Given that the surface gravity on the Moon is a fraction of Earth and there is virtually zero atmospheric pressure but I still want to wet our beloved moon with whatever liquid that is known to science. It can be liquid nitrogen or a cocktail of acids etc, assuming the present day tech level and the only handwavium is awarded to transportation of the liquid and nothing else how can we cover the entire (at least 95% of lunar surfaces) with liquid solution so that the shallowest depth is about 1 m for at least 1 year? (I'm trying to figure out how the new tug of war between a planet and its moon will play out.)


  • Money isn't an issue.
  • Foams are not allowed.
  • It can be any kind of liquid not necessary water
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    $\begingroup$ The highest mountain on the Moon is Mons Huygens, at 4.7 km. To cover the entire surface of the Moon with approximately 5 km of liquid will not happen, in any kind of way, because you would need to transport about 190 million cubic kilometers of fluid there. And "how the new tug of war between a planet and its moon will play out"... the answer is: there will be no difference. Assuming water at 1 ton per cubic meter, you change the mass of the Moon by about 0.2%. This is next to negligible. $\endgroup$ – MichaelK Oct 24 '16 at 12:04
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    $\begingroup$ Are you intending to somehow seal the surface of the moon before you pour liquid on it? While it's theoretically possible to calculate the volume of fluid needed to cover the moon, it's difficult to estimate how much would soak in. I'm willing to bet you'll need a huge amount of fluid before it even begins to show on the surface. $\endgroup$ – user10945 Oct 24 '16 at 12:58
  • $\begingroup$ @Pete: thanks for reminding, foams are not allowed. $\endgroup$ – user6760 Oct 24 '16 at 13:02
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    $\begingroup$ Mandatory xkcd : what-if.xkcd.com/54 $\endgroup$ – Goufalite Oct 24 '16 at 14:06
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    $\begingroup$ I'm waiting for the molten lava surface answer (just melt the moon by a method of your choosing such as meteor bombardment) and radioactive material answer (radioactive decay will produce enough heat to keep the what ever material, with suitable melting and boiling points and half-life, liquid). Answers with math on temperature and cooling time of course, which is why I'm not writing these answers myself. $\endgroup$ – hyde Oct 24 '16 at 15:50

How much we need?

Rounding down Moon to a roughly flat ball with the average distance to the planetoid center and then making a hollow sphere to the highest elevation via math gives us the estimated volume of liquid we need to fill. So:

$V=\frac{4 \pi}{3}(R_{peak}^3-R_{sea}^3)$

Taking 1737.4 km for moon sea level, the highest point goes 10.786 km above that. This gives a volume of:

$V=411722695.8934 \text{ km³}$

That only fills to just about the top of the highest mountain, demanding 5 km coverage, brings it to a generous $V=604296935.9922 \text{ km³}$, so your preferred solution is somewhere between those two extremes.

In other words: it's a huge lot of liquid. If that would be water with a density of $\rho=1 \frac{\text t}{\text m^3}$, it would weigh ${4.1 \choose 6.0} \times 10^{16}\text t$. That is still much less than the weight of the moon (which ranges in the $10^{19}\text t$ area), but it is a considerable fraction: you would add about $\frac {1 \to 2} {1000}$ of the mass of the moon itself if you take water.

Adding that much weight would have a substantial effect on the Moon-Earth system though, resulting in a decaying orbit and crash on a long term timescale, but that would be a better question for Randall Munroe, who might dedicate that question of world destruction a blog entry. So I skip on that and go to what our ocean has to endure.

Environmental Problems...

Obviously, water will freeze due to being exposed to an environment of at coldest $26 \text K$ on the surface of the moon and $3 \text K$ at the surface to outer space. To complicate things, fluids start to evaporate much earlier (as much as close to $0 \text K$ for Hydrogen) due to reduced pressure. And above that, the Moon's surface gets very very close to $400 \text K$ in the sun, because rock is a very very bad thermal conductor.

In the end, pouring all the water onto moon will just help in creating a jagged ice crust that more resembles an uneroded Europa than a liquid. Why uneroded? Europa encounters daily tidal forces, just like Tethys, which keep the inner water layers of the planetoid in a liquid state due to the extreme magnetic fields of the gas giants they rotate around. Earth can't grant that field - and both these snowballs have an outer crust of ice, not a fully liquid ocean, so the water solution is kind of out.

What to take?

So we got to take something that doesn't boil away, gets blown off the moon by the solar wind and doesn't freeze to solid by exposure to the temperatures. But here is the problem: there are not too many substances that are in liquid state in an environment facing to the $3 \text K$ and near vacuum environment of outer space... actually I could come up only one single material that actually is a liquid in an environment close to that: Helium boils at $4.22 \text K$ and remains liquid till very very close to absolute 0. it would weigh 0.125 tons per m³... but that is at 1 Atmosphere pressure, while we got a 0 atmosphere out at the upper edge of our ocean, which again... makes our precious ocean boil away due to reduced boiling point.

Hydrogen (which boils at $20 \text K$) is not an option, as it freezes at $14 \text K$.

End of the line

No, you can't create a fluid ocean on moon, as it has neither a protective magnetic field (which would keep the sun wind away to allow the creation of an atmosphere and thus any pressure at all), nor is there a liquid that would be fluid in the whole $3 \to 24 \text K$ temperature range that we can expect on coldest point, and especially there is no substance that is liquid for the whole $3 \to 400 \text K$ range that we have to deal with on the day side. This does ignore the temperature averaging effect of the fluid though, but still:

No Atmosphere = No oceans in fully fluid state.

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    $\begingroup$ The protective magnetic field is important in the timescale of millions of years, not on the timescale of 'what would happen if I dumped a lot of water on the moon right now.' If you put an atmosphere on the Moon it would not disappear over night, and if you put a liquid ocean on the moon that would give it an atmosphere, at least for a while. $\endgroup$ – kingledion Oct 24 '16 at 14:07
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    $\begingroup$ Titan has less surface gravity than the moon, and an atmosphere denser than Earths. The Moon is hot in its core, and warm in its mantle, adding a 5km+ ocean would provide the temperature buffering to avoid the 400K to 3K extremes you are talking about. I don't disagree that there is pretty much no way to avoid an ice sheet completely covering this new ocean, but some of your numbers are a bit dubious, in my opinion. $\endgroup$ – kingledion Oct 24 '16 at 14:34
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    $\begingroup$ "Pouring" that amount of water would release enough gravitational potential energy to melt the lunar surface, I wager without doing the math... Unless you have some form of antigravity to get the water to the Moon without depositing the released energy there too. $\endgroup$ – hyde Oct 24 '16 at 15:55
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    $\begingroup$ All of the problems in the environmental section would be removed, lessened, or changed by the addition of all that water. Sure, the water boils because of low pressure. But unless the water is travelling above escape velocity, it doesn't go anywhere. It becomes atmosphere. Solar winds and even brownian motion will eventually strip it away, but the OP was talking about it lasting for a year, not forever. Heck, would that ice crust even form in a year? Not to mention how effective an ocean is at moderating temperature swings... $\endgroup$ – Shane Oct 25 '16 at 15:08
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    $\begingroup$ tl;dr While "No Atmosphere = No oceans in fully fluid state" is surely correct, I'm not sure how true the 'No atmosphere' part would be after you made the attempt. $\endgroup$ – Shane Oct 25 '16 at 15:15

If we are hand waving the transportation of the liquid, then the source is easy: Tethys.

Tethys' density of 980 kg/m$^3$ and spectroscopy of its surface indicates that it is almost pure water ice. And it weighs 6e20 kg, so plenty of water to go around. Assuming incompressible liquid water, it has enough payload to bury the moon under 16km of water. Even if you just slammed it into the moon, the probably has enough gravity to keep a good portion of that water. If you put it in orbit around the moon (note: do not attempt at home! gravitational effects on Earth-Moon system have not been validated!) you could break it off in chunks and throw it at the surface to liquidate the water without liquidating the moon itself.

Also, there are plenty of smaller ice moons or icy comets around to supply water for you. Also also of note, if you put that much water on the moon, a lot of it will evaporate due to low pressure and solar radiation. But only up to the partial pressure of water at current surface temperature. The result will be an atmosphere on the Moon, at least for a while until everything cools down and freezes.

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  • $\begingroup$ Put it in low lunar orbit and then lower it to moon-grazing heights. It will lose energy in collisions and come down, but rolling rather than yielding all it's energy at once. You'll have far less splash thrown off the moon in the process. $\endgroup$ – Loren Pechtel Oct 24 '16 at 19:03
  • $\begingroup$ Bonus points for just outright swapping Luna and Tethys. Good thing transporting is easy in this universe. $\endgroup$ – Kys Oct 24 '16 at 20:53

Reading on Terraforming the Moon a while ago, it does seem to be marginally possible by repeatedly peppering the surface with comets until enough gasses had accumulated to create an atmosphere, which would then allow a liquid water hydrosphere to form. Obviously, this would not submerge the entire surface of the Moon under water (other answers have already calculated the vast amount of water required), but so long as you had accumulated enough gasses to build an atmosphere which could hold liquid ocean and allow people to walk unprotected on the surface, it seem plausible that such an atmosphere could remain for over 10,000 years.

enter image description here

The low gravity means the Lunar atmosphere extends for hundreds of kilometres, resulting in spectacular cloud formations

While not long on a geological time scale, this is still twice the length of recorded history on Earth, so sufficient time to allow a civilization to grow and prosper.

Of course, any really smart people who were doing this would also arrange for an artificial magnetic field and probably seal the entire Moon in a vast "bubble" of artificial diamond to keep the air in, but brute force engineering could supply the solution.

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Doesn't have to be water? Great!

Water is stupid anyway, boils off, totally. For a short time you get residual ice and snow, and earth has a ring system, but you don't want to sell space postcards, do you?

So, to business: What you need is a liquid without vapour pressure. Ionic liquids. They're cheap, you can get them at a few hundred euros per liter nowadays. And I'll give you a real good discount for any order of more than 200 cubic kilometres.

Getting the feedstock for my lunar factories might be a bit tricky in the beginning. But I'm sure we can work something out with my friends who collect spent comets and stray asteroids to get all the necessary carbon and fluorine.

I really hope your bank will be up to it. Delivery is immediately stopped when monthly advance payments are interrupted.

As a plus, I guarantee without additional cost a minimal amount of foaming for at least six months. Can be extended to 12 month for a modest 25% surcharge, in advance.

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  • $\begingroup$ You seriously should try to work on your expression: this is not a forum, you shall give an answer, and half of this is not an answer but sounds like self- promotion. $\endgroup$ – Trish Oct 24 '16 at 19:19
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    $\begingroup$ @Trish You are free to ignore that this is the only positive answer to the OPs question, that i gave the link to the wikipedia article and addressed the resources problem. And if you think you cannot afford my services, fine, i'm not forcing anyone into buying. Get your fluid moon somewhere else if you can. $\endgroup$ – Karl Oct 24 '16 at 19:29
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    $\begingroup$ @Trish It's supposed to sound that way. I don't think they're trying to advertise for their Ionic Liquid business. A little humor is good. Huckster salesman is a fine sci-fi tradition, especially for the more absurd questions. See also worldbuilding.stackexchange.com/questions/58010/… $\endgroup$ – Schwern Oct 24 '16 at 19:32
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    $\begingroup$ @Schwern Thanks. And Trish: SMILE! My goodness. ;-) $\endgroup$ – Karl Oct 24 '16 at 19:35
  • $\begingroup$ Low vapor pressure you say? Like, how low are we talking? I have a 1 picobar near-vacuum to deal with here. And what's the operating temperature range? I have to work with 35 to 300K. I asked for ionic liquid, not ionic solid! What do you mean the warranty is void if I take it off planet?! $\endgroup$ – Schwern Oct 24 '16 at 19:38

You're going to need a lot more than current tech (it would be a huge industrial undertaking to make the factories to make the stuff to make the liquid) to 'generate' any type of liquid, even if you handwavium away the transport problem.

If you merely want to move existing liquids, and you're handwaviuming from Earth to the Moon (simplest case), your only possible viable option is seawater (well, perhaps if you were willing to kill everyone on Earth by stealing their freshwater... you'd have two options) - but that won't stay liquid for long.

Given enough handwavium transport, we could coat the Moon in cometary-ice, I suppose.

We don't have the tech to keep any of the waters (salt or fresh) from freezing, unless we handwavium a space industry (I hope we get to transport it and all of the materials needed with handwavium, too). It might be possible to set up enough mirrors (using existing technical ideas) to heat the dark-side of the moon - but that's a lot of material to make and place, and to run in order to keep all of this water liquid. ie: Again with making huge industries, and probably 10s of thousands of people (maybe we could get away with just under 10k people... but I doubt it).

Covering such a huge amount of the Moon's surface is going to be problematic. The lunar high point is 1938 meters higher than Everest. Granted we won't have to go that high (let's go with 8.848 km high), and surface area is 38 million km2 is 3.36224 × 10^14 m2 of liquid.

That's a substantial number.

Unless my math is wrong, all of the seawater on Earth only amounts to 1.386 x 10^12 m2, a couple of orders of magnitude less than you would need.

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  • $\begingroup$ you used an estimated flat area to extrapolate the dimension of the needed water, however, the Moon is a sphere and we will add a hollow sphere. however, taking into account the elevations above sea level, both estimations are likely too high and will result in coverage of 100% $\endgroup$ – Trish Oct 24 '16 at 13:29
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    $\begingroup$ The OP said "cover 95%", and your math about the water volume is off by several orders of magnitude. The earth has ten times as much ocean surface than the total moon surface. That makes for a ~30km deep lunar ocean. $\endgroup$ – Karl Oct 24 '16 at 18:01

Self-replicating nanobots that consume the regolith and burried ice deposits and produce nanoparticles that don’t adhere to each other, so it behaves as liquid at our scale.

The hard part is providing energy. If the biofilm is on the surface of the rock and below the growing layer of heavy fluid, the fluid should transmit sunlight. Large particles are usually black (think petrolium) or some opaque pigment. To make it clear the particles are engineered to support surface phonons in the manner of plexiglas.

The pool of heavy oil will Enhance the metabolism of the biofilm, protecting against damaging UV and passing the frequencies that can be used for power, blocking solar wind particles, and carying off heat. It will also serve as a medium to move different resources around as deposits of needed atoms will be found in different places.

It might be designed as an ocean of nanotech manufacuring bots! That’s why it was done. Once booted up to produce a biofilm and pool filling some lowland or crater depression, you can introduce instructions to produce a larger variety of nanobots which manufacture goods of all kinds.

Why let it fill the whole moon, overflowing the original basins and forming ever larger seas? Because it increases capacity and connects geologically diverse regions where different materials can be found automatically.

Perhaps the civilization that produced it falls or “ascends” and leaves it to go feral. A few million years later you have something like Code of the Lifemaker but based on nanotechnology it is more indistinguishable from the kind of ecosystem we are familiar with.

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  • $\begingroup$ Anonymous downvotes when there is nothing obviously wrong with the post to the standard stated for downvote is not helpful for the poster to address whatever problems were perceved. One can only presume that the voter meant “I don’t care for this idea” which is not what downvotes are for on this site. $\endgroup$ – JDługosz Oct 25 '16 at 18:47

Oh, that's easy.

Melt it.

Place giant mirrors in orbit of the moon and have them reflect and concentrate the sun onto the moon's surface. You are going to want to heat it slowly and evenly. Too much heat too quickly or heat applied to locally and you'll cause vaporization and/or stress fractures.

I'll add the math later tonight, but I'm sure it'll cost trillions of dollars.

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  • $\begingroup$ In two words: Um-possible. There is not even enough space in moon's orbit to place all those mirrors to heat the whole surface to >1000°C. $\endgroup$ – Karl Oct 25 '16 at 16:30
  • $\begingroup$ Or use high velocity asteroid impacts. $\endgroup$ – Peter Oct 25 '16 at 18:08

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