From the Cover: Fuel and oxygen harvesting from Martian regolithic brine

NASA’s current mandate is to land humans on Mars by 2033. Here, we demonstrate an approach to produce ultrapure H₂ and O₂ from liquid-phase Martian regolithic brine at ∼−36 °C. Utilizing a Pb₂Ru₂O_7−δ pyrochlore O₂-evolution electrocatalyst and a Pt/C H₂-evolution electrocatalyst, we demonstrate a brine electrolyzer with >25× the O₂ production rate of the Mars Oxygen In Situ Resource Utilization Experiment (MOXIE) from NASA’s Mars 2020 mission for the same input power under Martian terrestrial conditions. Given the Phoenix lander’s observation of an active water cycle on Mars and the extensive presence of perchlorate salts that depress water’s freezing point to ∼−60 °C, our approach provides a unique pathway to life-support and fuel production for future human missions to Mars.

Life-support O₂ and fuel (e.g., H₂) are indispensable for human space exploration. The electrolysis of extraterrestrial liquid water can be a significant concurrent source of H₂ and O₂. NASA’s Phoenix lander has found evidence of an active water cycle (1), extensive subsurface ice (2), and the presence of soluble perchlorates (3) on the Martian surface (SI Appendix, section S1). Spectral evidence from the Mars Odyssey Gamma Ray Spectrometer points to the existence of large quantities of water-ice in the northern polar region of Mars (4) and the Mars Reconnaissance Orbiter has also found indications of contemporary local flows of liquid regolithic brines shaping Martian geography (5). Martian regolithic brines with dissolved perchlorates (see “Martian regolith composition” in SI Appendix, Table S1) can exist in the liquid phase since perchlorates significantly depress the freezing point of water (6). Based on compositional analysis by the wet chemistry instrument on the Phoenix lander, Mg(ClO₄)₂ is reported to be a major constituent of the Martian regolith and its concentrated solutions remain in the liquid phase up to ∼−70 °C. This offers a temperature window for the existence of liquid brine on the Martian surface and subsurface as the mean annual terrestrial temperature on Mars is ∼−63 °C (7) with a wide (>100 °C) average diurnal variation (8). The hygroscopic nature of these perchlorates also enables the entrainment of atmospheric water vapor to produce concentrated brine solutions (9). Recently published data obtained by the Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument onboard the Mars Express spacecraft shows that multiple subglacial water bodies presently exist underneath the Martian south pole deposits at Ultimi Scopuli (10).In support of NASA’s mandate to send humans to Mars by 2033 (11), we demonstrate that the electrolysis of these brines at ultralow temperatures is a route to the concurrent production of H₂ as fuel and O₂ for life-support in practical quantities and rates under Martian conditions. NASA has incorporated the Mars Oxygen In-Situ Resource Utilization Experiment (MOXIE) (12) as a part of its Mars 2020 mission (13), as a feasibility study of the electrolysis of CO₂ into CO and O₂ (SI Appendix, section S2). As an alternative, we show that regolithic brine electrolysis under Martian conditions will enable the production of ultrapure O₂ for life-support and H₂ for energy production (SI Appendix, section S3), with no additional purification requirement for CO removal. The H₂ produced in tandem can serve as a clean-burning fuel with a superior calorific value to CO (SI Appendix, section S2). Our electrolyzer system has a 25-fold higher production rate of O₂ when compared to MOXIE while consuming the same amount of power (or, put another way, our system consumes 25× less power than MOXIE for the same O₂ production rate).