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When we think of terraforming a body in our galaxy Mars gets lots of consideration as the closest potential body. But what about the Moon?

It is currently beyond our ability to add an atmosphere to a planet or a moon, but it is not impossible, and will likely be done in the future.

If we add an atmosphere to Earth's Moon, how stable would it be? Would the Earth's Gravity steal it? Would the Earth's magnetic field protect it?

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    $\begingroup$ Question inspired by a VERY wrong answer to a recent question. You know who you are, thanks for the inspiration. $\endgroup$ – James Jenkins Nov 5 '15 at 11:58
  • $\begingroup$ My knowledge is limited, but the problem is mass. The moon doesn't have enough mass to maintain an atmosphere. $\endgroup$ – fotijr Nov 5 '15 at 13:23
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    $\begingroup$ @JamesJenkins: I was wondering how long it would be before someone did something about that particular answer... $\endgroup$ – PearsonArtPhoto Nov 5 '15 at 13:44
  • $\begingroup$ @PearsonArtPhoto and now I'm curious... $\endgroup$ – John Dvorak Nov 5 '15 at 16:43
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    $\begingroup$ @JanDvorak: Let's just say a person who should have known better though that space.stackexchange.com/q/12556/25 was about the Moon... $\endgroup$ – PearsonArtPhoto Nov 5 '15 at 16:45
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It can keep an atmosphere, and in fact does. The atmosphere is something akin to a high grade Earth-based vacuum. But that's probably not what you are looking for.

Okay, so what would happen with, say, an Earth sized atmosphere on the Moon? A lot of really interesting things would happen actually. First of all, in longest days of the night, the atmosphere might freeze. It would also have a high wind volume, at the day/night terminator. It would lose a lot of it's atmosphere (See this Physics Question) due to Solar Wind and simple lose by having too much energy.

Bottom line, the atmosphere would stay, but only for maybe 1000 years (See this article). Domes are probably the best way for the foreseeable future on the Moon.

To illustrate this, Wikipedia provides a really neat chart showing atmospheric particles that would stick vs temperature and size.

Chart illustrating what molecules will stay vs temperature and size

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  • $\begingroup$ Couldn't the Moon get an ionosphere that somewhat protects it against the Solar wind, like Venus? And wind speeds that super rotates the atmosphere relative to the Sun much faster, again as the weather on tardy Venus, than the Moon itself turns, to even out the day/night temperatures. 60% of the Moon's mass is oxygen, so while the varieties of potential volatiles is limited (no carbon or nitrogen), there's at least no lack of elemental raw material to refill a breathable atmosphere as it outgasses. The mass of Earth's atmosphere is but a millionth of the planet's mass. $\endgroup$ – LocalFluff Nov 5 '15 at 17:22
  • $\begingroup$ The Solar wind is a less serious effect, which main results in smaller particles. Smaller particles are easier to float away through normal processes. $\endgroup$ – PearsonArtPhoto Nov 5 '15 at 17:44
  • $\begingroup$ But O2 isn't that very light, and there's a refill potential anyway, lasting evolutionary times. Antelopic animals in herds could breathe and breed and jump high, and graze on fertilized fungi, on such a managed lunatic world made up. $\endgroup$ – LocalFluff Nov 5 '15 at 17:59
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    $\begingroup$ O2 is still too light enough for the moon. See my latest edit. It could be refilled, but Xenon is really the mass of particle required to stay on the Moon without replenishment. $\endgroup$ – PearsonArtPhoto Nov 5 '15 at 18:13
  • $\begingroup$ I think the day time temperature of the moon can reach ~400K, so even the Xenon would tend to get enough energy to escape. I suspect would be a slow process (millions of years?) but not indefinite. $\endgroup$ – James Jenkins Nov 5 '15 at 19:47
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PearsonArtPhoto covers the basic issue, which is that light molecules move so fast that they reach lunar escape velocity.

The average speed of the molecules of a particular gas is proportional to the square root of (temperature in Kelvin divided by molecular mass) see https://en.wikipedia.org/wiki/Root-mean-square_speed

I propose a couple of gases with a molecular weight higher than xenon (mw 54) that might be able to sustain an atmosphere on the moon, given that there is very little xenon in the solar system.

I think the chances of terraforming the moon with an atmosphere are very slim, but it's an interesting thought experiment.

Butane (mw 58.) Made of abundant elements. May decompose to lighter hydrocarbons over time, especially if interacting with hydrogen from the solar wind. Butenes or heavier, more unsaturated hydrocarbons (such as benzene) might last longer.

Chlorine Cl2 (mw 71.) Not one of the top ten most common elements in the solar system, so would need to be specifically mined, or imported (earth seawater is about 2% chlorine by mass.) Would be stable on its own, but would oxidise rocks and thus leave the atmosphere if the rocks were oxygen deficient. Forms a number of gaseous oxides, but these are unstable with respect to decomposition to chlorine and oxygen, so would not accumulate. Would react with hydrogen from the solar wind to form hydrogen chloride, which has a lower molecular mass.


EDIT 1: according to wikipedia, the solar wind is a mass flow of about 1E9 kg/s, most of which is hydrogen. At Earth's orbital radius of 150E6km, this is just 1E9 / (4*pi*(150E6)^2) = 3.54E-9kg/km2s or 0.11kg/km2 year. The moon's radius is 1740km, so it presents a 9.511E6km2 surface to the solar wind. The total hydrogen received by the moon from the solar wind is therefore about 3.367E-2kg/s so the replenishment rate of chlorine Cl2 that has been converted to HCl would be quite trivial, in comparison to the unimaginably large task of putting a chlorine atmosphere on the moon in the first place. I'm surprised this number is as small as it is.

Photodissociation of Cl2 into two Cl radicals (which each have a lower mass than CO2) may be a far more significant mechanism of loss of chlorine, but I don't know how to quantify it.

EDIT 2: another possibility is Sulphur Dioxide SO2 (MW 64.) This is probably chemically stabler than either of my previous proposals. There's also a possible non-intentional source for it: If massive building programs on the moon were undertaken using sulphur concrete, it might in the presence of oxygen degrade to SO2. Sulphur concrete is similar to regular concrete in that it contains aggregate, but molten sulphur is used as a binder instead of traditional cement. https://en.wikipedia.org/wiki/Sulfur_concrete

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  • $\begingroup$ Wouldn't the butane photodissociate badly? $\endgroup$ – Loren Pechtel Nov 6 '15 at 4:17
  • $\begingroup$ @LorenPechtel stability of butane is an issue, I already mentioned that. But chlorine photodissociates much more than hydrocarbons, partly because it is coloured, and partly because of its lower bond strength, see cem.msu.edu/~reusch/OrgPage/bndenrgy.htm and en.wikipedia.org/wiki/Bond-dissociation_energy $\endgroup$ – Level River St Nov 6 '15 at 5:45
  • $\begingroup$ I posted space.stackexchange.com/questions/12597/… with a focus on Xenon as it is a noble gas, but I would encourage you to post an answer based on the gas of your choice. $\endgroup$ – James Jenkins Nov 6 '15 at 12:28
  • $\begingroup$ @James - I assume this is a consequence of solar system formation. In other words, the temperature differences in the protoplanetary disk. I can't find the temperature charts but that may be why xenon is more rare in certain parts of the solar system. $\endgroup$ – spacer Nov 6 '15 at 12:46
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    $\begingroup$ @steveverrill and after introducing SO2 atmosphere we would rename Moon to Stinky :). $\endgroup$ – Matas Vaitkevicius Nov 11 '15 at 11:29
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If I remember correctly, Stephen Baxter's novel Moonseed (1998) explores this question and answers yes, it could maintain an atmosphere sufficient to support human life on the surface for a couple of hundred years before it attrited away. Hardly definitive, but he's no slouch when it comes to research.

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