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A photo taken inside Biosphere 2 in March 2025
The first impression you get when visiting Biosphere 2 is that it would be a great place to start a cult. The futuristic greenhouse complex is built in the hills north of Tucson and has a beautiful view of the Santa Catalina mountains. The environment contains dangerously high levels of Arizona woo—you can practically feel the earth currents flowing down your energy meridians and making your various chakras and nadis vibrate in sympathy with Mother Gaia.
And the people who ran the place were kind of cultlike! They practiced mandatory weekly meditation and radical theater, were based out of a place called Synergia Ranch near Santa Fe, and had a charismatic and domineering leader named John Allen, who would harangue them for hours about world history and the noösphere. It didn’t help that the eight biospherians who locked themselves inside for a two-year stint in 1992 dressed in Star Trek-like uniforms and had names like Laser and Harlequin. There was a distinct Hale-Boppness to the whole enterprise that people at the time picked up on, years before the Heaven’s Gate cult finally gave wearing matching track suits a bad name. This was not a group of people whose mortal shells you would have been surprised to find arranged in a mandala on the floor when the doors finally re-opened on their great experiment in 1993.
But in truth, the biospherians were sane and competent, and they achieved something that has eluded everybody else, then and since. They built a large-scale habitat and lived inside it for two years in conditions of full material closure, growing their own food, recycling their own waste, and maintaining a breathable atmosphere in a completely sealed environment. Their large complex of greenhouses leaked less air than even the Space Shuttle1, and the 17 months they spent inside before needing to supplement the atmosphere with outside oxygen remains the absolute record for survival in an environmentally closed system.
Biosphere 2 was built as a complex of conjoined greenhouses, each hosting a different type of biome. The site included a miniature ocean, complete with wave machine, and a living coral reef. There was a savannah room, a tropical rain forest, a fog desert (an arid microclimate with high humidity) along with a farming area and the biospherians’ living quarters. The group shared a communal kitchen, but each resident had a private space he or she could retire to. Underneath the whole complex was a machine room, also part of the enclosure. Two large rooms with weighted bellows, called the lungs, equalized air pressure with the outside world and served as storage rooms for excess biomass (cut grass) when carbon dioxide levels got too high.
Today the complex is run as an ecological lab by the University of Arizona, and you are free to go inside on a tour. The whole facility feels a bit like a video game level, or a place where you might have to frantically search for the murderer while your fellow biospherians dropped one by one, with the police watching helplessly from outside.
The biospherians ate only what they could grow, a diet built around sweet potatoes, bananas, beets, goat milk, and a particularly vile-tasting bean called the lablab. They spent about three quarters of their time doing farm work, meal prep or cleanup, and approximately all of their time being hangry, which contributed to tensions that split the crew into two irreconcilable factions within a year of starting the experiment.
The meal schedule had crew members taking turns to cook lunch and dinner for their shipmates, which ensured that the two worst cooks on the crew each prepared a quarter of the meals. Their diet was mostly vegetarian, supplemented with small amounts of eggs, milk, and occasional meat and fish. Breakfast was invariably porridge sweetened with papaya, and any attempt at innovation around this meal (the only chance during the day to feel full) met with threats of violence. Lunch was a soup. There was a midmorning snack of peanuts, counted out carefully so everyone got the same portion; the hungry crew ate them with the shells on.
Like in any subsistence society, life in the biosphere became entirely centered on food. There was a regular schedule of feast days (American holidays and crew birthdays) to offset the constant gnawing hunger. The biospherians ate so many sweet potatoes their skin turned orange from the beta carotene.
Pest control was a battle. Stowaway Australian cockroaches covered the floors at night and destroyed everything they could get their mandibles on, including much of the salad crop and both of the kitchen’s microwave ovens (they ate the wiring). The broad mite, a pest that normally afflicts tea plants, broadened its horizons in this special setting and killed the entire potato crop. In the Ocean, the biospherians engaged in a battle of wits with stowaway octopi who had been accidentally introduced into the habitat and whose voracity was only eclipsed by their cunning. After the first few weeks they were never seen, but their depredations continued.
By objective criteria the crew were aggressively healthy—everyone lost weight, blood pressures and serum cholesterol dropped, and they lived the dream of a modern life extension influencer. But they were also grouchy, beset by cravings for sugar and coffee, and towards the end of the mission (as oxygen levels in the sealed habitat approached 14%) chronically hypoxic, to the point of sleep apnea. Early in the mission one crew member destroyed most of the coffee crop by damaging the flowers on the young coffee plants. In her book on the experiment, Jane Poynter recounts being so hungry that she stole a handful of monkey chow (it tasted sweet), along with multiple occasions when she and her boyfriend cooked themselves a illicit handful of sorghum late at night in the lab. The only locked room in the whole biosphere was the door to the banana pantry; no one trusted themselves to resist the sweet fruit at night.
The Biosphere missions were meant to keep going, but after the first experiment, the project’s billionaire benefactor got into conflict with management and pulled the plug. For a while the Biosphere was run by Steve Bannon(!), then got dumped on Columbia University, and finally landed in the hands of the University of Arizona, who operates it today. There are no more experiments with environmental closure, but the unique facility has enabled a lot of ecological research that would be difficult or impossible to do in a less controlled environment. You can go visit it today—get there early in the morning, and you’ll practically have the place to yourself.
Biosphere 2 was a bit of a punch line, but it deserves a better reputation than it got. This unlikely project remains our best window into what living in a Martian habitat would be like.
There are two conceptual approaches you can take to off-world farming. One is the NASA approach of treating agriculture as an engineering problem, where plants are just a specialized green tool in a mechanical toolkit. In this paradigm, you want to minimize the number of constituent parts needed to keep the crew alive, while excluding all extraneous factors. Your goal will be to cultivate hardy single-species plants that are free of pests, microorganisms, or any other complications, in known and reproducible conditions.
The term for this kind of minimalist farming is gnotobiology, from the Greek root gnosis. The idea is that everything in your biological system is fully characterized, down to the microbe level. At the extreme end, you might choose to grow organisms that are axenic, with no other species present at all. (This is particularly easy to do for fish, since the outside of their eggs can be sterilized before they hatch.) But since many plants need bacterial partners to thrive, it often describes systems where you’ve excluded everything except a short whitelist of chosen, beneficial organisms.
The other approach to farming is ecological. Instead of building up from a minimal set of species, you start from the other end, trying to capture as much of the natural community around an organically grown crop as you can fit in a sealed enclosure. This will necessarily include a lot of mystery organisms, including all kinds of insects, some pests, and a vast and unknown microbiome. The motivating idea is that you stand a better chance of preserving the balance that exists in a natural ecosystem on Earth by sizing it down than you would by trying to build it up one species at a time, from first principles.
Biosphere 2 incorporated both approaches. For habitats that were well-understood (savannah, desert, and of course the farm plot), biospherians carefully curated the set of species, from plants to insects, that they wanted to have in their world. But for the most complex habitats (ocean and rain forest) they used the other approach, collecting bulk soil and water samples from natural sources, and then trying to create conditions that would keep them thriving in captivity.
The advantage of the gnotobiological approach is that it lets you exclude all kinds of pests and nuisances from your space farm. The disadvantage is that your crops then grow in conditions that are unlike anything in nature or human experience, while still embedded in the relatively septic environment of a manned space station. In those conditions, what you thought was a harmless symbiotic bacterium might step up and become an opportunistic pathogen. Or you might discover that an organism you left out was a crucial symbiote necessary to the health of the parent plant.
And there’s evidence that plants and animals need a baseline level of pathogens to adequately develop. Sheltered plants might stand no chance in situations where a more grizzled, street-smart plant could easily fend off a challenge. Or they might simply not be able to produce offspring.
The risk of the ecological approach is also clear. You have no good sense of what organisms are living on your space farm, or how they might behave in an artificial environment. There is also the permanent risk of stowaways (or volunteers, as they were charmingly called by the Biosphere 2 crew). But at least you’re starting with a known balanced system and trying to keep it stable, rather than staking everything on a few chosen species adapting to an alien environment.
The difference in approach also affects your mindset. Are you an engineer or a farmer? If it’s the latter, you gain access to a very deep store of expertise and knowledge, but at the price of full control and understanding. If it’s the former, then you are hanging a giant KICK ME sign on your back to taunt Mother Nature, like every Silicon Valley startup that has so far ventured into vertical farming and found itself obliterated.
The interesting thing about Biosphere 2 is that it was built by a bunch of barely rehabilitated theater majors, but has so far blown all comparable attempts by European and American space agencies out of the water. Until someone can do better at closed system living, the burden of proof will be on NASA to show why hippie-in-a-van organic farming should not be the default approach for long-term living on the Moon or Mars.
With all this philosophical underbrush cleared, let’s build ourselves a Martian farm!
NASA says it takes 50 square meters to grow enough calories to feed one person. That area is more than twice what you need to generate oxygen and absorb carbon dioxide for one human, so in planning our farm we can treat food as the limiting factor, confident that air replenishment will take care of itself.
Biosphere 2 was able to adequately feed a crew of seven (and struggled to feed eight) on half an acre of farmland. That comes out to about 280 square meters per person. Much of the discrepancy with the NASA figure comes from crop failures (broad mites ate the potato crop, mildew got the soybeans) and insufficient lighting. In what follows, I’ll assume a figure of 100 square meters per astronaut, to allow for a safety factor and some variety in what is grown.
Sunlight on Mars is not bright enough for staple crops to thrive. Full Martian sunlight is about the equivalent of a cloudy day in the American heartland, enough for shade-loving plants, but inadequate for growing a proper crop of wheat, potatoes, or rice. The diagram below shows that even great Martian weather is marginal for potatoes, and if there’s any dust in the air, first the potatoes and then you are going to die.
Daily light amount received from the perspective of a potato
Another variable of interest is atmospheric pressure. Experiments on Earth have shown that staple crops will grow normally in air pressures down to 10 kPa (about 1/10 of sea level pressure on Earth, the equivalent of 50,000 feet), as long as they are given enough carbon dioxide. But this is still about twenty times higher than the surface pressure on Mars. So a simple heated greenhouse is not going to cut it; you need some sort of pressurized habitat on Mars to grow stuff.
An official SpaceX rendering of ‘Mars Base Alpha’. Check out the dome!
Everyone wants a dome. I understand. I want a dome! The gorgeous SpaceX renderings of ‘Mars Base Alpha’, back before SpaceX became an AI hosting company, show a lovely transparent geodesic dome full of greenery. But this is an example of what I call the Utah Fallacy. Because Mars looks so much like the desert Southwest, there’s a tendency to underestimate the harshness of the environment there, and assume you can build desert Southwest-type things (like Biosphere 2!) right on the surface.
The trouble is that Mars is really more of a frozen, radioactive asteroid than a Mormon heartland. A geodesic dome full of Earth-standard air would act like a balloon squeezing with all its might against the Martian surface, trying to propel the dome high into the sky. The whole structure would have to be either bolted to bedrock or weighed down by many meters of rock and ice from above.
You can build such a dome on Mars (you can do anything!), but it’s hard to make a case for it. To build a sixty-meter dome, for example, you would need several hundred tons of carbon fiber struts and plastic panels2, along with another few hundred tons of bottled air to fill it3. This dome would initially leak at every joint, and the amount of work needed to seal it is completely unreasonable, even on Earth. De-leaking Biosphere 2 took months of sedulous effort (a lot of it was done by burning incense and following the smoke), and that was in a design where the pressure differential between inside and outside was tiny. Nor did anyone have to do the work while wearing a space suit.
A basic dome also has to be heated. If you’re one of those people who owns a vacation home that is mostly windows, you will be sympathetic to the heating problems on Mars. In full noon sunlight, a transparent 60-meter dome will hold an internal temperature of 23˚C, but at night or in dust storms it needs about a megawatt of power to keep the contents from freezing.
The worst thing is that you can’t even rely on the transparent dome for light. The solar flux on Mars is about 43% of what it is on Earth, and the dome structure will block another half of that. So you end up with less than 1/4 of Earth sunlight before factoring in dust storms, which can last for months and absorb 90% of incoming sunlight. This means that after all the work you did building a transparent dome, you still need basically the same number of grow lights as you would in an underground facility.
Those grow lights will also need power (although they help heat the dome!) and you can see how this all starts to get a little bit hard to assemble and maintain with robot labor 200 million miles from Earth.
The dome is also not a safe place to live. Besides the risk of sudden depressurization, there is the chronic radiation hazard from galactic cosmic rays. As I’ve written about before, the high-atomic weight component of this radiation poses a health hazard that is not captured by standard radiation models, so a crew living on Mars will have to spend most of its time hiding under several meters of rock to avoid getting all the cancer.
So what you’ve ended up building is an ornamental dome. At that point the sensible thing is to skip it entirely and move all of your farming into the underground tunnels you’re going to have to dig for your crew anyway.
To save on marsmoving equipment, it might make sense to find a lava tube or underground cavern that can be sealed off and converted to a pressurized space without all the bother of having to burrow into the soil. But the point is, for the first few decades at least, ain’t nobody going to be living in a dome.
Hydroponic zinnias destroyed by opportunistic fungus during an ISS experiment.
So now you’re underground, looking out at the sealed tunnels you will spend the rest of your life farming. The next choice you have to make is whether to grow your crops in soil, or try your hand at hydroponics.
The argument for soil is easy. We have ten millennia of experience growing food crops in dirt, we know how it behaves, and what can go wrong. An important property of soil is its high inertia. It takes time and effort to heat it up, dry it out, get it wet, enrich or deplete it in nutrients, or make it do anything else. You can render your soil unsuitable for plants, but it’s hard to do quickly.