Making oxygen and fuel more efficiently on Mars with Plasmas

The case for in situ resource utilisation for oxygen production on Mars by nonequilibrium plasmas

Herein, it is argued that Mars has nearly ideal conditions for CO2 decomposition by nonequilibrium plasmas. It is shown that the pressure and temperature ranges in the ~96% CO2 Martian atmosphere favour the vibrational excitation and subsequent up-pumping of the asymmetric stretching mode, which is believed to be a key factor for an efficient plasma dissociation, at the expense of the excitation of the other modes. Therefore, gas discharges operating at atmospheric pressure on Mars are extremely strong candidates to produce O2 efficiently from the locally available resources.

Local production of oxygen (O2) on Mars may help solve the problems of manufacturing fuel for coming back to Earth and of creating a breathable environment for a future outpost. In fact, the main component of the Martian atmosphere is carbon dioxide (CO2) (95.9%), with smaller percentages of Ar (1.9%), N2 (1.9%) and other gases. CO2 can be converted into O2 and carbon monoxide (CO), which were proposed to be used in a propellant mixture in rocket vehicles. Such in-situ resource utilisation (ISRU) will diminish the needs of additional launch or lander mass. Accordingly, it will minimise risks to the crew and mission, as well as reduce logistics, making it possible to increase the space-craft shielding and provide increased self-sufficiency. Moreover, it will reduce costs by demanding less launch vehicles to complete the mission.

Plasma reforming of CO2 on Earth is also a growing field of research, prompted by the problems of climate change and the production of solar fuels. Indeed, low-temperature plasmas constitute one of the best media for CO2 dissociation, both by direct electron impact and, especially, by transferring electron energy into vibrational excitation.

Plasma technologies for CO2 reforming on Earth are already competitive nowadays with solid oxide electroliser cells (SOEC). Therefore, our investigation evinces that a nonequilibrium plasma process can probably perform better than SOEC for O2 production on Mars, the technology proposed by the exciting MOXIE programme. In fact, while the efficiency of plasma dissociation of CO2 on Mars is likely to increase compared to that on Earth, as demonstrated in this work, the efficiency of solid oxide electrolysis is likely to decrease, because extra energy is necessary to heat the gas up to ∼1100 K and to compress it up to ∼1 atm. In addition, any estimation based on typical gas flows and CO2 conversion rates obtained on Earth points out that the throughput anticipated by the MOXIE experiment, of about 10 g per hour for a power of 300 W, is perfectly within the reach of an optimised plasma device.

A system needing only 150 to 200 Watts for 4 hours each 25-hour Mars day could produce 8 to 16 kilograms of oxygen. The International Space Station currently consumes oxygen in the range of 2 to 5 kilograms per day, so this would be enough to support a small settlement.

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