Domes are over-rated

Part of my series on common misconceptions in space journalism.

It is an unwritten rule of space journalism that any article about Moon or Mars bases needs to have a conceptual drawing of habitation domes. Little scintillating blisters of breathable air clustered between pointy antennas.

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Look, I get it. Domes are cool. I’ve built several. And while I don’t regard myself as an expert on Mars urban planning, I believe domes are not a very good solution for building cities on Mars.

I’m going to motivate this post by describing constraints on “the mission”. There is a time and place for discussion of short term exploration missions with a few plucky astronauts, but as far as I’m concerned, the SpaceX Mars vision is the biggest, baddest vision for exploration and industrialization, and the obvious design reference for this blog.

The goal is to build a self-sufficient city on Mars. This will require enormous quantities of money, time, cargo, and people. The goal is to understand ways to reduce the requirements while increasing the probability of success. Given that human capacity to solve problems isn’t infinite, a good mission architecture allows the technical teams to focus on the core self-sufficiency problem rather than working around counterproductive constraints. As an example, one of the main weaknesses of the Lunar Gateway is that so much engineering effort is required to build the space station, which diverts attention and resources away from the more direct challenges of building landers and surface base hardware. Similarly for Mars – the last thing anyone needs is an architecture that requires undue design work and places hard limits on future growth.

For generations, engineers have internalized a hard mass constraint on Mars missions, resulting in scores of proposals that struggle to keep a handful of humans alive for a few years in a spaceship that weighs just dozens of tons. But while it may be *just* possible to do an Apollo-style human Mars landing with a few SLS-loads of stuff, building a city requires either non-existent self-replicating robots or much, much more cargo. Hence the significance of the SpaceX Starship, which plausibly can launch not tens, but millions of tonnes of cargo to Mars.

The Starship provides a mechanism to retire the Mars city mass constraint, just as Starlink can retire the capital constraint. What are some other important constraints?

Consider the raw material constraint. A self-sufficient city can be either open or closed, but total recycling of all materials with either huge reserve stocks or perfect efficiency, such as would be required for a slow interstellar spaceship, is much much more difficult than an open system. In an open system, an effectively infinite supply of natural raw materials can be used, and wasted, as needed. The Mars city solves this problem by building on the surface of a planet that is made of all the raw materials it could need.

The next constraint of concern is the labor constraint. Self sufficiency means different things to different people, but a sufficiently healthy and well-trained individual human can survive indefinitely in many places on Earth. Indeed, small groups of cooperating humans have been capable of survival since the dawn of technology, and even crossed oceans before the wide-scale adoption of metals.

That said, the surface of Mars is next level in terms of its sheer hostility to life. It’s a pitiless frozen vacuum. The Earth’s south pole in the middle of winter is closer to a beach in Hawaii than the nicest place on Mars on the nicest day of the year. So when we think of self-sufficiency on Mars, the traditional “pioneer with 20 acres and a mule” just won’t cut it. Like the bottom of the ocean or the stratosphere, human survival is possible only with advanced technology. Self sufficiency on Mars means the ability to build all the requisite advanced technology, and to do it faster than it wears out.

Instead of a small group of generalists, the Mars city will need teams of specialists to replicate every part of the modern industrial stack, from raw material extraction and processing through to advanced lithography, and everything in between. This process is enormously labor intensive, requiring on the order of 100 million humans on Earth. On Mars, SpaceX hopes to get by with “only” a million people and a lot of manufacturing automation. Even then, the high marginal cost of keeping an extra human alive changes almost everything we take for granted about how jobs are done here on Earth. For more in this vein, check out the relevant chapter in my book in Mars industrialization.

For the purposes of this blog, however, we need merely agree that humans building a self-sustaining city on Mars will be extremely busy. And so we come to the space constraint.

No matter what people are doing on Mars, the mission designers will do everything they can to make their jobs as easy as possible, to maximize productivity. The next most important constraint on productivity is space. On the domestic level, Marie Kondo can help us make the best of our inability to avoid accumulating worthless possessions. On an industrial scale, it turns out that manufacturing difficulty is exacerbated by not having enough space. Factories need room to move things, store things, and lay out assembly lines.

When cars were first built, factories were multilevel buildings in cities with elevators and total vertical integration. Today, most manufacturing plants are laid out over a single level in areas with lower land value, and cover millions of square feet.

So it must be on Mars. We cannot industrialize a new planet in pressurized trailer homes or tuna cans or subterranean tunnels. Nor can we operate efficient factories in space suits. The city will need unimaginably enormous climate controlled spaces to enable millions of people to work efficiently in a shirt sleeves environment.

What about domes?

A prefabricated dome assembled on Mars would certainly have more interior volume than a lander or short tunnel. On the other hand, domes have significant drawbacks that are underappreciated by their advocates, many of whom haven’t actually ever tried to build one!

Domes feature compound curvature, which complicates manufacturing. If assembled from triangular panels, junctions contain multiple intersecting acute angled parts, which makes sealing a nightmare. In fact, even residential dome houses are notoriously difficult to insulate and seal! A rectangular room has 6 faces and 12 edges, which can be framed, sealed, and painted in a day or two. A dome room has a new wall every few feet, all with weird triangular faces and angles, and enormously increased labor overhead.

It turns out that the main advantage of domes – no internal supports – becomes a major liability on Mars. While rigid geodesic domes on Earth are compressive structures, on Mars, a pressurized dome actually supports its own weight and then some. As a result, the structure is under tension and the dome is attempting to tear itself out of the ground. Since lifting force scales with area, while anchoring force scales with circumference, domes on Mars can’t be much wider than about 150 feet, and even then would require extensive foundation engineering.

Once a dome is built and the interior occupied, it can’t be extended. Allocation of space within the dome is zero sum, and much of the volume is occupied by weird wedge-shaped segments that are hard to use. Instead, more domes will be required, but since they don’t tesselate tunnels of some kind would be needed to connect to other structures. Each tunnel has to mate with curved walls, a rigid structure that must accept variable mechanical tolerances, be broad enough to enable large vehicles to pass, yet narrow enough to enable a bulkhead to be sealed in the event of an inevitable seal failure. Since it’s a rigid structure, it has to be structurally capable of enduring pressure cycling across areas with variable radii of curvature without fatigue, creep, or deflection mismatch.

Does this sound like an engineering nightmare? High tolerances, excessive weight, finicky foundations which are a single point of failure, major excavation, poor scaling, limited interior space, limited local production capability. At the end of the day, enormous effort will be expended to build a handful of rather limited structures with fundamental mechanical vulnerabilities, prohibitively high scaling costs, and no path to bigger future versions.

Is there a better alternative? I think so.

What we need is a method for pressurizing vast areas of the Martian surface with relatively little hassle, labor, and raw material. For a long time, I thought the key might be gigantic masonry vaults, but I’m increasingly convinced that tensile structures are inherently better due to much lower mass requirements. The same goes for digging tunnels, which is so labor intensive that almost noone lives underground. In contrast, a thin, flexible tensile membrane supported by its own pressure seems to be a step in the right direction. But how would this work?

UV resistant polymers as as ETFE are routinely used for exterior cladding of structures that need transparent, curved, lightweight, waterproof barriers. Some of these structures are even pressure stabilized. A multilayer ETFE fabric incorporating Kevlar fibers is an ideal material for both performance yacht sails and transparent pressurized structures on Mars. This material can be thermally or chemically welded in the field for ease of integration, modification, and repair.

To transmit pressure load into the ground, a pressurized ETFE membrane must be periodically anchored to the ground by steel cables that can be arbitrarily long, supporting ceilings high enough to permit cloud formation! Unlike a dome foundation, cable anchors can be pile driven from the surface with generic hardware and without trench excavation. Both ETFE and steel can be readily produced from local resources. Importantly, the cables are located in discrete areas and offload all the pressure, so that couplings to the surface at the perimeter don’t have to resist enormous pullout forces and can be done with a set of relatively simple buried steel membranes.

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Conceptual sketch of section of tented area.

Within the pressurized structure, membrane tunnels, shelters, and bulkheads can be readily deployed to ensure any desired level of redundancy or compartmentalization. Tall structures can be supported from the ceiling in tension, reducing material requirements.

Despite its light weight and versatility, inflatable tensile structures have a long history and is not as exotic as it may sound. It is used for both Zodiac dinghies and inflatable mattresses! It has even been used to make a fully functional inflatable plane!

Screenshot from 2019-11-27 22:54:26.png

Such an approach has many advantages over rigid dome construction, but the most important one is that it crushes the space constraint. What is the per capita area requirement on Mars? In a future post I’ll estimate this more rigorously but I believe it’s on the order of 10,000 sqft. If a Mars base is doubling its population every launch window, then the 5000->10000 person increment requires the addition of about a thousand acres of enclosed area, in just two years. This is about 3000 standard prefabricated domes (roughly one per person!), or less than two square miles of additional membrane, with distributed concurrent anchor construction. Only one of these methods scales easily!

Within the pressurized volume, people can build houses, schools, factories, farms, forests, or anything else they want. The space is versatile and flexible. For more in this vein, see my earlier post on Mars urban planning.

What are the potential drawbacks of such an approach? Thanks to great comments on Hacker News and Twitter, I have a good idea of what people are worried about.

Anchoring. Some have suggested anchoring to a second membrane on the ground, to avoid anchors and to isolate from Mars dirt (that may have toxic chemicals) and to avoid perfusion of gas through the ground. While enclosing membranes are great for bridges, corridors, and air handling, I think building directly on the dirt is the best approach. Provided the perimeter walls are deep enough, the risk of substantial leaks are low. All pressure structures leak, so the key is to ensure that there’s a ready supply of new gasses to make up the difference. As for perchlorates, they decompose in water at room temperatures. In other words, spraying a newly pressurized area with warm water should be adequate to neutralize the extremely thin layer of dust. Finally, farming on directly covered land won’t require moving millions of tonnes of dirt through an airlock.

Radiation. As explained in this blog post, I believe that unshielded radiation exposure on Mars is not one of the major problems to deal with. While it’s possible to build some kind of laminated inflatable structure with pockets of transparent water, in practice living and sleeping spaces will have modest shielding, and exposure in the “outdoors” will be part of life, just as excessive sun exposure on Earth can cause increased risk of cancer.

Underground living. Quite a number of readers pointed out that natural caverns, canyons, and Boring Company-dug tunnels are options. While I agree these have their uses, they also have significant drawbacks. Underground spaces are incredibly labor and energy intensive to build – to the point that basically no private underground structures of any size are routinely constructed. Even natural caverns are location specific, difficult to expand, difficult to ensure structural stability, and lack natural light. I think TBMs will be used extensively to operate pressurized underground mines for minerals not abundant on the surface. But in general the easiest level to build is at ground level.

How does redundancy work? What happens if there’s a hole? Fiber-reinforced ETFE has really good ripstop properties, so a hole won’t suddenly spread over the whole area, leading to instant catastrophic failure. Still, the air will leak out stopped only by choked flow. If the leak rate is lower than the maximum replacement rate, no problems. The hole is found and patched. If the leak rate exceeds the maximum replacement rate, then the pressure will start to drop – which also reduces the leak rate. If the pressure drops below the level needed to tension the anchoring cables, then the whole ceiling will gradually deflate like a bouncy house at the end of a party. As the pressure drops, occupants will need to evacuate to “air shelters”, which would take the form of sealable corridors within the main structure. The membrane will gradually drape itself over whatever structures/former trees/non-spikey supports remain.

Additionally, the pressurized volume must be segmented by vertical bulkheads into separate compartments, so even total collapse (or toxic chemical leak) in one section doesn’t affect other sections. A collapsed membrane could still be repaired by robots or workers in space suits, and once the hole is patched, repressurized.

For particularly important areas, multiple membranes or layered membranes can be deployed to reduce the odds of a leak all the way through.

How much steel is required? Pressure at sea level on Earth is around 100 kPa, or 100 kN/m^2.  On Mars, we can make do with lower pressure and enriched oxygen, reducing structural pressure loads, but the gravity is also lower so the total amount of rock needed to anchor is about the same. A 40 kPa atmosphere on Mars needs about 3-4 m of rock to completely react out the pressure load, though to be on the safe side the anchors would be driven somewhat deeper than this. According to The Engineering Toolbox, a 20 mm steel cable is adequate to lift 40 kN, implying an average steel fill fraction of 0.05% to anchor the roof, regardless of cable configuration. Span between anchors is determined by ground conditions and overall desired membrane tension, but I think 50 m is about the sweet spot. With a mass of 1.5 kg/m, the membrane could be flown as high as 6 km before the mass of the cable (to say nothing of its cost) was enough to hold down the membrane without an anchor.

Conclusion

So when one thinks of a city on Mars, don’t think of some quaint potato patch in a dome that’s too small for a game of tennis. Think instead of a gently puckered transparent plastic sky that stretches over the horizon in all directions, supported by a sparse forest of steel cables with endless open space between. A new savanna.

47 thoughts on “Domes are over-rated

  1. Would more familiar, hermetically-sealed skyscraper-like structures be any good? Seems like then you could bulkhead off lots of sections to protect against leaks and such. Maybe excavate some big underground cavities? I love the amount of thought put into thinking about these Mars colonies outside of the usual imagery, thanks for writing!
    Of course, just in case you hadn’t seen it yet—gotta plug my favorite web resource for hard science fiction/real designs for serious manned space travel:
    http://www.projectrho.com/public_html/rocket/
    Has some sections on planetary bases and other kinds of habitats. Super interesting! Don’t start reading it if you have something important to do later! It’s a serious wormhole.

    Liked by 2 people

    1. Towers, bridges, and other structures are also possible with tensile membranes, or hybrid structures. But I don’t see any reason for highrise when land is infinitely available.

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  2. I’ve been thinking about this. I can’t imagine people wanting to live somewhere that doesn’t even have an open area big enough for a soccer field. Knowing how a baby will develop in such a low g environment is a completely virgin area. Would muscular dystrophy provide some pieces to the model? The “fake” exercise on the ISS seems doesn’t appear to be a livable option for a whole city

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    1. You can use a combination of this and circular tunnels. The author The article dismisses the boring machine, prehaps too quickly. One solution need not be used for all things. Circular tunnels (don’t recall the author of this idea) allow moving modules to be installed underground. these can spin on maglev (this will cost some power but will simulate any gravity that is required (let us say 1.05g so that exercise is not really required to mitigate bone and muscle atriphication. If deep enough, these tunnels will be effectively impervious to radiation and small meteors

      Assuming work and sleep time is spent in these areas, one can then use the low G surface habitat for vistas and to alleviate any claustrophobic tenancies.

      Note that we Already spend most of our days in enclosed office buildings, so there would be minimal change from normal earth life for one’s working day (assuming one is an office worker). You can use OLED panels to simulate windows to “open up” spaces.

      One can move between rings by a module that is ejected, moves down a shaft to the next ring and inserts into that spinning ring – much like an elevator ride between floors.

      Note that the boring machine creates tunnel liners made from the excavated rock as it goes. (The rock is processed into tunnel segments elsewhere). This process can be automated to a large degree to cut down on costs.

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  3. This is great, but what about evacuation if there’s a leak? If you have large open spaces, then getting to a pressurized sector from a leaking one might take a lot of time.

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  4. A great blog you run here Casey! Thanks.

    When I saw your tensioned grid membrane above I thought the tension cells could also be pressurized mesocosms (often used in marine and lake research), maybe growing high-nutrition crops (also handy refuges).

    Following on from Linus’s question it may become possible to achieve a level of seal on more porous bases using mycorrhiza – there are quite a few advances in this field at the moment with people considering sealing broken concrete, capping landfills and creating subsurface membranes without excavation, all with natural organic textiles that weave themselves.

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    1. I think moisture will migrate downwards and outwards along with residual leaks until it hits the frost line. Provided the outside remains super cold leaks will self heal with permafrost.

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  5. Agree about domes
    But Mars or Luna are terrible places to go
    There is no point in going back down a hole when everything you need is out there
    https://en.wikipedia.org/wiki/3753_Cruithne
    Cruithne would be a much much better place to put a colony – 100 Billion tons of material
    24/7 sun power
    And no hole to climb down
    Use tethers and spin for gravity

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    1. I agree… much rather occupy the orbitals… but Casey makes a good point:

      | recycling of all materials with either huge reserve stocks or perfect efficiency

      That’s hard. Each person needs about 2.8 kg/day of oxygen, so an open system with a perfect way of pulling the CO2 out (which we don’t have) needs to produce that much a day. 80 tons a lifetime. A million people will breath away all the volatiles of that rock pretty quickly.

      This is not a surprise, you’ve gotta recycle the oxygen, and your food crops will help you do that. Open systems become closed systems over time – or as closed as we care to make them.

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    2. Dreams pay the bills, and the first thing a space colony can sell on Earth is tickets to go live there.

      I think living at the bottom of a hole is a bad idea unless you happen to have started there. But I’m not the market for tickets. Most people think of planets when they imagine people going to space, and education is fundamentally more difficult than engineering.

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    3. Cruithne is not a good target because of delta-v and travel time problems. It lacks adequate gravity for an Oberth effect assist, and there are no Hohmann-like transfer orbits to visit it due to the 1:1 orbit.

      In contrast, Deimos or Phobos require relatively little delta-v to/from Earth C3, with reasonably frequent transfer windows (every two years). The benefit of being close to Mars is that you get a good Oberth effect, as well as some help from aerobraking.

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  6. I think I’ll do a before and after on this. Here’s what I think before reading the post.

    I’ve never taken domes seriously. They seem like just something that people draw to make the picture look nice. But now that I think about it, I like them. I think the Martian city should have some. Probably three.

    There’s only one thing that a new Mars colony can sell on Earth: tickets to Mars.

    Setting up a Mars colony is hard. It will be insanely expensive. So the founders will have to do almost everything they can to maximize the revenue from those tickets. That includes monumental architecture. More people will want to go to a shiny city in a sparkling diamond done, than would pay a gazillion bucks to go spend the rest of their lives in something that looks like a utilitarian cluster of industrial facilities.

    And a dome seems as though it would be a good way of doing monumental architecture on Mars.

    The Martian atmosphere, as far as I know, contains only one thing that’s deadly poison: carbon monoxide. If a building has a catastrophic failure of its pressure seals, I think people could survive with just emergency oxygen masks if it were just CO2 and the non-CO trace gases. But carbon monoxide binds hemoglobin with such high affinity that the amount you would be getting around a mask would be toxic rather quickly. So a secondary pressurized vessel with carbon-monoxide-scrubbed Martian air would provide useful protection.

    Another use for a dome is as a visible example of something built of Martian materials. Ordinary rock contains decent amounts of silica and aluminum, so glass and an aluminum framework should be fairly easy to make.

    Three seems like a good number, so that the Great Dome can have something to be bigger than, to help it fulfill its function of looking good in the pictures.

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  7. “The Earth’s south pole in the middle of winter is closer to a beach in Hawaii than the nicest place on Mars on the nicest day of the year.”

    According to NASA’s website, highs can reach 20 degrees Celsius. Basic point still stands, though. Buildings have to be pressurized and insulated.

    “Nor can we operate efficient factories in space suits. ”

    Robots are vastly cheaper than humans to support on Mars. The cheapest thing to send across interplanetary space, in fact the *only* cheap thing to send across interplanetary space, is information. Work on Mars should be done by remotely operated semi-autonomous machines, 99% of them operated by people on Earth. Whole systems will be designed to function with a 24-minute lag. Because expensive as it is to redesign an entire production system, having people on Mars is even more expensive. When a machine encounters a situation that doesn’t fit any of the scenarios it’s prepared for, it will seek real-time operation from one of the relatively few people there.

    Martian autarky is a dream. That’s fine. On Mars, dreams are the only thing that pays the bills, and the push for autarky pays more than its share. But if it’s going to coexist with reality, it will have to give a little.

    Re your alternative —

    When you’re talking about forests and similarly profligate uses of pressurized space, I don’t want to do it with steel sent from Earth. Compressive structures can be built of locally quarried rock. It’s a lot of rock to move, but the cost of sending stuff to Mars is always going to be high unless we have a propellant source that’s not at the bottom of a 5+ km/s gravity well. A better option than compressive structures would be to produce tensile material locally. My favorite is rock glass fiber. Melt some rock, extrude it under the right conditions, and you have a decent approximation to glass fiber. If it has the right chemistry, it’s stronger.

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      1. Oops. Saw that after I posted the comment. I should have known you wouldn’t have suggested using steel from Earth beyond the point where it makes sense.

        I’m still fond of basalt fiber. Good tensile strength, and it seems as though you could make it with less mass sent from Earth per newton of load.

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  8. Yes. You can buy a bag of it on Amazon as chopped fiber for reinforced concrete. It’s also sold as a roll of fabric, or as tape for wrapping motorcycle exhaust pipes. For bobbins of fiber, you apparently have to contact the manufacturer. It’s a low-volume product, because its main advantage (from our point of view) isn’t particularly relevant on Earth.

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  9. This concept doesn’t take into consideration radiation shielding. You’d want to have perhaps 5 tons per square meter of regolith on top, to emulate the long term radiation shielding Earth’s atmosphere provides. This would be relatively inexpensive regolith and/or dry ice, of course, but it’s still going to make it desirable to go higher rather than just flat and across.

    So here’s the thing. This level of radiation shielding still only weighs a fraction of the air pressure pushing up on the membrane. One obvious solution to this problem is to layer on more weight, so you don’t need any steel cables.

    But let’s suppose you have steel cables. To me, it seems more desirable to hang tall buildings using them than to drive them into the ground. Unlike tall buildings here on Earth, there is no wind to worry about. Hanging from above also eliminates a lot of engineering issues with compression structures. Extremely little of the building’s volume is consumed by the support tethers.

    I think that it makes sense to have sides that slope upward – the slope is gentle enough to easily layer regolith and/or dry ice on top, but it still gets higher so you get multiple stories for use. So, the habitat looks like a buried worm, expanding with new segments at both ends.

    The point is – multiple floors gives you a lot more bang for your buck.

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      1. Do you mean that the radiation shielding layer of regolith/dry ice spoils the view?

        Well … I suppose an alternative could be liquid water. Liquid water is clear enough that you could still have a decent view of the sky through 5m of it. If you site your habitat near a pole, you could have access to large amounts of water ice.

        Using liquid water, your pressure membrane would actually be the outer membrane. You wouldn’t need sloping sides to make it easier to cover with regolith/dry ice. Instead, the sides could be vertical.

        So, in this case the colony is a half pipe shape, growing by adding segments to the ends. As before, the weight of the water is still only a fraction of air pressure. Thus, you still have plenty of weight bearing potential for hanging stuff.

        But the vertical sides let you directly support floors via the sides, rather than using hanging cables.

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  10. Domes are indeed over rated.
    However, I do have a few comments regarding the tenting design, perhaps a few tweeks. One, I would imagine that the anchoring cables would be placed in a close packed hexagonal pattern, not the orthogonal arrangement shown in the conceptual illustration. Further, the cables might be brought together mid length, in a “Y” fan to a single cable to the ground to reduce obstructions at ground level.
    A couple commenters mentioned the atmosphere underneath the tent, one even suggesting using the unmodified, but pressurized native atmosphere, but mentioned the danger of CO. While CO is indeed poisonous, I would be FAR more concerned about leakage of CO2 into the mask, as it too is poisonous and is the dominant gas of the native Martian atmosphere. If this gas perfuses into the breathing mask, it will give rise to hypercapnia… a dangerous condition that is unpleasant and gives rise to feelings of suffication and panic, and eventually death.
    While the unmodified atmosphere may be pressured during construction and testing of a newly tented area, it would be best to fractionate the atmosphere, removing the vast bulk of the CO2, leaving the N2 & Ar, supplimenting the O2 with electrolytically cracked H20 as the source.
    Dr. Handmer suggested using a lower than standard atm. pressure but with a larger percentage of O2 to increase its partial pressure. I would be very cautious with such. The ratio of O2 to non-reactive gasses is critical to fire safety. Non-reactive gasses cool flames. Too little of the non-reactive gas can cause hot, fast moving flames. This is especially concerning when using ETFE, which when it combusts, releases HF gas which is corrosive, especially to human lungs. Also, in areas with parkland / forests under the tents, a high percentage of O2 presents a very high danger of fast spreading fires. I’ve heard it said that the danger point is found with any O2 percentage approaching 30%.

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    1. Oxygen is probably going to be a lot more abundant than water, so hydrolyzing water doesn’t make sense as a source of O2. Ordinary rock has formulas like Mg2SiO4, with more atoms of oxygen than everything else combined. We’ll be making structural materials, such as plastic and steel. Plastic means getting the C from CO2, and steel means getting the Fe from stuff like MgFeSiO4. In each case, oxygen is a byproduct. Of course, plastic also means getting the H from H2O, so we’ll be getting some O2 from water, but as a byproduct.

      My understanding of CO2 vs CO is that CO2 makes you feel an urgent need to breathe, so you would adjust your mask if it leaks some CO2. But CO just accumulates in your blood until it makes you get sleepy and die, and CO is deadly at a concentration orders of magnitude lower than what it takes with CO2.

      For fire safety, what about having a moderately lower partial pressure of oxygen, comparable to what people deal with when they acclimate to living at high altitude?

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      1. I think the water scarcity problem will need to be solved, and it’s much easier to get oxygen from water than slag. But I’m not precious about any particular approach.

        Re ppO2, Codyslab has a great video where he shows that combustion scales like ppO2 and nothing else. It surprised me but 0.2 bar pure O2 burned like regular air. Some reduction of oxygen availability is possible but we want to keep the occupants at peak performance.

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      2. As I imagine it, O2 wouldn’t come from slag, except insofar as we need silicon. Rock can be thought of as a combination of oxides: instead of writing formulas MgFeSiO4 (which appropriately describe how the atoms are grouped in the crystal structures), we can write it as MgO+FeO+SiO2. If we want that Fe atom, we need to remove both it and the O that goes with it.

        On Earth, we start with ore that’s mostly FeO+Fe2O3, and we get slag that’s got lots of SiO2 but not much MgO or Al2O3. Most of the separation has been done for us by millions of years of
        geology: weathering, erosion, precipitation of dissolved material from seawater, partial melting of rock followed by slow gravitational separation, and so on. On Mars, we’ll have to do all of that ourselves, starting with the kinds of rock that exist there.

        I don’t know the best way to extract useful materials (Al, Fe, SiO2 pure enough for window panes, etc.) from ordinary basalt, because it’s not something that makes sense to do on Earth. We may have to cook crushed rock to separate the more soluble components (alkali metals, alkali earth metals, halides), before smelting the results to separate the transition metals from the silica. My guess is that we will want to do multiple stages of cooking at different temperature and pH. Then we’ll heat it with carbon to pull off enough oxygen that we get metal and slag phases, then we’ll blast it with excess oxygen to pull out the carbon. Then we’ll burn the mostly-iron to get mostly-iron oxides, then we’ll separate the oxides again by solubility. Then we’ll smelt them again to get iron pure enough to make decent steel. Then we’ll combine it with selected trace elements that we had to pull out of the solutions will ion-exchange resins, because Mars doesn’t have ores of those elements either. If I’m right, it’s a whole lot of work, which explains my fondness for just melting the whole rock and extruding it to make basalt fiber as our main tensile material. And it explains why I thought you might be talking about bringing substantial amounts of steel from Earth.

        Anyway, the point is that any time you make metals or carbon, you also make O2.

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    2. You are correct WRT the dangers of CO2, but nevertheless I think filling the tent with pressurized native atmosphere would be the obvious initial choice from a simplicity and energy requirement perspective. It is true that inhabitants would then likely still have to wear full-body containment suits. This doesn’t quite bring you to a “shirtsleeve” environment, but it does solve the issue of having to wear balky, expensive and highly restrictive pressure suits.

      Overtime the atmospheric mix could be shifted to one that would be safe for breathing masks, and eventually complete habitability.

      A CO2 rich atmosphere may initially be preferable for farming as well, and the O2 produced would over time facilitate respiration. The limiting factor for complete habitability under the tent would thus be sourcing enough inert gas (N2 + noble gases) to complete an atmospheric mixture that is breathable and safe – i.e. high enough partial pressure O2, low enough partial pressure CO and CO2, and high enough total pressure.

      The area under the tent would likely also be further compartmentalized into areas of full and partial habitability. Settlements and industrial zones being constructed in fully habitable zones, with high partial pressures of inert gases, and agriculture being conducted in areas of low (or nonexistent) inert gas partial pressure. This would greatly reduce the requirements for scarce inert gases, while also likely maximizing crop yields.

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  11. Mars has never had the full set of plate-tectonic processes that were necessary for the creation of high-quality ores on Earth. So a Martian city will need to process much larger amounts of rock for some of the materials it will need. It may make sense to use the waste as weight and as radiation shielding for core living spaces where people spend most of their time, while covering most of the city with tensile structures.

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      1. As far as I’ve been able to find, the crust everywhere on Mars is almost all basalt/gabbro. There are a few patches of andesitic rock but nothing like the distinction between oceanic and continental crust that we have here: no ocean trenches, no subduction of entire slabs of crust, no subduction-zone arc volcanism, no way for water to move from the surface to the mantle and create chalcophile-concentrating ore.

        There are deltas, with evaporite deposits. There are shallow hydrothermal features. But I don’t see how those can produce anything like the range of ores that we have on Earth. We may be able to mine gypsum and halite, so that we can have a concentrated source of sodium, potassium, calcium, sulfur, and chlorine. Or we may not.

        The most important thing that’s missing for mining may be ocean-going shipping. Even on Earth, where we have a huge portfolio of mineral resources, we don’t have most combinations of them close enough together to be practical without putting them on a ship.

        I think we’re going to have to site the city according to either orbital parameters or water availability, and get everything else from air, from quarrying and processing ordinary rock, and from picking up a tiny quantity of meteorites — not from mining ore.

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      2. You may be right. I am still hopeful we can find a site with water, sun, or a heinous vein of iron ore or uranium or something.

        On Mars there are probably plutonic intrusions, sedimentary sorting of minerals, breccia pipes, and skarns. Also all the good stuff won’t have been pilferred by Bronze age hordes.

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  12. Maybe I am out of line but if we are thinking out of the box, how about using the asteroid belt for some your needs, metal, water. It may not be easy to obtain but neither is building on Mars.

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      1. If you’re talking about a Mars city doing it from the ground on Mars, to the ground on Mars, then I would expect the same result: ground to orbit and orbit to ground are a big chunk of delta-v, even if not as big as on Earth. For bulk materials like steel or water, that you’re going to use on the ground, I don’t think it would be worth sending an empty Starship from ground to orbit to bring them down, even if the delta-v from Mars escape to the asteroid is zero.

        But how about a not-at-all-assembled space castle plus lots of propellant, for use in Mars capture orbit to receive stuff from Earth, with the stuff to get there and extract the propellant coming not from the ground on Mars, but from Earth as a substitute for some of the supplies that the Martian city would be receiving from Earth, and the propellant that would be bringing them? The delta-v is effectively negative, because you’re getting your propellant closer to the middle of your route than if you had gotten it from either Earth or Mars.

        Ordinary rock is full of oxygen (aka propellant) in the asteroids, same as it is on Mars. I’m assuming that you can pry some O2 loose from the rock entirely with remotely-operated equipment that works across a substantial light-speed lag, rather than needing people close enough to operate it in real time.

        I would also expect different profiles of elements at high concentration in asteroids and Mars dirt. So if it turns out that you have to move an extra million tons of rock to get each ounce of gallium on Mars (or whatever element is hardest to get locally), but it’s almost a byproduct of propellant production from a suitable asteroid, then my guess is that it would be well worth bringing down from orbit in the quantities you use a most-expensive element in. And I expect that it would be cheaper to do it from a near-Mars asteroid than to send the material from Earth and the propellant from the surface of Mars, where the initial cost of the material is negligible but transport costs are sky-high.

        The main downside I see is complexity. More things that can go wrong mean more cost in planning, engineering, and ground support.

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      2. Aerobraking means only a few percent of the fuel load is required for the landing, as opposed to TMI.

        Rocks have lots of oxygen in them but it’s as hard to get out as, eg, the thermite reaction is hot. That energy has to come from somewhere, and Mars orbit is pretty low on infrastructure.

        Remember that large swaths of Mars’ surface are covered with pre-disassembled asteroids already!

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      3. Yeah, it would be better to simply use raw rocks/regolith for propellant directly, rather than extract oxygen. Since asteroids are lacking in gravity well, the most suitable propulsion is electric thrusters anyway. Blasting a rock with a pulsed electron beam for thrust would be good.

        But … what even is the point? We have no expectation that the poor rocks available in asteroids would be any better than the poor rocks available on Mars.

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      4. We do have reason to believe that rocky asteroids will be a better source of some elements than the surface of Mars. As a proto-planet differentiates, immiscible fluids percolate through the solid material with high melting temperature, and through each other. Some trace elements fit well in the crystal structures of the refractory minerals, so they remain in place. Some are more soluble in molten iron (or iron sulfide, I think, for proto-planets that formed farther from the sun). Those elements tend to wind up in the core. Some are more soluble in water/CO2 (they’re miscible). Those wind up in ore, when the fluid reaches a depth where the the temperature and pressure are low enough.

        Billions of years later, the stuff that headed to the core is inaccessible — unless the proto-planet got smashed to bits and is still floating around in many little pieces.

        The rock on the surface of Mars is basically all just basalt, or stuff extracted from basalt by a limited set of processes and left as evaporite or hydrothermal deposits. Some elements are not going to be easy to get from Mars rock.

        I don’t know how much asteroid material is available on the surface of Mars.

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      5. The place that really makes sense as a source of scarce-on-Mars elements is Earth. Thermal tile is made of silica, but it could perfectly well be made of zircon or chromium oxide or something, whatever element is hard to purify from Martian basalt and useful for the Martians. Steel in the Starships can be reformulated to contain more of the scarce-on-Mars stuff than necessary. Even the small stuff may be worth reformulating, to increase it’s value as scrap on Mars.

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  13. You’re concerned about building a habitable structure cheaply, but in terms of resources and labor, I would expect that the stuff that goes inside of the structure would be more expensive to produce. Maybe the structure itself is not an important part of the cost equation. For agriculture, maybe your structure makes sense, though.

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