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Premise

I’m trying to design a planet with atmospheric conditions nearly identical to Earth, but with roughly the same size and mass of Titan. Titan lacks the conditions to maintain an atmosphere on its own like Earth, it depends on the magnetosphere of Saturn. The conditions on Titan aren’t really compatible with human life or the story I’m writing, because it’s tidally locked and far too cold. So, using the magnetosphere of a gas giant isn’t an ideal situation for the planet I wish to create. Instead, I want to explore the idea of an artificially created atmosphere.

Humans regularly produce artificial atmospheres in sealed chambers. I think a sufficiently advanced space faring civilization could conceivably put a bubble around a planet and pump it full of gasses.

Actual Question

Could a gigantic gas-filled bubble substitute the gravity and magnetosphere required to maintain an atmosphere?

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    $\begingroup$ Broadly no, but why is that a problem? Bubbles can't substitute gravity or magneto-anything and what would that change? Humans produce artificial atmospheres in relatively tiny sealed chambers and a "sufficiently" advanced civilization could by definition do anything. If you want a planet with atmospheric conditions like Earth, but roughly the size or mass of Titan, write that. Most of your audience/ readers/ viewers won't notice what you don't draw their attention to. The same size and mass of Titan should be a problem and still, who will notice what you don't signpost? $\endgroup$ Nov 16 '20 at 23:59
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    $\begingroup$ If you are replenishing the atmosphere at roughly the same rate it is being lost, it will find an equilibrium. Remember, Titan's atmospheric pressure is 50% higher than Earth's at the surface. $\endgroup$ Nov 17 '20 at 0:18
  • $\begingroup$ Is this not a duplicate? worldbuilding.stackexchange.com/questions/106234/… $\endgroup$
    – rek
    Nov 19 '20 at 4:32
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A pressurised bubble held by a strong global shell should be possible

In this model there's a semi-flexible shell (of a strong, light, airtight material - eg graphene), that's been inflated with oxygen.

Its mounted using 2 attachment points at the poles, 100m-1km tall, back of the envelope calculations show graphene is easily strong enough to hold a 0.5atm atmosphere pushing up, or titans 1-2m/s/s gravity pulling it down. At 130gpa (~200 times stronger than steel) it should be able to stand up to most low-medium speed impacts, and the odd high speed meteorite would be decelerated by the shell considerably, but still require a patch.

enter image description here

The shell, cylinders, and atmosphere are synchronised to the planets rotation, so no strong winds or rotating parts. The cylinders could be hollow in order to implement an airlock allowing ships to land "air tightly".

(Kevlar, Carbon Fibre, Carbon nanotubes, and a few other materials should also be strong enough to implement this structure, but I'm not sure these can be made sufficiently airtight easily).

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    $\begingroup$ This raises the question of what the inhabitants of the planet would SEE on their world.In other words, how much light would pass through this membrane and what would the effects be on life on this world? $\endgroup$
    – Henry
    Nov 16 '20 at 21:30
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    $\begingroup$ @Henry There'll be a slight lensing effect but will otherwise be minimal occlusion of light. Eg photo at technologyreview.com/2008/04/14/221065/how-to-make-graphene/amp $\endgroup$
    – Ash
    Nov 16 '20 at 22:21
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    $\begingroup$ "Its mounted using 2 attachment points at the poles" - no need, the shell will be stable without that, it does not need support besides provided by the atmosphere itself. $\endgroup$
    – MolbOrg
    Nov 17 '20 at 3:09
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    $\begingroup$ @MolbOrg: But I would guess (anyone care to do the math?) that it would not actually be stable, and would eventually get off-centered from the planet, and crash into it. See for instance Ringworld instability: physics.stackexchange.com/questions/41254/… $\endgroup$
    – jamesqf
    Nov 19 '20 at 4:00
  • $\begingroup$ @jamesqf stability in the case isn't just gravity interaction, but also pressure on underlying media(aka atmosphere) - it's stable for similar reasons as the atmosphere itself - pressure "repulsion" forces grow faster (to displacements towards center point direction) than gravity, so as it drops faster than gravity in the opposite direction. what one has to care about is maintaining shell integrity(bursts), and it is doable if proper tech involved in the creation of the shell. there is potential for oscillation to negate, but that is relatively easy compared to the rest. $\endgroup$
    – MolbOrg
    Nov 22 '20 at 15:26
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It's certainly possible

Domes

It depends on what exactly you are tying to achieve. Doming over the planet, either with small bubble domes or a global bubble is certainly possible. The small domes are best if the settlement is gradual, the global dome needs to be supported. A global dome might be held up by air pressure alone, given you use some aerogel like substance, a robotic repair ecosystem and an orbita point defense network. Landing spacecraft could either go through airlocks between dynamically suspended Atlas-towers or just punch through the dome, if it has advanced self repair capabilities.

Dynamic support structures would definetly come in handy for keeping the dome up. Check out orbital rings and atlas towers. Nanotechnology-heavy civilisations might favor the aerogel dome, and ones with advanced biotec could plant a global forest of vacuum hardened Dyson trees, producing habitable bubbles, which connect to each other on the ground.

Other Ideas

You don't nessesarily need a dome. If the moon was give an earthlike atmosphere, it would retain it for millions of years. What we call rock is mostly made of oxygen, Earth is an "oxygen-metal-planet". If your civilisation has a abundant energy, it could just thermally break down rock to counterbalance atmospheric losses. One could just use the facilities one used to build the atmosphere in the first place. Of cause, this means that a pure, 0.25 atm oxygen atmosphere would be best for humans here. This isn't dramatic, there will be a slight increase in firehazards and one would need be vigilant about supplying nitrogen to plants (probably via irrigation in richly greened areas, in other areas plants adapted for nitrogen conservation might be a good idea).

Another option would be to ditch the surface altogether and either dig out or settle natural caverns. Especially on icy moons this mightv be the preferred option, as a few meters or kilometers of ice or rock beat an atmosphere for protection any time. Geothermal energy or deuterium fusion could very well be better energy sources.

A rather radical option would be to ditch the idea of an atmosphere altogether and adapt the ecosphere for life in vacuum. Using genetic engineering and cybernetic augmentation might result in a fascinating biosphere, where solar forests grow capacitor fruits, which symbiotic robots and cyborgs can drink from. Cybernetic animals might dig up resources and take care of the trees.

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I think it will be simplier to build a lot of really big (about 1 km heigh) linked together geodomes (see picture) on surface of your planet, and fill them with breathable atmosphere and place plants, bees and animals in them to make ecosystem self sustaining and suitable for humans to live in it. I think its quite simple approach for space faring civilisation, since it can use materials found on surface of planet to achieve it. Artificial lights in domes can emulate Earth day and nights and even seasons, alongside allowing plants to perform photosyntheses.

Unfortunately, this approach only allows us to emulate atmosphere of Earth, not gravity. But, after few generations all lifeforms will adapt to lower gravity.

geodomes

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Variations:

  1. Artificial magnetic field. Some powerline-like setup around the equator, some power stations (solar is probably OK), some maintenance effort and you are set. The atmosphere will have to be ticker than Earth's, just like Titan one is, in order to get acceptable pressure.

  2. Thin shell (supported by its own tensile strength). Discussed in other answers.

  3. Thick shell. Like the thin one, but supported (mostly) by the weight of a material and structures piled over, instead of its own tensile strength.

Advantages: a great deal of protection against small meteorites, cheaper materials, lower technology, lower heat loses.

Disadvantages: dark inside. May be possible to use "light wells" like in medieval buildings with the added possibility of using heliostat mirrors to feed light into them.

  1. Thick transparent shell. As above, but glass or glacier-like material above. Like above, but allows for Earth-like agriculture. Glacier-like material may be self-healing to some extent.

  2. Domes - discussed in other answers. Works like either the thin or the thick shell, but over small area.

  3. A patchwork of 2, 3, 4 and 5. Diverse (allowing for different activities) and fault-tolerant. They can be even combined with 1 for much less dense, but still somewhat survivable atmosphere above.

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  • $\begingroup$ it's Saturn, there isn't much natural light, to begin with - 1.2-1.4% of that on earth (1/80th'ish) $\endgroup$
    – MolbOrg
    Nov 17 '20 at 3:21
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I don't think it is possible. I see two issues with your idea:

  1. How to sustain a planet wide structure without having it collapse under its own weight. You are talking about a shell which has to wrap the entire planet to hold its atmosphere. It won't weight peanuts, and you want to make it robust enough to withstand impacts with space debris, which will slowly make it look like a fishnet, piercing holes through which the gas will leak out.
  2. Those working with vacuum chambers know that, after a certain vacuum, materials start to become leaky to gases. So, in your cases having an atmosphere on one side and the deep vacuum of space on the other will put you in the same situation.
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    $\begingroup$ Point of comparison: The mass of the 250,000 square meters of paint alone on the Eiffel tower is about 60 tons. By my estimates, if you covered Titan in the same paint, to the same thickness, The paint would mass 20 billion metric tons. More if you added the air-gap OP wants. Titan has only 1/9th the surface gravity of earth, but that's still a big number to support. $\endgroup$
    – Ruadhan
    Nov 16 '20 at 16:03
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    $\begingroup$ for any reader of the A, I have to note that the author of the A misunderstands the situation and effects in (1) and so as the effects happening in (2). $\endgroup$
    – MolbOrg
    Nov 22 '20 at 15:35
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The OP, Mahaus, is wrong about Titan. Titan is unsuited for being habitable for humans because it has an excape velocity which is too low to retain gases such as oxygen, not because of its lack of a magnetosphere t o prevent the solar wind fron knocking molecules out of its atmosphere.

The first necessity to maintain an atmosphere on a world is sufficient escape velocity. Having a strong magnetosphere to defect particles of the salor wind away from the world and its atmosphere is a secondary consideration.

Note that the planet Venus has a highly dense atmosphere, despite having a very weak magnetossphere compared to Earth.

In 1967, Venera 4 found Venus' magnetic field to be much weaker than that of Earth. This magnetic field is induced by an interaction between the ionosphere and the solar wind,[106][107] rather than by an internal dynamo as in the Earth's core. Venus' small induced magnetosphere provides negligible protection to the atmosphere against cosmic radiation.

The weak magnetosphere around Venus means that the solar wind is interacting directly with its outer atmosphere. Here, ions of hydrogen and oxygen are being created by the dissociation of neutral molecules from ultraviolet radiation. The solar wind then supplies energy that gives some of these ions sufficient velocity to escape Venus' gravity field. This erosion process results in a steady loss of low-mass hydrogen, helium, and oxygen ions, whereas higher-mass molecules, such as carbon dioxide, are more likely to be retained. Atmospheric erosion by the solar wind probably led to the loss of most of Venus' water during the first billion years after it formed.[112] The erosion has increased the ratio of higher-mass deuterium to lower-mass hydrogen in the atmosphere 100 times compared to the rest of the solar system.[113]

https://en.wikipedia.org/wiki/Venus#Magnetic_field_and_core[1]

Vwnus has lost a lot of atoms of lighter elements from its atmosphere due to a week magnetosphere.

But Venus still retains a very dense and massive atmosphere:

Venus has an extremely dense atmosphere composed of 96.5% carbon dioxide, 3.5% nitrogen, and traces of other gases including sulfur dioxide.[64] The mass of its atmosphere is 93 times that of Earth's, whereas the pressure at its surface is about 92 times that at Earth's—a pressure equivalent to that at a depth of nearly 1 km (5⁄8 mi) under Earth's oceans. The density at the surface is 65 kg/m3, 6.5% that of water or 50 times as dense as Earth's atmosphere at 293 K (20 °C; 68 °F) at sea level. The CO2-rich atmosphere generates the strongest greenhouse effect in the Solar System, creating surface temperatures of at least 735 K (462 °C; 864 °F).[17][65] This makes Venus' surface hotter than Mercury's, which has a minimum surface temperature of 53 K (−220 °C; −364 °F) and maximum surface temperature of 700 K (427 °C; 801 °F),[66][67] even though Venus is nearly twice Mercury's distance from the Sun and thus receives only 25% of Mercury's solar irradiance. This temperature is higher than that used for sterilization.

https://en.wikipedia.org/wiki/Venus#Atmosphere_and_climate[2]

Venus is obviously not going to lose its atmosphere to space anytime soon. It has kept an atmosphere many times as dense as Earth's for billions of years.

The less massive an astronomical object is, the lower its escept e velocity is likely to be. And the lower the escape velocity, the faster the object looses whatever atmosphere it has. And the lower the escape velocity, and the stronger the solar wind hitting the atmosphere is, the faster the solar wind will accelerate ions to the lower escape velocity.

So on a low mass world with a low escape velocity, the weaker the magnetosphere is the faster the solar wind will accelerate the loss of atmsophere. A weak magnetosphere is most effective in removing atmosphere from a world that has so low an escape velocity that it is losing atmosphere anyway. A weak magnetosphere makes a bad situation worse.

And as a general rule, the more massive a world is, the stronger its magnetosphere is likely to be.

Someone interested in writing about habitable planets, moons and other worlds should read Habitable planets for Man Stephen H. Dole, 1964, 2007.

https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf[3]

It includes scientific discussions of many habitability related factors including the possible mass range of a habitable planet.

Note that your example of a low mass world, Titan, has a mass of 0.0225 Earth, while Dole's calculated minimum mass for a planet to retain a dense oxygen rich atmosphere is 0.195 Earth, 8.6666 times as massive as Titan. So that explains why Titan is basically airless.

Actually, of course, Titan has a significant atmosphere, with a surface pressure greater than the surface pressure of Earth's atmosphere. Like Earth's atmosphere, Titan's atmosphere is mostly nitrogen, but unlike Earth's atmosphere, Titan's contains no free oxygen.

One major reason why Titan has such a dense atmosphere billions of years after forming is that Titan orbits Saturn, which orbits the sun at a distance of 9.5 Astronomical units, which is 13.194 times the distance of Venus from the Sun and 9.5 times the distance of Earth from the Sun. So at Titan's distance from the Sun, it receives only 0.0110 times as much solar radiation as Earth, and only 0.005744 times as much solar radiation as Venus.

That means that the average temperatures in the upper layers of Titan's atmosphere are much lower than the average temperatures in the upper layers of Earth's atmosphere. So atoms move much slower in the upper layers of Titan's atmosphere, the layers that lose atmosphere, than they are in the upper layers of Earth's atmopshere. This enables the lower escape velocity of Titan to retain atmsphere much longer than it would if Titan had Earth's temperature.

I also note that if Titan receives only 0.0110 as much radiation from the Sun as Earth and only 0.005744 as much a Venus, that includes the solar wind. The solar wind would obvious take a much longer time to knock away Titan's atmosphere at the distance of Saturn than it would take at the distances of Earth or Venus.

Anyone interested in the possibility of habitable exomoons orbiting giant exoplanets in other star systems should read:

Heller, Rene, and Barnes, Roy "Exomoon habitability Constrained by Illumination and Tidal Heating" 2013.

https://arxiv.org/ftp/arxiv/papers/1209/1209.5323.pdf[4]

and:

Heller, René (September 2013). "Magnetic shielding of exomoons beyond the circumplanetary habitable edge". The Astrophysical Journal Letters. 776 (2): L33.

https://iopscience.iop.org/article/10.1088/2041-8205/776/2/L33/pdf[5]

Acccording to the later paper, exomoons orbitating larger giant planets at distances between 5 and 20 planetary radii will be within the planetary magnetosphere.

Saturn has an equatorial radius of 62.268 kilometers or 36,184 miles, so moons orbiting Saturn at distances of 311,340 to 1,245,360 kilometers should be within the planetary magnetosphere. Tital orbits Saturn at a distance of 1,221,630 kilometers and so may have been protected from losing atmosphere to the solar wind by Saturn's magnetosphere.

In any case, Titan does have a dense atmosphere, despite it's low mass and escape velocity,perhaps being able to produce or otherwise acquire atmosphere faster than it is losing it.

Of course if the story involves a low gravity world with a dense atmosphere which is breathable to humans at the surface and has a temperature suitable for humans at the surface, there is a problem. Titan's doesn't satisfy either requirement, and probably would not be able to retain its atmosphere if it was at Earth's distance from the Sun.

What is needed is a world with the surface gravity and escape velocity of Titan, and with temperatures at the surface similar to those of Earth, but almost as cold as those of Titan at the outer layers of it's atomsphere where atoms escape into space, and with a breathable atmosphere at the surface.

One way to do so might be the make the world an exomoon orbiting a giant exoplanet in another star system. The giant exoplanet and its exomoon orbit their star at such a distance that the amount of radiation they receive from their star is much less than Earth gets from the Sun, but more than Titan gets from the Sun.

Thus possibly the outer layers of the exomoon's atmosphere will be could enough that the exomoon will lose atmosphere faster than Earth, but slow enugh to retain it from billions of years. But then, if the exomoon is heated only by radiation from the star, it's surface should be for too cold for humans or similar life forms.

Thus the surface of the exomoon should be heated to temperatures suitable for Earth life by internal heat, probably produced by tidal heating due to the tidal forces exerted on the exomoon by the giant exoplanet and by any other large exomoons it might have.

And possibly the lower atmosphere of the exomoon contains enough greenhouse gases like carbond dixode and water vapor to retain a significant percentage of the tidal heating, so that the upper atmosphere is not much heated by escaping tidal heating - but not enough of those gases to make the lower atmosphere unbreathable for humans or similar beings.

And also see my answer to this question:

https://worldbuilding.stackexchange.com/questions/189995/what-is-the-smallest-a-planet-can-be-whilst-retaining-a-venus-like-atmosphere/190021#190021[6]

I believe in the later article there is a discussion of the proper distance for a habitable exomoon orbiting a giant exoplanet.

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  • $\begingroup$ Useful information, thank you! I did not say that the magnetosphere was the problem for human life though, just that the conditions of Titan could not support human life. $\endgroup$
    – Mahahus
    Nov 20 '20 at 5:00

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