Life on Earth owes its existence to photosynthesis, a process that is 2.3 billion years old. This immensely fascinating (and still not fully understood) reaction allows plants and other organisms to harvest sunlight, water and carbon dioxide while converting them into oxygen and energy in the form of sugar.
Photosynthesis is such an integral part of how the Earth works that we take it practically for granted. But as we look beyond our planet for places to explore and settle, it’s obvious how rare and precious the process is.
As my colleagues and I investigated in a new paper, published in Nature Communications, recent advances in making artificial photosynthesis could hold the key to surviving and thriving away from Earth.
The human need for oxygen makes space travel complicated. Fuel constraints limit the amount of oxygen we can take with us, particularly if we want to make long-haul trips to the Moon and Mars. A one-way trip to Mars usually takes on the order of two years, meaning we can’t easily send resource supplies from Earth.
There are already ways to produce oxygen by recycling carbon dioxide on the International Space Station. Most of the ISS’s oxygen comes from a process called electrolysis, which uses electricity from the station’s solar panels to split water into hydrogen gas and oxygen gas, which astronauts inhale. It also has a separate system that converts the carbon dioxide in which astronauts exhale. water and methane.
But these technologies are unreliable, inefficient, cumbersome and difficult to maintain. The process of generating oxygen, for example, requires about a third of the total energy required to operate the entire ISS system supporting environmental control and life support.
Ways to follow
The search for alternative systems that can be used on the Moon and on trips to Mars continues. One possibility is to harvest solar energy (which is abundant in space) and use it directly for oxygen production and carbon dioxide recycling in one device.
The only other input into such a device would be water, similar to the photosynthesis process that occurs in nature. This would circumvent complex setups where the two processes of light harvesting and chemical production are separate, such as on the ISS.
This is interesting as it could reduce the weight and volume of the system, two key criteria for space exploration. But it would also be more efficient.
We could use the additional thermal energy (heat) released during the process of capturing solar energy directly to catalyze (ignite) chemical reactions, thereby speeding them up. Also, complex wiring and maintenance could be greatly reduced.
We produced a theoretical framework to analyze and predict the performance of such integrated artificial photosynthesis devices for applications on the Moon and Mars.
Instead of chlorophyll, which is responsible for absorbing light in plants and algae, these devices use semiconductor materials that can be directly coated with simple metal catalysts that support the desired chemical reaction.
Our analysis shows that these devices would actually be feasible to complement existing life support technologies, such as the oxygen generator set employed on the ISS. This is especially true when combined with devices that focus solar energy to power reactions (essentially large mirrors that focus incoming sunlight).
There are other approaches as well. For example, we can produce oxygen directly from lunar soil (regolith). But this requires high temperatures to work.
Artificial photosynthesis devices, on the other hand, could operate at room temperature at the pressures found on Mars and the Moon. This means they could be used directly in habitats and using water as the main resource.
This is particularly interesting given the established presence of water ice in the lunar crater Shackleton, which is a planned landing site in future lunar missions.
On Mars, the atmosphere is almost 96% carbon dioxide, apparently ideal for an artificial photosynthesis device. But the light intensity on the red planet is weaker than on Earth due to its greater distance from the Sun.
So would this be a problem? We have actually calculated the intensity of available sunlight on Mars. We’ve shown that we can actually use these devices there, although solar mirrors become even more important.
The efficient and reliable production of oxygen and other chemicals, as well as the recycling of carbon dioxide aboard spacecraft and in habitats, is a tremendous challenge we face for long-term space missions.
Existing electrolysis systems, operating at high temperatures, require a significant amount of input energy. And devices for converting carbon dioxide into oxygen on Mars are still in their infancy, whether based on photosynthesis or not.
Several years of intense research are therefore needed to be able to use this technology in space. Copying the essential bits from nature’s photosynthesis could give us benefits, helping us make them happen in the not-too-distant future.
The returns would be huge. For example, we could actually create artificial atmospheres in space and produce chemicals that we need on long-term missions, such as fertilizers, polymers or pharmaceuticals.
Furthermore, the insights we gain from designing and manufacturing these devices could help us meet the challenge of green energy on Earth.
We are lucky enough to have plants and algae to produce oxygen. But artificial photosynthesis devices could be used to produce hydrogen or carbon-based fuels (instead of sugars), opening a green way for producing energy-rich chemicals that we can store and use in transportation.
Space exploration and our future energy economy have a very similar long-term goal: sustainability. Artificial photosynthesis devices could become a key part of its implementation.
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