How would a future humanity "terraform" the moon?

Question

Let's say, hypothetically (perhaps in the setting of a book series), Earth is temporarily destroyed. Antimatter weapons have been used in self-defense during a violent conflict. Now, thousands of years later, humanity has turned to a multitude of alternatives for a home. Venus has been terraformed to have a full 25 hour long day, Mars is once again seeing an (artificial) magnetic field, with the return of oceans and a healthy atmosphere. Not to mention the multitude of O'Neill cylinders have been constructed from the excess resources and energy from using Mercury to construct a Dyson Swarm.

However, there's another location that's been neglected. The Moon, aka good ol' Luna. It was a major front during the antimatter echange that brought an end to Earth, and is currently the oldest standing collection of settlements and homes in the Solar System. They get the brilliant idea of adding an atmosphere to the moon, and keeping it there.

Let's lay down some assumptions. Humanity has access to 1.5% of the Sun's solar energy output, a planet's worth of elements and mass, as well as water (from Saturn's Ice Moons), excess gasses from the gas giants (Titan is a natural park of sorts) such as methane and nitrogen, and the ability to deploy an artificial magnetosphere to shield Luna. The moon has a daily period of ~70 hours, so it won't freeze at night, and if you have a problem with that then you can take it up with Venus.

There would be obvious advantages, especially liquid surface water. There's also the possibility that a lower surface pressure can still be beneficial, as at very low elevations (in deep caves, for example) cities can enjoy lower maintenance and less risk from any breaches in their hulls. But this only goes so far. If the effort needed to introduce and maintain an atmosphere exceeds the benefits provided.

Thus, my question: How would humanity "terraform" the moon, i.e. give it a friendly atmosphere? Where a friendly atmosphere would be an Earth mix of gasses with surface pressure within 10% of Earth's. Lasting within tolerance at least a span of 100 Kyears.

Answer 1

Yes. There are two broad classes of options:

1. A "worldhouse", or paraterraforming. Surround the entire Moon in a spherical dome, or in a collection of domes tiling the surface, to keep the air in.
2. Open-sky terraforming. The Moon cannot retain a breathable atmosphere over geologic time periods... but the timescales for human engineering and lifetimes are much, much lower. If you just keep dumping air onto the Moon, you can indeed build up enough of it to get 1 bar of pressure on the surface. The resulting atmosphere will have a much larger scale height than Earth, and it will continuously leak away into space, but not so fast that you couldn't keep it topped off with regular imports. And if the imports stop, it'll stay remain habitable for a couple million years, so you've plenty of time to fix the problem.

Answer 2

How much atmosphere do you need, really?

A 'friendly' atmosphere with ~20 kPA O2 and overall 90 kPa pressure would be very challenging with Luna's low gravity and her high-insolation orbit.

Useful but less friendly atmospheres are less difficult to engineer.

101 kPa: Earth at sea-level

90 kPa: "Friendly" as per OP

50 kPa: "Unfriendly" - air pressure at the highest permanently inhabited town on Earth

6.3 kPa: "Armstrong Limit" - the lowest pressure at which humans can breath unpressurised

0.6 kPa: "Triple Point of Water" - the lowest pressure at which liquid water can exist in the open

For example, aiming for 10 kPa (1/10th of Earth), humans could walk around the surface in casual clothes plus respirators breathing pure oxygen, giving an oxygen partial pressure comparable to high-altitude areas on Earth (drink plenty of fluids!). Simple plants like mosses and lichens could grow on land, and Earth-normal flora in water bodies.

At 5 kPa, pressurised respirators and elastic counterpressure clothing would be needed (comparable to scuba gear). Swelling (odema) of the hands and face would be an issue on extended stays outside, but could be relieved by returning to (pressurised) habitats.

What gasses to use?

According to https://en.wikipedia.org/wiki/Atmospheric_escape, Earth's atmospheric erosion at present is "charge exchange escape (~60–90%), Jeans escape (~10–40%), and polar wind escape (~10–15%)".

Charge exchange is overwhelmingly just hydrogen and other very light gases. Jean's Escape and Polar Wind Escape are strongly moderated by the gasses' molecular weight and the temperature of the upper atmosphere (exobase). From memory residence time is proportional to the third power of the molecular weight, but I can't locate the reference just now.

The temperature of the upper atmosphere might be the most "easily" engineered variable - incoming solar UV heat's Earth's exobase to around 1000K. If your setting's humans filter most of that out somehow, you could keep Luna's exobase to a moderate temperature, and reduce both Jean's Escape and other methods of loss.

Buffer Gasses

Most of Earth's atmosphere is nitrogen (N2), molecular weight ~32 g/mol, biologically mostly inert. But not useless - it contributes to the overall pressure, and it provides cooling in the event of fire (pure oxygen is a terrifyingly dangerous atmosphere, look up Apollo 1). Your atmosphere needs a buffer gas, and there are many you could choose.

If you have a look at the figure on page 4 of The escape of planetary atmospheres (https://web.archive.org/web/20230220215020/https://geosci.uchicago.edu/~kite/doc/Catling2009.pdf), you will see that Luna could almost hold onto CO2 against Jean's Escape over geological timescales even today. CO2 has a molecular weight of ~44 g/mol, but there are heavier gasses we could use. I will limit my suggestions to those that are non-toxic and non-reactive with common materials, and which provide flame-cooling as good or better than nitrogen.

Xenon is ~131 g/mol, and a noble gas. It cannot be photolysed, it doesn't react with anything except under the most extreme conditions, and it is nontoxic. That would do nicely, but it is rare and exotic, and not found usefully concentrated anywhere in the solar system that we know of, other than the atmospheres of Earth, Venus and Mars.

Sulphur Hexafluoride (SFl6) is manufactured today in thousands of tonnes per year from readily available precursors. Its molecular weight is ~146 g/mol, even heavier than Xenon. It is nontoxic, very resistant to photolysis, and doesn't spontaneously react with anything much except elemental alkali metals. The drawback of SFl6 is that if something does manage to ionise it (x-rays, extreme UV, a high-voltage electrical arc) the decomposition products are toxic fluro-analogs of phosgene. That will react with water if there is any around and form safer compounds, so this might be manageable if high-energy radiation can be limited on your future Luna (orbiting sunshades tuned to filter UV?).

A more fragile but biologically-safer alternative would be octofluoropropane (freon-128). Molecular weight ~188, but breaks down via reactions with UV and water vapor as per https://www.fluorocarbons.org/environment/climate-change/atmospheric-lifetimes/.

Biologically-Active Gasses

Oxygen (O2), CO2 and H2O are the big ones. If you have an open-air biosphere at all, you will have at least 1 kPa of each of O2 and H20, and 0.2 to 1 kPa of CO2. Add another 1 kPa of N2 and/or derivatives such as NO for a sustainable green plant nitrogen cycle. These biologically active gasses will escape over time and need to be replenished due to their low molecular weights (H20; ~18 g/mol, O2: ~32 g/mol, CO2: ~44 g/mol), but the smaller fraction they are of your atmosphere the less problematic this becomes.

My 2 Cents

Yes, a Lunar atmosphere is definitely possible, but consider reducing expectations (and pressure). Orbiting macro-scale sunshades can help by filtering UV.

Answer 3

Now, thousands of years later, humanity has turned to a multitude of alternatives for a home. Venus has been terraformed to have a full 25 hour long day, Mars is once again seeing an (artificial) magnetic field, with the return of oceans and a healthy atmosphere. Not to mention the multitude of O'Neill cylinders have been constructed from the excess resources and energy from using Mercury to construct a Dyson Swarm.

One) it would take an incredible amount of energy to change the rotation period of Venus.

It would be much easier to build giant sun shades orbiting Venus. Their orbital period would be such that the Sun facing side on Venus would have alternating light and dark each roughly 12 hours long.

The inner sides of those sun shades, facing Venus, would also be reflective. So the Sun shades passing over the dark side of Venus would reflect light down onto the surface, and there would be alternating 12 hour periods of light and dark, just as on the sunward face side.

Two) Giving Mars an artificial magnetic field would shield its atmosphere from being knocked off by the solar wind. But atmospheric loss due to solar wind is a rather minor effect compared to atmosphere loss due to the low escape velocity of Mars, 5.027 kilometers per second.

According to the discussion on pages 33 to 39 and 54 of Habitable planets for Man, Stephen H. Dole, 1964, an escape velocity of 5.027 kilometers per second would be about 4 times the velocity of atomic oxygen in the escape layer of the exosphere of Mars, and so Mars would lose oxygen at a rate which would reduce the amount to 0.368 of the original amount in a few thousand years. So to keep the oxygen in the Martian atmosphere constant about 62 percent would have be replaced every few thousand years. About 0.02 percent of the oxygen would have to be replaced every year. In a mere million years of human habitation on terraformed Mars, the oxygen in the atmosphere would have to be replaced over two hundred times.

And that seems very wasteful to me.

Instead Mars should be made a shellworld.

A shellworld[1][2][3] is any of several types of hypothetical megastructures:

A planet or a planetoid turned into series of concentric matryoshka doll-like layers supported by massive pillars. A shellworld of this type features prominently in Iain M. Banks' novel Matter.

A megastructure consisting of multiple layers of shells suspended above each other by orbital rings supported by hypothetical mass stream technology. This type of shellworld can be theoretically suspended above any type of stellar body, including planets, gas giants, stars and black holes. The most massive type of shellworld could be built around supermassive black holes at the center of galaxies.

An inflated canopy holding high pressure air around an otherwise airless world to create a breathable atmosphere.[4] The pressure of the contained air supports the weight of the shell.

Completely hollow shell worlds can also be created on a planetary or larger scale by contained gas alone, also called bubbleworlds or gravitational balloons, as long as the outward pressure from the contained gas balances the gravitational contraction of the entire structure, resulting in no net force on the shell. The scale is limited only by the mass of gas enclosed; the shell can be made of any mundane material. The shell can have an additional atmosphere on the outside.[5][6]

https://en.wikipedia.org/wiki/Shellworld

So Mars can be made the third type of shellworld, with the shell keeping the dense atmosphere below it from escaping, with giant airlocks in the shell for spaceships, and a thinner atmosphere above the shell to stop meteoroids. The shell would also prevent the solar wind from knocking away atmospheric particles.

Three) The Moon.

You wrote:

Let's lay down some assumptions. Humanity has access to 1.5% of the Sun's solar energy output, a planet's worth of elements and mass, as well as water (from Saturn's Ice Moons), excess gasses from the gas giants (Titan is a natural park of sorts) such as methane and nitrogen, and the ability to deploy an artificial magnetosphere to shield Luna. The moon has a daily period of ~70 hours, so it won't freeze at night, and if you have a problem with that then you can take it up with Venus.

The Moon has a sidereal rotation period of 27.321661 days or 655.71986 hours, and in its present orbit around the Earth and the Sun it has a synodic rotation period and day of 29.830589 days or 715.93413 hours.

It would be incredibly wasteful of energy to change the rotation period of the Moon to about 70 hours.

It would be much easier to build giant sun shades orbiting the Moon. Their orbital period would be such that the Sun facing side on the Moon would have alternating light and dark each roughly 12 hours long.

The inner sides of those sun shades, facing the Moon, would also be reflective. So the Sun shades passing over the dark side of the Moon would reflect light down onto the surface, and there would be alternating 12 hour periods of light and dark, just as on the sunward facing side.

The Moon has an escape velocity of only 2.38 kilometers per second, less than half of Mars's. So gases would escape from the Moon much faster than from Mas, so it would be even more necessary to make the Moon a shellworld with a shell keeping the atmosphere from escaping into space.

Answer 4

Thus, my question: How would humanity "terraform" the moon, i.e. give it a friendly atmosphere?

Short answer: they wouldn't.

Even if it were possible, it would be so horribly uneconomical and slow that it would make no sense whatsoever. (It would probably be much easier to bioengineer humans so they don't need outside air and can tolerate vacuum, or transfer our consciousness into AI which doesn't need those pesky planets or whatever. But I digress).

Also, you say "terraform" which usually means making it fit for unprotected human life for many generations - which is usually much more than just having right amount of breathable gases in the atmosphere. To mention just a few:

• there are big radiation issues (there is no magnetosphere to protect from either the sun SPE or more energetic GCR sources),
• prolonged exposure to hugely reduced gravity has serious health issues for humans (there is a reason why we try to keep astronauts stay on ISS less than a year - and those are prime specimens. While we haven't tried staying on moon for long periods of time yet, it is quite unlikely it would be harmless),
• (biggest issue) that regolith is deadly microdust which will tear your unprotected lungs to shreds in days if not hours even if tiny amounts are raised in the breathable air (and there will be incredibly huge amounts even produced by one man walking, unless you put tarmac like substance over 100% of the moon)

So there would literally be hundreds of better ideas to use those resources available if you want to make habitats for humans, or terraform other planets / moons, or build generational ships to move to other star systems to avoid all-eggs-in-one-basket situations in the future etc.

---

But let's say they're doing it for sentimental reasons, and they don't care about huge amount of resources wasted, and they only care about "i.e. give it a friendly atmosphere" part of your question, and not about making it actually livable unprotected outside (i.e. "terraforming").

With that level of technology (1.5% sun output, harvesting outer planets, antimatter war) you'd likely be able to collect appropriately sized primordial black hole or few, and put them in the middle of the moon to gain enough gravity.

That would allow you to easily keep the gasses and reduce many of the problems mentioned above. The biggest one that still remains is the regolith - even the tiny activity of moving humans/vehicles, or a tiniest wind (now that you have atmosphere, you have that problem too) would raise that razor-sharp particles from the regolith into the air and make it exact opposite of "friendly atmosphere".

I mean, even without air to carry it around, moon dust would destroy sturdy EVA suits in a matter of months. Now imagine its power being amplified several orders of magnitude by atmospheric winds, and unprotected humans... Gruesome scene, they would've likely been better if they were directly hit directly with those antimatter weapons in the first place.

So you'd still have that "tiny detail" of needing to remove or contain all that regolith surface of the moon and replace it with something less deadly.
Which is nigh impossible with technology levels you mention - assuming war surplus of antimatter is not enough to turn all surface of moon to glass? Even it were, you'd first have to evacuate moon, nuke it and let it cool, and then put the gases in and re-settle it again. Hardly economical or practical. Some sort of insanely capable and quick reproducing nanobots might be able to do the job non-destructively given enough supplement materials, energy and at least a few dozen thousands years (or more likely millennia), but until that is completed your population would have to stay inside moon domes.