# Could Mars' air pressure (assuming terraforming) be high enough to sustain humans without pressure suits but low enough to prohibit flight?

I'm looking for a reason to keep humans living on Mars on the ground, and - apart from storms - the very slowly rising air pressure is the best I can come up with, since planes do need significant air pressure to create lift.

But I also don't want humans have to don full pressure suits every time they go outside, I want a warm overall and a breath mask to be sufficient.

If any of you can think of a reason why rockets wouldn't work either, I'd be delighted!

• Hi Tobi. Braydon's answer is good but if you hold off accepting that answer for a bit, you might get other good ones. Sometimes the green check scares people off. – Willk Sep 2 '17 at 0:45
• Haha, okay, cool. Will do. – Kirby Sep 2 '17 at 4:18
• Electric field(solar wind) + ions(upper layer of atmosphere) = ions(gains kinetic energy) - weight(escape velocity), you can easily tell from this equation that solar wind is the problem! – user6760 Sep 2 '17 at 4:49
• If you took an earth plane and moved it to a mars with an atmospheric pressure equivalent to mt everest (roughly the minimum people could breath), the plane will fly better than on earth since it weighs almost a third of what it weighs on earth. – John Sep 2 '17 at 15:07

Dust

1. Dust is bad for planes.

Mars has beaucoup dust. The movie The Martian starts with a sweet dust storm. It is bad to fly your aircraft through a dust storm. Dust chews up machines. The parallel situation on Earth which was recently in the news is clouds of volcanic ash - comparably tiny, sharp and high flying mineral bits.

Aircraft avoid any airspace that has volcanic ash in it for a simple reason: the ash can wreck the function of propeller or jet aircraft, because it is so fine that it will invade the spaces between rotating machinery and jam it – the silica melts at about 1,100C and fuses on to the turbine blades and nozzle guide vanes (another part of the turbine assembly), which in modern aircraft operate at 1,400C.

That, in turn, can be catastrophic – as the crew of two aircraft, including a British Airways Boeing 747, discovered in 1982 when they flew through an ash cloud from the Galunggung volcano in Indonesia. On both planes, all four engines stopped; they dived from 36,000ft (11km) to 12,000ft before they could restart them and make emergency landings.

That's not the only problem. Ash can pit the windscreens of the pilot's cabin, damage the fuselage and light covers, and even coat the plane so much that it becomes tail-heavy. At runways, ash creates an extra problem because takeoffs and landings will throw it into the air again – where the engines can suck it in and it will create horrific damage to moving parts that suddenly find themselves in contact.

1. Dry dust is sticky and electrically active.

Lunar explorers were not trying to fly planes, but ran into adhesion and static electricity problems from moon dust. from https://www.space.com/3080-lunar-explorers-face-moon-dust-dilemma.html

Halekas recounted a technical debrief by Apollo 17's Gene Cernan after his 1972 Moon voyage.

Cernan said that "one of the most aggravating, restricting facets of lunar surface exploration is the dust and its adherence to everything no matter what kind ... and its restrictive friction-like action to everything it gets on." The astronaut added: "You have to live with it but you're continually fighting the dust problem both outside and inside the spacecraft."

Electrically active

Although the lunar environment is often considered to be essentially static, Halekas and his fellow researchers reported at the workshop that, in fact, it is very electrically active.

The surface of the Moon charges in response to currents incident on its surface, and is exposed to a variety of different charging environments during its orbit around the Earth. Those charging currents span several orders of magnitude, he said.

Dust adhesion is likely increased by the angular barbed shapes of lunar dust, found to quickly and effectively coat all surfaces it comes into contact with. Additionally, that clinging is possibly due to electrostatic charging, Halekas explained.

You could have the static electricity generated by dust inactivate any electronics. Also there would be constant discharges of static electricity within the dust cloud - lightning.

1. Terraforming will make Martian dust storms 100 times worse.

Consider wind. It can exert force because air has mass and velocity. The force exerted by the wind is what lifts dust and blows it along. The force exerted by a mass (m) of air at velocity v is 1/2 mv^2.

The Martian atmosphere is 0.6% as dense as that of Earth at sea level. At 20,000 feet elevation in Earth you could get away with no mask, maybe; at that elevation atmospheric pressure is half of that at sea level so Mars is 1.2% of that. You need to increase the air density or mass of the Martian atmosphere by 2 orders of magnitude to get to where your pioneers can wear a mask outside. The force exerted by the Martian atmosphere on the dust will also increase by 2 orders of magnitude. The wind will be able to lift 100 times as much dust as it does now.

1. Monsters. I will stick to the known stuff here, and leave to your imagination the tenacious airborne filaments of protein-hungry Martian dust fungus.

These are daunting impediments. But not totally insurmountable. In my mind there are comparably great impediments to fishing the North Atlantic in an open boat, and people have done it for millennia. The rare Martian planes that there are would have to be robust in special ways, and their pilots insane in special ways. That makes for fun narrative!

ADDENDUM from comments - /Do you have ideas for how to work around the static thing?/

Sharp points made of conductive materials will bleed off charge into the atmosphere. It is one way lightning rods work. You will see these points on planes. You can see the effect on a child who gets charged by sliding down a plastic slide - her hair will stand on end. In small items prone to accumulating charge, you can put copper foil strips or this copper fuzz. The tiny threads of copper serve as miniature lightning rods to dispel accumulated charge.

I will restrain myself from writing prose but I can envision your Mars pioneers, covered with fuzzy copper threads and strips as they move along. A cloud of dust moves near and suddenly all the threads and strips stand straight out. They curse.

• OH YES! I love this! Thank you so much! Do you have ideas for how to work around the static thing? I'd like to have my protagonists (and antagonists) move around on the surface at least a bit (I was thinking trains and Mechs, tbh). – Kirby Sep 2 '17 at 17:41

Creating a significantly low pressure environment to prohibit flight would harm humans. Planes already fly in a very rarefied section of atmosphere because the thinner air means less wind resistance. Lowering the atmospheric pressure at any level humans can handle would just cause planes to fly lower. Also rockets should work in any air oxygen dense enough to allow humans to breath. Plus having an inability to use rockets would mean any humans who went to Mars would be permanently trapped.

I know you said you don't want storms, but the logical answer is storms. Storms, solar radiation storms or dust storms.

Another solution, if you want to restrict access to only a few remote areas, is to make it too difficult to land on site. Planes cannot land in excessively rocky areas. There is still the possibility of helicopters, but they do handle less well in thin atmosphere. You could possibly combine a lack of runways to stop planes and frequent but more minor dust storms to discourage helicopters.

• Rockets do not need air oxygen to work. Else we wouldn't have been to the Moon and back. – L.Dutch Sep 2 '17 at 3:05
• @L.Dutch Oh, I know. I was just hoping someone might come up with something cool ;) – Kirby Sep 2 '17 at 4:18
• @L.Dutch My bad, was just thinking combustion requires oxygen. Forgot for a minute that rockets carry their own oxygen. (In chemical form.) – Braydon Sep 2 '17 at 4:20
• Storms are frequent because mars has a thin atmosphere, with a thinker atmosphere they will be only as frequent as they are on earth. – John Sep 2 '17 at 14:57
• @John not if something goes wrong with the terraforming *evil laughter* – Kirby Sep 3 '17 at 17:12

As Braydon says, pressure alone can't be the answer, since planes can operate in much thinner air than people can (especially on Mars).

But that assumes that the atmosphere works like Earth's, i.e. that air is simply dumped on Mars until the surface pressure and temperature can support human life everywhere. It's not at all clear that that's physically possible. For people to live on the surface of Mars, they might have to be in domes, or at the bottom of kilometers-deep craters, while most of the planet has an atmosphere barely denser than today's. Either case would make air travel between distant settlements difficult (at least for heavily-laden planes).

Rockets are stupendously inefficient anyway, and barely justifiable even on Earth with all its resources; it's hard to imagine Martian colonists expending that kind of energy and ecological capital just to avoid a 3-week desert trek.

It's already possible for small rockets and ultra-light aircraft to travel near the edge of space, so it'd be hard to rule out air travel on Mars completely, but you could easily imagine it to be well beyond normal use.

• Planes can fly at high altitudes, but takeoffs and landings are more problematic. Basically, the thinner the air, the faster you have to be moving to fly. So even on Earth, a typical commercial jet can fly at 30K ft because it's moving at ~600 kts. but it can't take off with a full load from high-elevation and/or high temperature airports. The key concept here is called "density altitude". – jamesqf Sep 2 '17 at 4:20
• Thanks! I think I'll go with storms, though, Mars is already known for those :D If there's prohibitive storms 4 weeks out of every 10, people will just avoid planes out of habit. The train is right there and you don't have to check the weather report! – Kirby Sep 2 '17 at 4:20
• @jamesqf Yeah, but that's probably just a matter of engineering, I'd think. If you really wanted to you could probably build a large landing strip or special plane? – Kirby Sep 2 '17 at 4:22
• @jamesqf Yes but you can get around this issue. You just use larger wingspan, a launcher like that on an aircraft carrier, or a brief rocket boost. Also the lower martian gravity makes it easier as well. – Braydon Sep 2 '17 at 4:27
• @Braydon: Sure, you most likely can get around the problem with engineering, but can you do so cost-effectively? As an example, consider that we have the engineering skills needed to build & operate supersonic passenger planes (like the Concorde), we just can't do so profitably. – jamesqf Sep 2 '17 at 17:01

Do they need to be humans?

Lets take a very different approach: The "humans" inhabiting Mars are a group that wanted to cut themselves off from the rest of humanity. To this end they did some genetic modification to tolerate life in a far higher pressure than a mod 0 human can tolerate. The Martian terraforming consisted of building up the Martian atmosphere to a tremendous density.

This provides far better impact protection than Martian colonists would currently experience and makes flight impractical (planes work, they just have to go very slowly--high speed transit will be by hyperloops) and rockets impossible (they work, the drag is so bad they can't reach orbit) and thus any trip to the Martian surface is a one way voyage.

• That's an interesting idea (going in the opposite direction), but I'm not sure it would work with air. Leaving aside the atmosphere-retention and greenhouse-effect issues, you'd need VERY high air pressure to make it so that air friction melts steel at 90mph. However, it'd be a good answer if these post-humans modified themselves to live in water... – bobtato Sep 2 '17 at 15:18
• Interesting approach, but it makes the part where the Martians throw off their Earthican oppressors a bit unfasible ;) – Kirby Sep 2 '17 at 16:08
• @bobtato I wasn't trying to melt steel, just make it impractical due to drag. Look at ships--AFIAK the only things that do 90mph in the water are supercavitating torpedoes and they don't do it for long. (Yes, there are ships with that sort of speed but they do it by pretty much getting out of the water.) Atmosphere retention isn't an issue--it would stick around on a human time scale. Avoid CO2 and I don't think you'll have too much warming and some would be good. – Loren Pechtel Sep 3 '17 at 4:22

Robert Zubrin talks a lot about this kind of problem in his book "The case for Mars".

As other people have already noted, airplanes can operate at much higher altitudes than humans can.

In real life, Mars' atmosphere is sufficiently sparse today that it would make a very creditable attempt at creating a hard vacuum in a laboratory here on Earth. While the storms kick up a lot of dust, you would hardly even feel them because the air pressure is so low. the major hazards they pose are visibility and dust contamination.

Between its low gravity and low air pressure, the biggest part of Mars' attraction for development lies in the cheapness of transportation. Long distance travel by suborbital rocket is incredibly cheap compared to air travel on Earth. Travel to Earth's moon is actually cheaper from Mars than from Earth.

If you thicken the atmosphere sufficiently to prevent explosive decompression, you lose the advantages that make Mars desirable.

There are two approaches to building a low pressure environment suit. The first is the "bubble", which is how current generation space suits work. You seal the suit and then pressurize it with air to something tolerable by a human. The second approach is to use the suit fabric to apply mechanical pressure directly to the wearers skin. What you need is the latter, a mechanical counter pressure (MCP) suit.

Robert Heinlein actually had some pretty good ideas about how a quasi "shirt sleeve" environment might work in his 1950's novel "Red Planet". While some of his ideas are definitely based on a faulty knowledge of Mars' true environment (canals, martians), his engineering skills were working at 100%. Reading the first chapter might give you some ideas.

The mass manufacture of mechanical counter pressure (MCP) suits and rebreather packs, so very little air actually has to be carried except under prolonged or heavy exertion, make it possible for something that is manageable for a normal family to enjoy Mars in comfort.

In appearance, the MCP suit looks like the flash-suits in the "Enders Game" movie. Compare the MIT BioSuit .

In practice, people trying working prototypes of MCP suits report that they fit like a diver's dry suit that is about two sizes too small.

TL:DR Answer: I coined a phrase: technology dichotomy. I was a micro publisher for a decade and you'd be surprised at the number of books that assumed (e.g.) time travel without first inventing the wheel. You're in that same spot. People had to fly just to get to Mars. Therefore, your people (a) have the technology and/or skills and/or engineering background for flight and (b) want to fly. Oh, they definitely want to fly. It's difficult to imagine a condition when humanity wouldn't want to fly. So, as you write your story, you need to remember that your characters will be looking for every way possible to circumvent whatever limitation you throw at them. Anything less and the story would be unbelievable.

OK, let's do a little math.

The air pressure at sea level here on Earth is give-or-take 14.7 PSI (pounds-per-square-inch).

Now, the "death zone" on Mt. Everest, which is the altitude above which average people conditioned to breathing sea-level air must have oxygen tanks or they can't make it to the summit and back alive, is around 26,000 feet. At that altitude, the air pressure is give-or-take 5.16 PSI. Let's consider this the bare minimum air pressure you'd need on Mars to survive without a pressure suit.

I'm going to ignore the fact that astronauts and terraformers are more likely to be fit and therefore would likely withstand a smaller air pressure — but it wouldn't be that much smaller.

You need three things to fly:

• Thrust

• Wing area

• Atmosphere (usually "air," a mix of oxygen and other things).

Ignoring the hard math, if you have low atmosphere, you need more thrust and/or wing area. However, adding wing area and/or thrust often means adding weight. The engine is heavy. The wings and infrastructure are heavy. Fuel is really heavy. So there's only so much you can do to overcome low atmosphere.

World record altitude holders for powered flight include 300,000+ feet for the X-15 and SpaceShipOne rocket-powered air craft, 100,000+ feet for the F-104 and similar jet fighter/bombers, 85,000 feet for the SR-71 Blackbird, and 60,000 feet for the highest flying propeller-driven bird. But you're starting with 1/3 the air pressure at "sea level."

Air pressure decreases exponentially. Consequently, dropping the pressure by 2/3 at "sea level" means dropping the maximum altitudes by more than 2/3. This is a honking' rough estimate, but let's cut all those records by 75%:

• Best propeller altitude: 15,000 feet (the highest altitude city in the U.S. is Denver at 5,282ft. We're only talking 3 miles off the ground.).

• Best jet: ~25,000 feet.

• Best rocket plane: ~75,000 feet.

This assumes the same wing area and thrust as used on Earth, and assuming any of those birds can get off the ground in the first place with such low air pressure. (And assuming the rocket planes don't enter low orbit due to the lower gravity....)

Could it be done? Sure. I would expect a booming electrically-driven ultralight aircraft industry. Is it practical? Only if there are some fabulous advances in ultra-light, ultra-strong materials to bring down the weight. But, sadly, yes, there would be flight.

As for rockets... The only reason they couldn't use rockets on Mars is because you need an oxidant (e.g., oxygen) to mix with reactant to get combustion for lift. No oxidant... no boom. Since air is such a rare commodity, it would be tantamount to a sin to liquify it just to throw a rocket into space. Assuming there isn't a minable mineral that would substitute for oxygen as an oxidant, the only way you're getting a rocket off of mars is to import the oxidant.

• you forgot to account for the fact the plane weights 40% of what it would on earth. – John Sep 2 '17 at 15:13
• The X-15 and SpaceShip One weren't flying, at least in the boost and re-entry phases. They were rockets, not significantly different from the Space Shuttle (except for having much less power). Also, it's perfectly possible to get a normally-aspirated prop plane to fly over 15K ft - I've done it it my Cherokee. (OK, it's technically illegal...) Turbocharged & pressurized prop planes can go a good deal higher. – jamesqf Sep 2 '17 at 17:08
• @jamesqf, you didn't get your Cherokee above 15,000 feet on Mars. Read my post more closely. – JBH Sep 2 '17 at 18:15
• @JBH: I think I did. I admit it's been a few years since I was in Denver, but I think if it had suddenly been teleported to Mars, the event would have made the news. – jamesqf Sep 3 '17 at 17:29

No. Because you could already fly aeroplanes in the current Martian atmosphere. We have several design prototypes that can do it, including ones that have been tested on Earth at altitudes comparable to Martian atmospheric pressures. The biggest hurdle is the extremely scary takeoff and landing speed (close to mach 1), but that is merely an engineering problem which can be solved with long enough runway.

Human body needs an Oxygen pressure of about 0.2 atm to survive. That's what is used in space mission. This set the lower limit to the pressure you can set (assuming it is an Oxygen only atmosphere).

Now, can an airplane fly with such a thin atmosphere? On Earth the height record is about 30000 meters (not counting rockets), where the air pressure is about 0.29 atm.

So, you cannot maybe count on commercial flights, but still that thin air can sustain flight.

# Yes, sort of.

Mars, unlike Earth has a tremendous difference in heights, as much as 30,000m. As you begin to increase air pressure, it will be greatest at the lowest elevations. Keep your humans in the lowest of elevations and don't try to have a fully inhabitable Mars.

Planes will fly in the low elevations, of course, but be unable to fly long distances in the near-zero atmosphere upper elevations.