Designing the bioproduction of Martian rocket propellant via a biotechnology-enabled in situ resource utilization strategy

The paper addresses how to produce rocket propellant on Mars to enable a human Mars Ascent Vehicle (MAV) without transporting large masses from Earth. Given Mars’ low gravity, CO₂-rich atmosphere, and scarce O₂, the authors propose designing a Mars-specific propellant and an in situ bioproduction process. They analyze limitations of existing chemical ISRU strategies focused on methane and highlight the advantages of oxygenated fuels that require less liquid oxygen for combustion. They hypothesize that a biotechnology-enabled ISRU producing 2,3-butanediol (2,3-BDO) from Martian CO₂, water, and sunlight via cyanobacteria and engineered E. coli can meet MAV propellant needs while co-producing substantial oxygen to support broader exploration activities.

Prior ISRU concepts (NASA DRA 5.0) prioritize methane via SOCE, Sabatier reaction, and water electrolysis, but face challenges including large hydrogen shipments and supplemental O₂ needs. Biological approaches can convert CO₂ to fuels and materials; methanogens achieve high-purity CH₄ production comparable to ISS Sabatier throughput. Cyanobacteria can fix CO₂ into biomass and evolve O₂, but product titers are generally lower than those of engineered heterotrophs like E. coli or S. cerevisiae. Previous work shows heterotrophs can utilize cyanobacterial biomass or sugars to produce fuels, suggesting a hybrid autotroph–heterotroph scheme may offer higher productivity and flexibility. The authors also review fuel candidates and identify C3–C4 diols as promising Martian propellants due to favorable O₂/fuel ratios, energy density, phase behavior, and demonstrated microbial production.

The authors develop a process model for a bio-ISRU producing 2,3-BDO on Mars, composed of four modules: (1) continuous cyanobacterial cultivation (Arthrospira platensis) using Martian CO₂ and sunlight; (2) biomass preprocessing via concentration (for suspended cultures) and enzymatic digestion (lysozyme, α-amylase, glucoamylase) to release sugars and nutrients; (3) engineered E. coli fermentation converting glucose to 2,3-BDO; and (4) 2,3-BDO extraction and purification to ~95% purity via liquid–liquid extraction (butanol) and membrane separation. Key Martian constraints and assumptions: process temperature 25 °C; protection from UV while allowing PAR transmission; reduced incident photon flux on Mars (57% of Earth) modeled as the primary growth limitation; potential dust storm impacts; water availability with likely pretreatment; shipment of nitrogen and phosphorus (as (NH₄)₂PO₄ and NH₃) due to low Martian N₂ partial pressure; shipment of infrastructure and power sources from Earth. Cyanobacteria cultivation: Two modes evaluated—suspended growth in LDPE hanging-bag photobioreactors (PBRs) and biofilm growth on porous, hydrophilic substrate (cotton-like). Growth modeled with Monod kinetics adapted for light-limited growth under Martian photon flux, optimized reactor spacing (1 m), and temperature-dependent parameters. Suspended growth assumed 1 g/L culture; biofilm assumed ~7.5 g/m² areal concentration. Modeled land-productivity ~6.5–6.6 g/m²/day for both modes under Martian light. Biomass handling: Suspended mode uses cross-flow ultrafiltration (40 kDa, ~40 L/m²/h, 6-week stability) to concentrate biomass to 20 g/L; biofilm mode collected at ~20 g/L equivalent by scraping. Enzymatic digestion residence time (24–48 h) traded off against digester size and cyanobacterial farm size; 48 h increases glucose yield (up to ~45 wt% assumed baseline) and reduces water and payload in cultivation. Fermentation: Engineered E. coli producing 2,3-BDO with state-of-the-art steady-state productivity 1.17 g/L/h and yield 0.432 g/g glucose (≈80% of theoretical). O₂ requirement in fermentation is supplied by O₂ evolved during cyanobacterial growth (~12 tons over 500 sols). Separations: Liquid–liquid extraction using butanol, simulated with Phasepy and Aspen-regressed binaries, followed by PDMS/PVDF membrane-based pervaporation (~0.5 m² area) to concentrate/dewater to ~95% purity. Solvent recycle implemented; butanol carryover to aqueous stream ~0.01% with attention to E. coli tolerance. Water recycling: Reverse osmosis (thin-film composite, 25 L/m²/h) sized at ~8.7 m² for 3.64 L/min water-rich stream; unit mass ~0.008 tons (membrane + housing). Waste streams considered for nutrient recycle or other ISRU uses. Rocket performance and sizing: Theoretical specific impulse (Isp) computed from simplified exhaust velocity-energy relationships; 2,3-BDO theoretical Isp ~420 s; methane theoretical Isp ~459 s, but literature methane Isp ~369 s. Ideal rocket equation used to estimate propellant needs: for 2,3-BDO, 8.4 tons fuel + 16.5 tons LOX theoretically; conservatively target 10 tons 2,3-BDO (assuming Isp ~383 s) requiring ~19.6 tons LOX. Process modeling framework: Python-based model calculates land area, power, water, and mass for given biological/material parameters. Thermodynamics with Phasepy; membrane performance from literature differential-equation models; recycle solved via SciPy root solver. Methods section details equations, parameter values, and unit sizing assumptions, including mixing energy reductions under Martian gravity, and payload mass breakdowns for structures, tanks, membranes, enzymes, solvents, nutrients, and materials. Optimization studies: Biological optimization targets—doubling cyanobacterial productivity (to ~13.28 g/m²/day), increasing enzymatic glucose yield to 60 wt%, and improving 2,3-BDO yield to 0.51 g/g (95% theoretical). Materials optimization—replace heavy cotton substrate with LDPE-like substrate; replace steel vessels with aluminum or HDPE to reduce payload mass.

• Feasible Mars-specific propellant: Short-chain diols, especially 2,3-butanediol (2,3-BDO), meet Martian constraints with favorable O₂/fuel ratios and adequate Isp (~420 s theoretical), enabling MAV launch with lower oxidizer mass than methane.
• Production target: Although 8.4 tons of 2,3-BDO would suffice theoretically, the study conservatively targets 10 tons over 500 sols, requiring ~19.6 tons LOX; cyanobacterial O₂ evolution also supplies ~12 tons for fermentation.
• Biofilm vs suspended cyanobacteria cultivation: Similar productivities (~6.6 g/m²/day), but biofilm uses 89% less water, 65% less power, and 15% less payload mass than suspended growth; eliminating mixing and biomass concentration significantly cuts power demand.
• State-of-the-art bio-ISRU vs chemical ISRU (DRA 5.0 O₂-only): • Power: Bio-ISRU uses 32% less power (17.64 kW vs 26.08 kW). • Payload: Requires 2.8× higher payload mass (≈20.94 tons vs 7.51 tons). • Oxygen: Generates ≈43.81–44 tons of excess clean O₂ over 500 sols, beyond 19.6 tons for launch and ~12 tons for fermentation.
• Biological optimization (plausible improvements): Increasing cyanobacterial productivity to 13.28 g/m²/day, enzymatic glucose yield to 60 wt%, and fermentation yield to 0.51 g/g decreases payload mass by ~56% (driven by ~69% farm-size reduction) and reduces water and power by ~25% and ~16%, respectively.
• Materials optimization: Replacing cotton substrate with LDPE-like material and steel reactors with aluminum/HDPE further reduces payload mass.
• Fully optimized bio-ISRU: Requires 59% less power and 13% lower payload mass than the DRA 5.0 O₂-only strategy (payload ~6.53 tons vs 7.51; power ~10.80 kW vs 26.08), while still producing ~20.35 tons excess O₂.
• Logistics for subsequent missions: Resupply payload estimated at ~3.73 tons (notably less than ~6.5 tons methane for the DRA 5.0 O₂-only strategy).

By tailoring propellant chemistry to Martian conditions and leveraging biological CO₂ fixation and conversion, the proposed bio-ISRU directly addresses the challenge of producing return propellant on Mars without transporting massive quantities from Earth. The 2,3-BDO route balances fuel performance with reduced oxidizer needs and practical phase behavior for storage. The state-of-the-art process already reduces power compared to chemical O₂-only ISRU and uniquely provides large quantities of clean oxygen that support not just launch, but also life support and other operations. With feasible biological and materials advances, the optimized bio-ISRU becomes competitive or superior to chemical ISRU in both payload and power. The analysis highlights where engineering efforts should focus—improved cyanobacterial productivity under Martian light, higher enzymatic sugar release, modest gains in fermentation yield, and lighter materials—to achieve mission-relevant efficiencies. The co-production of oxygen and the modularity of the system (potentially swappable heterotrophs) broaden its relevance beyond propellant to chemicals and food, enhancing the robustness of Martian outposts.

The study proposes and evaluates a biotechnology-enabled ISRU for producing a Mars-specific propellant, 2,3-BDO, from Martian CO₂, sunlight, and water using cyanobacteria and engineered E. coli. The approach is feasible with current technologies, offering lower power consumption than chemical O₂-only ISRU and significant surplus oxygen generation. Model-guided biological and materials optimizations can further reduce power and payload mass, making the system competitive while still producing >20 tons of excess O₂. The work outlines clear engineering targets and suggests future directions: enhancing cyanobacterial photosynthetic efficiency or growth at 25 °C, increasing enzymatic sugar yields (or eliminating digestion via engineered cyanobacterial sugar secretion), modest fermentation yield improvements, lighter reactor and substrate materials, and integration of nutrient recycling. The platform’s flexibility could support broader biomanufacturing for Mars colonization.

• Environmental control: Maintaining ~25 °C process temperature on Mars is assumed but unresolved; potential solutions include greenhouse domes and reactor jacketing.
• Radiation and UV: While cyanobacteria and E. coli tolerate ionizing radiation levels, high surface UV risks mutations; requires UV-reflecting, PAR-transmitting materials.
• Light availability: Martian photon flux is 57% of Earth’s and subject to dust storms; growth modeled as light-limited with potential need for CO₂ pressurization and site selection to mitigate dust events.
• Water quality and treatment: Martian water may require desalting; process assumes effective pretreatment and high-rate recycling.
• Nutrient supply: N and P shipped from Earth due to low Martian N₂; trace elements partly recyclable but initial payload includes nutrients and enzymes; assumptions about recycling efficacy affect resupply estimates.
• Process scale-up uncertainties: Continuous biofilm harvesting and long-term membrane performance require further validation at relevant scales; several unit sizing and performance parameters are extrapolated from terrestrial data.
• Propellant performance uncertainty: 2,3-BDO Isp is theoretical; conservative fuel mass target used, but eventual experimental validation may alter requirements.
• Planetary protection: Current policies prohibit releasing Earth microbes on Mars; robust containment and biological safeguards (e.g., kill switches, auxotrophies) are prerequisites.
• Modeling assumptions: Growth assumed light-limited; mixing power reductions with lower gravity; complete water recycle; solvent losses minimal—deviations could impact power and payload metrics.