Scientists have been working on plasma technologies to split CO2 into oxygen and carbon monoxide, primarily prompted by the persistent problems of climate change.

"I started wondering if the CO2 reforming that we were working on could somehow be adapted to the production of oxygen on Mars. I shared these thoughts with my colleagues the next day...the more we mused about it, the better the idea looked," Guerra tells IE.

Fast forward to 2022. Guerra and his team from the University of Lisbon, along with an international team of researchers from the Massachusetts Institute of Technology, Sorbonne University, Eindhoven University of Technology, and the Dutch Institute for Fundamental Energy Research, presented a method for harnessing and processing local resources to generate oxygen on Mars.

Their weapon of choice? Plasma itself.

The plasma-based way to produce and separate oxygen within the Martian environment could function as a complementary approach to NASA's Mars Oxygen In-Situ Resource Utilization Experiment (MOXIE) and deliver high rates of molecule production per kilogram of instrumentation sent to space.

The research was published Tuesday in the Journal of Applied Physics, from AIP Publishing.

Summoning the fourth state of matter

The system could play a major role in the development of life-support systems on Mars, as well as the feed-stock and base chemicals necessary for processing fuels, building materials, and fertilizers.

So, how does this work?

Since the Martian atmosphere is primarily comprised of carbon dioxide that can be split to produce oxygen and its pressure is favorable for plasma ignition, conditions on the Red Planet could be almost ideal for in-situ resource utilization by plasmas.

"To extract the oxygen from the CO2 molecule, we need to decompose it. This requires energy. Plasma provides an effective way to channel the energy [acquired from its free electrons] into the desired output," says Guerra.

In other words, the available energy can be used selectively. "We also need to separate the produced oxygen from a gas mixture that also contains, for instance, carbon dioxide and carbon monoxide, for which we use an oxygen permeation membrane," he continues.

Plasma contains free-charged particles, like ions and electrons, that are light and easily accelerated up to very high energies with electric fields. "When these "bullet-like" electrons collide with a CO2 molecule, they can directly decompose it or transfer some energy to put it to vibrate," says Guerra. To a large extent, this energy can be channeled for the decomposition of CO2. "In addition, the heat generated in the plasma and other plasma effects will also be beneficial for the separation of oxygen by increasing the performance of the separation membrane," explains Guerra.

How does it differ from MOXIE?

MOXIE, a toaster-size, experimental instrument located aboard the Perseverance rover, converts the Red Planet's carbon dioxide-rich atmosphere into oxygen. It isolates and stores oxygen on Mars to help power rockets that could lift astronauts off the planet's surface. The device could someday provide air astronauts themselves can breathe.

Currently, it is the only concrete proposal for oxygen production on Mars to date.

"NASA’s MOXIE experiment starts with a working technology on Earth and its adaptation to Martian conditions (of pressure and temperature, for instance). A plasma-based method directly takes advantage of the natural conditions on Mars, which are nearly ideal for plasma operation. In particular, the atmospheric composition, the ambient pressure, and temperature on Mars - all play in favor of a plasma process. This was our first speculation, supported by theoretical arguments and a few numerical simulations, and focusing only on CO2 decomposition," explains Guerra.

In particular, the atmospheric composition, the ambient pressure, and temperature on Mars - all play in favor of a plasma process.

With their colleagues at Ecole Polytechnique in France and the Dutch Institute for Fundamental Energy Research (DIFFER) in the Netherlands, Guerra and his team later demonstrated the validity of this theory.

However, there are hurdles.

Guerra mentions two significant challenges to producing oxygen on Mars. "Firstly, the very decomposition of CO2 (into CO and O), as CO2 is a very difficult molecule to break. The second is the separation of the produced oxygen from a gas mixture that also contains, for instance, carbon dioxide and carbon monoxide. We are actually trying to look at the two steps in a holistic way, solving both challenges at the same time," he says.

The team will also need to iron out scientific challenges, the primary of which is to demonstrate that their concept works. "We understood and solved many aspects and have strong indications and very promising preliminary results. We have some funding from the European Space Agency and other national projects to do so and are quite confident we will manage," says Vasco.

An engineering challenge also arises in the picture as the concept is to be done with a compact and lightweight device. "We will need to optimize the reactor and show that it can deliver high rates of production of molecules per kilogram of instrumentation sent to space," adds Guerra.

Crucial for sustainable missions

How far would it help reduce the logistics of a mission to Mars?

"Any gram and any cubic centimeter we can spare in a space mission are important," says Guerra. "Oxygen is key to creating a breathing environment, of course. But, oxygen and carbon monoxide can also be used to manufacture liquid propellants for rockets. The rover Perseverance is already collecting samples to be returned to Earth. Solid fuel rockets will be used in the forthcoming Mars Sample Return missions, but local production of fuels will be important for similar future missions. The impact in reducing the logistics of a mission to Mars can therefore be very significant, by reducing the amount of fuel and breathable oxygen to be transported," he says.

Also, oxygen and nitrogen — also available in the Martian atmosphere — are the building blocks for synthesizing nitrogen-based fertilizers for future Martian agriculture.

"Together with oxygen for life support and the production of fuels, we are no longer discussing only the reduction of the logistics of a mission, but in creating the conditions for a future human settlement as well," stresses Guerra.

The team is now progressing towards the assemblage of a proof-of-concept prototype. "Then we need to optimize the reactor and show it can be competitive. And, finally, to make it fly to Mars one day," says Guerra.

Beyond Mars

These results can also significantly impact life on Earth. The study author states that "the adaptation of our research to the conditions on Earth (that we are also pursuing) is part of the 'carbon capture and utilization' (CCU) strategy."

In this approach, carbon dioxide is used as a raw material to be converted into value-added chemicals and green fuels. "Any technology that recycles CO2 for further usage contributes to the reduction of greenhouse gas emissions and can play an essential role in the transition to a prosperous net-zero economy," adds Guerra.

Harnessing resources in the target site instead of transporting them from Earth is definitely the first step towards the self-sufficiency of space bases and missions. According to the study, the highest-impact in-situ resource utilization is extremely crucial to such endeavors - reducing the logistics and expense of the trip and the risk to the crew.

Abstract:

This work discusses the potential of combining non-thermal plasmas and conducting membranes for in-situ resource utilization (ISRU) on Mars. By converting different molecules directly from the Martian atmosphere, plasmas can create the necessary feed-stock and base chemicals for processing fuels, breathing oxygen, buildingmaterials and fertilizers. Different plasma sources operate according to different principles and are associated with distinct dominant physicochemical mechanisms. This diversity allows exploring different energy transfer pathways leading to CO2 dissociation, including direct electron impact processes, plasma chemistry mediated by vibrationally and electronically excited states, and thermally-driven dissociation. The coupling of plasmas with membranes is still a technology under development, but a synergistic effect between plasma decomposition and oxygen permeation across conducting membranes is anticipated. The emerging technology is versatile, scalable, and has the potential to deliver high rates of production of molecules per kilogram of instrumentation sent to space. Therefore, it will likely play a very relevant role in future ISRU strategies.