In situ resource utilization (ISRU) is still a very early science. Therefore, the technology utilized in it could be improved upon. One such technology that created one of the most useful materials for ISRU (oxygen) is MOXIE – the Mars OXygen In-situ Resource Utilization Experiment. A small-scale model of a MOXIE was recently tested on the Perseverance last year. Its primary goal is to create oxygen out of the Martian atmosphere.
Oxygen is useful in various ways – as rocket fuel for the Mars Ascent Vehicle, part of the Mars Sample Return Mission, and a necessary life support gas. Any source of pure oxygen on Mars, or any other planet, would be welcomed by any human explorers there. Now a competing technology known as a “thermal swing sorption/desorption” (TSSD) cycle is being supported by NASA’s NASA Innovative Advanced Concepts (NIAC) program. Could it really be that much better?
To answer that question, first, it’s best to understand the capabilities of the MOXIE. The underlying technology, which utilizes a solid oxide electrolyzer cell, has been well understood and used on Earth for years. To make it work on Mars, engineers added an impeller to concentrate the Martian atmosphere. After it is pressurized, the atmosphere would heat up to 800 degrees C, and then the CO2 would undergo electrolysis to separate the oxygen from the carbon.
This results in pure oxygen, but it also has some significant disadvantages. First is the other element this process results in – carbon. Partly because it is so common, carbon isn’t a valuable material for ISRU, so it is considered a waste rather than something to retrieve. Therefore, it sometimes jams up the electrolyzer itself. Carbon is not the only source of potential jamming. Martian dust could clog up the mechanical pump needed to concentrate the sparse Martian atmosphere enough to get the electrolyzer running in the first place. That pump also contributes to the overall electrical power requirements of the system, which also includes the electrolyzer having to run at a blister 800 degrees celsius.
All that power consumption makes for an expensive system. A MOXIE machine that can create 2 kg O2 per hour (enough to support two explorers) would require around 25 kW of power, or slightly less than the average American house uses per day. While that may not seem like a lot, utilizing solar energy is a much more difficult prospect on Mars. Any early solar farm built to run the MOXIE system would dwarf the habitat it could supply with oxygen for just two astronauts.
Enter the TSSD, which nicely eliminates all three major problems with MOXIE. The system itself relies on a thermochemical pumping system, which relies on heat differentials to move the atmosphere to the appropriate place, eliminating the need for a mechanical pump. It also doesn’t suffer from carbon fouling as it doesn’t break apart CO2. Lastly, it doesn’t require too much energy, with Dr. Ivan Ermanoski, the PI on the NIAC funded project and a research professor at Arizona State University, expecting that it will be 90% more efficient than MOXIE.
Details on how it will do all of this still need to be fleshed out, but conceptually the idea was backed up by a paper published back in March 2020. The NIAC project itself will continue that process. It seems to be a material science question of whether the right kinds of sorption materials can be found that work in the proper environmental conditions on Mars. However, if even half of the system’s advantages are realized, it will mean a great leap forward in ISRU technology for Mars and beyond.
NASA – Breathing Mars Air: Stationary and Portable O2 Generation
Ermanoski et al – Thermally–driven adsorption/desorption cycle for oxygen pumping in thermochemical fuel production
ASU – Physicist joins ASU LightWorks to help solarize society
Sandia – Concentrating on Sunshine to Advance the Hydrogen Economy
Graphic depiction of the TSSD cycle.
Credit – Ivan Ermanoski