How to Make Oxygen on Mars
Humans use oxygen for respiration, making it essential to their existence. Oxygen is also a key component of rocket fuel. On Earth, oxygen makes up 21% of the atmosphere. On Mars, oxygen only makes up 0.13% of the atmosphere. Not only that, but the atmosphere is less than 1% as dense as Earth’s. Therefore, it is simply not possible for a colony to be built on Mars without an effective and reliable in-situ oxygen production system in place.
Option 1: MOXIE
The MOXIE, developed by Michael Hecht, is a first generation oxygen generator which has been developed in order to test whether technology can provide the required oxygen for a human Mars colony. It stands for Mars Oxygen In-situ Resource Utilisation Experiment. Since 2014, the MOXIE has been the most well-known and likely candidate for in-situ oxygen production. It uses the fact that Mars’ atmosphere, although lacking in pure oxygen, consists of 96% carbon dioxide. Each CO2 molecule consists of one carbon atom and two oxygen atoms, resulting in a net oxygen percentage of 72% per molecule. Thus, the MOXIE pulls in the Martian air (of which there is an ‘unlimited’ supply) and uses a process called solid oxide electrolysis to electrochemically split the CO2 molecules and liberate the oxygen (it is named a ‘reverse fuel cell’).
The thin ceramic cells have a chemical property that allows them to conduct electricity using oxygen as the charge carrier. The ‘hurdle’ of this process is that the cells must be heated to roughly 800 degrees Celsius, thus requiring a lot of energy. The byproducts (carbon monoxide and other inert gases) are exhausted, while the oxygen is analysed for purity before being vented for direct consumption.
The current ‘standard’ MOXIE is a highly compact system. It has dimensions of 23.9 x 23.9 x 30.9 centimetres, roughly the size of a car battery. Each unit can produce up to 10 grams of oxygen per hour, enough to support one human. In fact, one was built into the 2021 Mars Perseverance Rover. The rover successfully used the MOXIE to convert carbon dioxide into breathable oxygen, confirming the claims of models, simulations and tests run by experts since the MOXIE was first developed. Despite this, one must still consider the large power demand of this machine, for energy will not be as accessible on Mars as it is on Earth (at least to begin with).
Option 2: Brine electrolysis
A recent study has demonstrated how hydrogen and oxygen can be produced efficiently from the Martian brine (salty water) via electrolysis. This is important because there are soluble salts in the Martian regolith known as perchlorates, which have been described as ‘global’ and ‘ubiquitous’ across Mars. These salts are excellent at absorbing water, even in environments as dry as Mars. Therefore, perchlorate abundant sites are indicative of brine hotspots. Given this brine can be electrolysed/split into hydrogen and oxygen, such sites become a potentially vast source of oxygen.
The image below shows liquid droplets of brine which formed on the struts of NASA’s Phoenix lander. It was believed to be a solution of magnesium perchlorate, with the three images corresponding to days 8, 31 and 44 (respectively) after the lander touched down in 2008. The fact that a perchlorate abundant site such as this was found unintentionally demonstrates how much there could be across Mars. Although, there is some potential for this to be an anomaly; further research will have to be done to determine this.
As explained when evaluating the MOXIE, it requires vast amounts of energy to produce oxygen, making it a highly inefficient system. Whereas, this study demonstrated how brine electrolysis is theoretically 25 times as efficient as the MOXIE. That is, it can produce up to 25 times the amount of oxygen for the same input power. This increase in efficiency would be invaluable on a self-sufficient Mars colony, because almost every aspect of living will require electricity. The extra energy saved by not using the MOXIE could be distributed to these other processes, improving the feasibility of self-sufficiency.
So, which one?
Clearly, the brine electrolyser is more appealing in terms of performance, and to many, this makes it the obvious choice. However, the fact that the MOXIE has been popular for so long gives it the edge (at this stage). The MOXIE has been the most likely candidate for this length of time because it has been tested rigorously, and although its power composition is higher than ideal, there is the security of knowing that it works. On the other hand, the brine electrolyser is, as I said, a veryrecent proposal, and thus has not had anywhere near the level of testing as the MOXIE. On a self-sufficient Mars colony, it is imperative that all machines and technologies have the highest level of integrity as possible, as they are what prevents colonists from succumbing to the inhospitable conditions of Mars.
Extending the importance of integrity further: the problem with sending either a MOXIE or brine electrolyserwith humans on their journey to the Red Planet, is that if such machines were to fail, they’d be helpless. Indeed, they may bring back-ups, but this attitude quickly becomes excessively expensive and loses integrity. In fact, Thomas Clayson explains how the largest problem with Mars missions is simply the challenge of getting there. Being roughly 60 million kilometres at its closest, it takes about 6 months to get there. Consequently, if a certain system were to fail, ‘people will almost definitely die’. In a similar fashion to Robert Zubrin’s Mars Direct plan, a sensible decision would be to send large-scale MOXIEs in advance of human missions.
In the Mars Direct plan, Zubrin proposes that one human mission should be sent every two years, with a cargo mission over the same interval but beginning two years prior. This, however, was written before the MOXIE’s time. And so, an ideal improvement would be to do the exact same but include a large MOXIE in each cargo mission (as opposed to just supplies), up until MOXIEs can be constructed directly on Mars. The advantage of this is that the MOXIE can produce oxygen over the period before humans arrive, meaning there is a definite supply on their arrival, thus minimising the potential catastrophe associated with relying on ‘live’ oxygen consumption. Such a supply can be designed to last them beyond the time it would take to construct a new one on Mars. In this way, oxygen self-sufficiency on Mars is achieved.
Hi, I’m Will! I’m a sixth form student at Sherborne School. This October I’ll be going to Oxford to study physics, and so, in my quest to learn more about this fascinating subject… I’m writing this blog! View all posts by Will Fahie
Originally published at http://thephysicsfootprint.com on February 7, 2022.
