, and abrasive. Atmospheric intake filters at ground level see continuous dust loading that ramps under storms.
Mitigation: Two-stage filtration (electrostatic precipitator + HEPA), elevated intake snorkel (1+ m above grade), reverse-flow self-cleaning cycles. MOXIE intake provides flight-validated design.
Seasonal CO₂ availability[1]
Impact: Mars atmospheric pressure swings ±25 % seasonally as CO₂ condenses/sublimes from polar caps. Capture throughput tracks this — winter production drops below summer at non-polar sites.
Mitigation: Size for winter throughput; buffer-storage methane production during summer to even out propellant output for transit windows.
Alternatives & substitutes
Direct CO₂ electrolysis (MOXIE)[8]
- Eliminates the Sabatier step entirely — produces O₂ directly from atmospheric CO₂
- MOXIE flight-proven on Perseverance since 2021
- No H₂ feedstock required
- Output is O₂ + CO, not propellant
- CO must be vented or used as Fischer-Tropsch feed
- Doesn't produce CH₄ — separate pathway needed for fuel
When preferred: When O₂ alone is the goal (ECLSS, EVA, oxidizer-only architectures).
In-situ atmospheric collection via airlock cycling[7]
- No dedicated infrastructure — uses existing airlock pumps
- Zero new mass on launch manifest
- Throughput orders of magnitude too low for propellant production
- Contaminates internal systems with Mars dust
- Only useful as a buffer or backup
Requires
Inputs
References
- (2017). The Atmosphere and Climate of Mars. Cambridge University Press. ISBN 978-1-107-01618-7. — Reference handbook for Mars atmospheric pressure, temperature, dust climatology.
- (2019). Perry's Chemical Engineers' Handbook, 9th Edition. McGraw-Hill Education. ISBN 978-0-07-183408-3. — Canonical chemical-engineering reference: thermodynamic calculations, equipment sizing, unit operations.
- (2024). NIST Chemistry WebBook, NIST Standard Reference Database Number 69. National Institute of Standards and Technology. doi:10.18434/T4D303 — Thermodynamic properties of H₂O, H₂, O₂. ΔH°, ΔG°, S° at standard state.
- (1999). Cryogenic Heat Transfer. Taylor & Francis. ISBN 978-1-56032-551-7. — Classic cryogenic engineering reference — heat-leak calculation, vacuum-jacketed vessel design, stratification.
- (2020). Initial SAM calibration gas experiments on Mars: Quadrupole mass spectrometer results and implications. Planetary and Space Science, 138, 44-54. doi:10.1016/j.pss.2017.01.014 — Mars atmospheric composition from Curiosity SAM — CO₂ 95.32 %, N₂ 2.7 %, Ar 1.6 %, O₂ 0.13 %.
- (2017). Carbon Dioxide Electrolysis for Mars ISRU. ECS Transactions, 78(1), 2953-2966. doi:10.1149/07801.2953ecst — MOXIE precursor work — solid-oxide CO₂ electrolysis at Mars conditions.
- (1996). The Case for Mars: The Plan to Settle the Red Planet and Why We Must. Free Press, New York. — Mars Direct mission architecture, in-situ propellant production, water electrolysis context.
- (2021). Mars Oxygen ISRU Experiment (MOXIE). Space Science Reviews, 217(1), 9. doi:10.1007/s11214-020-00782-8 — MOXIE flight instrument — first ISRU demonstration on Mars (2021-).
- (2005). International Space Station Carbon Dioxide Removal Assembly Testing. 35th International Conference on Environmental Systems, SAE 2005-01-2864. doi:10.4271/2005-01-2864 — CDRA architecture, zeolite 13X/5A duty cycle, ISS performance history.