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

The successful Perseverance mission signifies the dawn of Martian exploration. A major obstacle to sustained human presence on Mars is the return journey; transporting the roughly 30 tons of methane and liquid oxygen (LOX) needed for a Mars Ascent Vehicle (MAV) launch from Earth would be prohibitively expensive and logistically challenging. This research proposes a biotechnology-enabled in situ resource utilization (bio-ISRU) strategy to produce rocket propellant on Mars, leveraging the planet's unique conditions: low gravity, abundant CO2, water ice, and sunlight. This approach offers potential benefits including reduced launch mass, lander mass, and safety risks associated with transporting large quantities of propellant. Current ISRU strategies focus primarily on methane production via chemical processes, such as solid oxide carbon dioxide electrolysis (SOCE) to obtain O2, the Sabatier reaction to convert H2 and CO2 into CH4, and water electrolysis to produce H2 and O2. However, these strategies often involve shipping significant quantities of hydrogen from Earth, adding complexity and cost. This work explores the use of alternative propellants, designed specifically to account for the lower gravity and atmospheric composition of Mars. Oxygen-containing propellants require less LOX for combustion, a significant advantage given Mars' low oxygen levels. The specific impulse (Isp) is a key metric of rocket engine performance. Short-chain diols are identified as promising candidates, offering comparable Isp to methane while requiring less LOX. This paper details a bio-ISRU strategy using cyanobacteria to convert CO2 and sunlight into sugars, which serve as feedstock for engineered *E. coli* to produce 2,3-butanediol (2,3-BDO), a short-chain diol identified as a suitable propellant. This approach offers the added benefit of producing excess oxygen for various aspects of Martian exploration and human habitation.

Existing literature extensively documents chemical ISRU strategies for rocket propellant production on Mars. The NASA Human Exploration of Mars Design Reference Architecture (DRA) 5.0 primarily focuses on methane (CH4) due to its favorable heating value and specific impulse. DRA 5.0 outlines three chemical ISRU strategies: an O2-only strategy (shipping CH4 from Earth and producing O2 on Mars), an H2-only strategy (shipping H2 and using the Sabatier reaction), and a complete strategy (producing both CH4 and O2 on Mars). However, these strategies face limitations, particularly concerning the substantial mass of H2 that needs to be shipped in the latter two approaches and oxygen production being insufficient in the Sabatier approach. The literature also highlights the potential of bio-ISRU for producing various materials and fuels in space. Previous research demonstrates the use of methanogens to convert CO2 to CH4 and photosynthetic cyanobacteria to convert CO2 and sunlight into biomass and chemicals, producing O2 as a byproduct. However, the efficiency of cyanobacteria in producing specific chemicals has lagged behind other engineered microbes like *E. coli* and *S. cerevisiae*. This paper builds upon this existing knowledge by proposing a hybrid approach, using cyanobacteria as a feedstock for an engineered heterotrophic microbe (*E. coli*) for efficient 2,3-BDO production, thereby combining the advantages of both autotrophic and heterotrophic systems.

The researchers developed a process model for a bio-ISRU system to produce 2,3-BDO. The process comprises four main modules: (1) cyanobacteria cultivation, (2) biomass preprocessing (concentration and enzymatic digestion), (3) microbial fermentation (*E. coli* producing 2,3-BDO), and (4) 2,3-BDO extraction and purification. Two cyanobacteria cultivation methods were modeled: suspended growth in photobioreactors (PBRs) and biofilm growth on a porous substrate. The model incorporates various parameters relevant to Martian conditions, such as reduced sunlight intensity, temperature, and atmospheric pressure. The model also accounts for nutrient requirements (nitrogen and phosphorus primarily shipped from Earth, and trace metals recycled within the system), water usage and recycling via reverse osmosis, and power consumption for each unit operation. State-of-the-art parameters were utilized for the key biological components, including *A. platensis* growth kinetics, enzyme digestion efficiency, and *E. coli* 2,3-BDO production yield and productivity. The model incorporates mass and energy balances for each module, considering material properties, process efficiency, and equipment requirements, which were also modeled and integrated. The model was used to assess various cultivation methods (suspended vs. biofilm), enzymatic digestion times, and identify key optimization strategies for enhancing overall process efficiency. Theoretical Isp values were calculated, and the ideal rocket equation was used to determine the required propellant mass. Alternative materials for the system components were also evaluated to minimize payload mass. Finally, flux balance analysis (using the COBRA toolbox) was employed to determine the theoretical maximum yields of 1,2-PDO, 1,3-BDO, and 2,3-BDO from glucose in *E. coli*, aiding in the selection of 2,3-BDO as the target propellant.

The study's key findings are centered around comparing a bio-ISRU system for 2,3-BDO production with existing chemical ISRU strategies (primarily the DRA 5.0 O2-only strategy). The initial model using state-of-the-art biotechnology showed that the bio-ISRU, while using 32% less power, required a 2.8-fold higher payload mass. However, this was offset by the generation of 44 tons of excess oxygen. Further optimization strategies, focusing on improvements in cyanobacteria productivity (targeting 13.28 g/m²/day), enzymatic digestion yield (60% of total biomass), and *E. coli* 2,3-BDO fermentation yield (0.51 g/g glucose), significantly enhanced the bio-ISRU's performance. These biological optimizations, coupled with materials optimizations (replacing cotton biofilm substrate with LDPE and steel reactors with HDPE), led to a system with a 13% lower payload mass and a 59% lower power requirement than the DRA 5.0 O2-only strategy. The optimized bio-ISRU still generated over 20 tons of excess oxygen. The analysis also indicated that biofilm growth of cyanobacteria offers significant advantages over suspended growth, resulting in substantially less water usage and power consumption. Detailed modeling demonstrated how these optimizations reduced the need for water, decreased power usage across various unit operations, and significantly decreased the overall payload mass. The analysis specifically quantified the mass and energy requirements for each module of the bio-ISRU process, outlining potential challenges and opportunities for future improvement. The study thoroughly compares the bio-ISRU system with the DRA 5.0 O2-only strategy across multiple key metrics, demonstrating the potential superiority of the bio-ISRU in terms of resource efficiency and oxygen generation, despite higher initial payload mass.

The findings demonstrate the feasibility of bio-ISRU for rocket propellant production on Mars, offering a compelling alternative to existing chemical methods. The significant reduction in power consumption is a crucial advantage in the Martian environment, where energy sources are limited. The substantial production of excess oxygen is a key benefit, offering a valuable resource for various aspects of Martian colonization beyond propulsion. While the initial bio-ISRU model required a higher payload mass, the model-guided optimizations clearly show how the bio-ISRU can outperform chemical approaches. The study highlights the potential of integrated bio-systems for creating sustainable life-support infrastructure on Mars. The use of oxygen-containing propellants (diols) is shown to be advantageous for Martian conditions, and the selection of 2,3-BDO is well justified based on its high production yields, relatively high Isp, and established separation processes. The choice of biofilm cultivation, while requiring further development for large-scale continuous harvesting, demonstrates significant advantages over suspended growth, primarily in water and power consumption. The study identifies several research directions for further optimization, particularly focusing on improving cyanobacteria productivity and enzymatic digestion efficiency, as well as developing lighter, more robust materials for the system components.

This study demonstrates the potential of a bio-ISRU strategy for producing 2,3-BDO rocket propellant on Mars, using cyanobacteria and engineered *E. coli*. Through process modeling and optimization, the researchers showed that a bio-ISRU system can be more energy-efficient and produce excess oxygen compared to chemical ISRU methods. The proposed system, after optimizations, requires less payload mass and power than the DRA 5.0 O2 only strategy, while producing excess oxygen. Future research should focus on improving biological components, developing robust and lightweight materials for Martian conditions, and addressing potential challenges related to Martian environmental factors. The success of this bio-ISRU strategy could significantly advance the prospects of interplanetary space travel and Mars colonization.

The study relies on a process model, incorporating various assumptions about process efficiencies and material properties under Martian conditions. While based on existing data and literature, some parameters may require further experimental validation under simulated Martian environments. The model assumes certain technological advancements, such as efficient continuous harvesting of biofilm, which require further research and development. The analysis focuses primarily on propellant production but doesn't fully account for all aspects of system integration and long-term operation on Mars, such as potential equipment failure and maintenance requirements. The study assumes readily available water sources. The impact of Martian dust storms on photosynthetic efficiency is modeled, but its influence might need more thorough experimental investigation. Finally, planetary protection protocols must be considered when using microorganisms in situ.