On the moon and on Mars, it takes more energy to make oxygen from water than on Earth. Researchers led by Beth Lomax of the European Space Agency concluded this on the basis of an experiment they repeated at different gravitational strengths.
This insight could help space agencies and commercial companies that plan to send humans to Mars or the moon for extended periods of time. The researchers described their experiments in the magazine last week Nature Communications.
The moon hardly has an atmosphere and the amount of oxygen in the atmosphere of Mars is negligible: the atmosphere there consists of roughly 0.16 percent oxygen, that of the Earth 21 percent. One way to make your own oxygen on Mars and on the moon is by melting the water ice that can be found there and then using energy to convert the water (H2O) split into hydrogen (H) and oxygen (O2† That technique is called electrolysis.
Hydrogen cars
In electrolysis, two electrodes are placed in a container of water. Hydrogen is made at one electrode and oxygen at the other. This is done by pushing or pulling electrons into the water molecules using electricity. The hydrogen and oxygen molecules formed in this way manifest themselves as gas bubbles in the water. Not all water is used up; enough fluid remains for conduction. The bubbles bubble up and escape from the water; the gas is collected.
On earth, a small part of the hydrogen for hydrogen cars is made in this way, the majority comes from natural gas.
Scientists already knew that electrolysis is more efficient with a stronger gravity than with a weaker one. “This is because with a weak gravity the water is pressed down less hard, so that the bubbles can escape less well,” says Fokko Mulder. He is professor of renewable energy at Delft University of Technology and not involved in the study. The Moon and Mars, due to their relatively small masses, have a weaker gravitational force than Earth. Mars is about one-third that of Earth and the moon one-sixth. Therefore, Lomax and her colleagues investigated how efficiently electrolysis would proceed under gravity there.
Centrifuge
They repeated the same small electrolysis experiment, a set-up a few centimeters, under a broad spectrum of gravitational strengths: from almost no gravity to eight times that of Earth. To mimic a stronger gravity than that on Earth, the researchers placed the setup in a centrifuge with a radius of 25 centimeters in a lab. The rotation creates an outward force, which has much the same effect as a greater gravitational force.
Weaker gravity, the researchers mimicked in an airplane, an Airbus A310, which flew a special course. The aircraft followed a parabolic trajectory. At the top of the parabola, the engines turned off and the aircraft went into free fall for a few seconds. At that time, there was almost no gravity and the electrolysis experiment was performed.
During a number of flights, the gravity in the setup was turned up slightly by placing the experiment in a centrifuge during the free fall. Incidentally, it is prohibited to make hydrogen during a parabolic flight due to the risk of fire. The researchers therefore added copper sulphate in all experiments to prevent the formation of hydrogen.
It follows from the experiments that electrolysis on Mars and on the Moon produces eleven percent less oxygen than on Earth. This would then have to be compensated by pumping more electricity into the water.
Settlement
Willem Haverkort, electrolysis researcher at Delft University of Technology and also not involved in the study, is not yet convinced. “The experiments have been cleverly carried out and it is also expected that electrolysis will be less easy with weak gravity, but how efficiently the bubbles escape exactly depends on many more factors than gravity alone. To conduct the experiment in the plane, the researchers had to adjust the entire setup. This will make the efficiency different from that of the device that will eventually be on Mars or on the moon.”
“In addition, radiation on Mars or on the moon, or moon dust, could influence the functioning of the electrolysis machine, says Wim van Westrenen, moon researcher at the VU University Amsterdam and not involved in the study. “Before we send astronauts to Mars or to the moon for a long time, we also need electrolysis experiments on the moon and Mars.”
Several space agencies and companies are working on a settlement on Mars or on the moon. For example, NASA’s Artemis program is now training eighteen astronauts for an increasingly long-term occupation of the moon and perhaps then, in the more distant future, for a permanent base on Mars.
space cart
On Mars, electrolysis could also be done with carbon dioxide (CO .) instead of water2† That’s 96% of Mars’ atmosphere. MOXIE, an instrument aboard NASA’s space cart Perseverance, split CO . last year2 in oxygen (O2) and carbon monoxide (CO). After an hour of electrolysis, MOXIE produced enough oxygen for one astronaut to breathe for ten minutes.
“Another option is to use electrolysis to extract oxygen from rocks from Mars or the moon,” Mulder says. “But then you need extremely high temperatures to first liquefy the rock before you can use it for electrolysis. For now, electrolysis with water still seems to be the easiest source of oxygen.”