Comment on 'The case for in situ resource utilization for oxygen production on Mars by non-equilibrium plasmas'

The basic idea to use plasma technology for space travel in general and for the establishment of long term or even permant human settlements on Mars was first brought forward by Gruenwald in 2014 [2]. The author analyzed possible threats for astronauts by comparing the establishment of a human outpost on the red planed by historical data that was dealing with the dangers that the first European settlers faced in America. It turned out that plasma technology can play a key role in space hygiene and waste treatment because plasma disinfection or waste treatment has no reliance on liquid water or other scarce raw materials on Mars. After publishing this general idea, there was a follow up paper that was devoted to the efficiency of plasma assisted carbon dioxide dissociation directly from the Mars atmosphere [3]. This paper also suggested the separation of breathable oxygen from other by-products like carbon monoxide by centrifugal force. It was also pointed out that the carbon, which is produced by the plasma dissociation process, can be used to create raw materials like graphene or diamond directly on the red planet. Furthermore, available data on the dissociation rates for different plasma sources were compared and it turned out that the highest dissociation rate can be achieved with an rf discharge working at 13.56 MHz and 1 kW input power (up to 90% CO₂ dissociation). The available data suggested that a 3 kW rf plasma source suffices to generate enough oxygen for the daily consumption of an average adult human. This is considerably lower than the power consumption of biological life support systems as, for example, suggested by Wheeler [4], which would use between 10 and 100 kW. In addition, it was shown that it is possible to connect a plasma technology based life support system to other biological systems that generate hydrogen as a by-product and use this combination to also synthesize drinking water and hydrocarbons with higher complexity.

Two years later, another follow up paper was published on this topic [5]. This paper dealt with optimizing the geometry of the plasma life support system and the materials, which are suitable for space applications. Particularly the gas separation coefficients due to centrifugal forces and the oxygen generation capability as a function of the reactor size were calculated. The author was able to show that a reactor with half a meter in diameter and one meter in height can produce enough oxygen for nine humans at an input power of 40 kW. As the gas separation is done by centrifugal force, the rotation speed of the centrifuge for such a life support system was determined to be 1700 rad s^–1, which is well within the capabilities of a modern gas centrifuge that can handle up to 10⁵ rpm [6]. In the same work, materials suitable for space technology were identified according to the SpaceMat database [7] and their usage for the different parts of the life support system was examined. The optimal geometry for a plasma based life support system was found to be of elliptical shape in which the rf source is located in one focus, while the substrate for producing the carbon based by-products is located in the other one. This guarantees a very dense plasma for the CO₂ dissociation in a very well-defined spatial region of the reactor chamber.

These earlier studies are now complemented by the recent work of Guerra et al [1], who focused rather on pulsed DC discharges than on rf or microwave power for the plasma generation. The authors used kinetic codes to study the time evolution of different vibrational temperatures and the gas temperatures on Earth and in the Mars atmosphere. These simulations were done for electron temperatures in the range between 1.6 and 1.7 eV and electron densities between 5.5 × 10⁹ and 7.1 × 10⁹ cm⁻³. They were able to show that pulsed DC discharges under Mars' atmospheric conditions are able to efficiently dissociate carbon dioxide into carbon, carbon monoxide and oxygen by establishing a sever vibrational non-equilibrium. This promising research offers another possible tool to construct plasma based life support systems for long term Mars missions besides the already studied rf and microwave discharges.