Atmospheric Creation on the Moon

[-Callisto-]

First post, long time lurker.

Supposing that mankind were to first choose the Moon as the celestial body for colonisation (no questions at this point, those in favour of Mars pretend for a second it has some sort of life on it!), how would we go about creating a significant atmosphere on the Moon? Lack of significant magnetic field aside, what short and long term problems would be faced?

Is the Moon a large enough body to hold onto an atmosphere conducive to life as we know it? And if so, what is the lower bound for a planet/moon in terms of mass for a significant atmosphere?

Apologies for all of the questions, look forward to a good discussion getting going.

 

[Whovian]

I'd guess that any atmosphere with breathable gas ratios that we could form and the Moon would be able to "hold" on to would be both too thin to be breathable and be destroyed by cosmic effects (such as the famous Solar Wind.) Unless the Earth's magnetic field is still strong enough out there, I'm not too much of a cosmologist, so I'm not sure how strong it is. Not very, I doubt it would be strong enough.

 

[-Callisto-]

I'd guess that any atmosphere with breathable gas ratios that we could form and the Moon would be able to "hold" on to would be both too thin to be breathable and be destroyed by cosmic effects (such as the famous Solar Wind.) Unless the Earth's magnetic field is still strong enough out there, I'm not too much of a cosmologist, so I'm not sure how strong it is. Not very, I doubt it would be strong enough.

Thanks for the reply. Though I'm no expert on the issue I thought the current thinking on the matter was that the magnetic field isn't as pivotal for atmospheric retention as once thought and even then it's a long term issue as opposed to a short term one, any stripping away would surely operate on time-scales of hundreds of thousands of years?

 

[Whovian]

Thanks for the reply. Though I'm no expert on the issue I thought the current thinking on the matter was that the magnetic field isn't as pivotal for atmospheric retention as once thought and even then it's a long term issue as opposed to a short term one, any stripping away would surely operate on time-scales of hundreds of thousands of years?

I'm no expert either. I'm not sure what sorts of time scales these things happen on, so perhaps the magnetic field bit on my other post is quite ignorable. (I'd still think it would be too thin.)

 

[Chronos]

The martian atmosphere is believed to have been lost over the ages mainly due to atmospheric sputtering. And Mars is much more massive than the moon

 

[onomatomanic]

Let's see if I can remember how to do this...

To figure out whether the gravity of a planetary body is sufficient to retain a given component of the atmosphere in the long run, we compare the thermal kinetic energy and the gravitational potential energy of a molecule. If the kinetic energy is greater, the element is lost pretty much as soon as it is released. If the potential energy is greater, the element is held on to in the short run, but there'll always be some leakage from to the "long tail" of the thermal energy distribution. What that means is that, at any given time, while most molecules will have kinetic energies which correspond more or less to the temperature of the atmosphere, there will always be a few that happened to pick up a lot of excess kinetic energy and so correspond to a much higher temperature.

So, on the one hand, the mean kinetic energy is given by (3/2) k T, where k is Boltzmann's constant.

On the other hand, the gravitational potential is given by G M m / R, where G is the Gravitational constant, M and R the mass and radius of the planetary body, and m the mass of the molecule.

Equating and solving for the temperature, that gives T = (2G/3k) (M/R) m
Plugging in the values for Earth, we get T ~ 5,000 K * RM
Plugging in the values for the Moon, we get T ~ 250 K * RM

In these expressions, RM is the relative molecular weight for the atmospheric component in question, which is simply the total count of protons and neutrons in a molecule. For diatomic hydrogen, that's 2, for helium, 4, for diatomic oxygen, 32.

As mentioned, if these expressions yield a temperature which is less than the temperature of the atmosphere, it means that the element is lost straight away. If it's more, molecules will still leak out, and the ratio of the two temperatures indicates how long it'll take for most of them to be gone. Doing this rigorously takes quite a bit of statistical physics, but IIRC the rule of thumb is that temperatures within one order of magnitude above that of the atmosphere mean that the component is lost "very quickly" (within a year?), while temperatures in excess of two orders of magnitude above that of the atmosphere mean that the component is retained "almost indefinitely" (for billions of years).

So, let's see what that gives us. For Earth:

T_H2 ~ 10,000 K
T_He ~ 20,000 K
T_O2 ~ 160,000 K

Our atmosphere has a temperature between 200 K and 300 K, so this tells us that Hydrogen and Helium do escape, though only slowly, while oxygen is retained "almost indefinitely" - which agrees with what we observe.

For the Moon:

T_H2 ~ 500 K
T_He ~ 1,000 K
T_O2 ~ 8,000 K

Assuming the same atmospheric temperature, that means that Hydrogen and Helium escape after a negligible amount of time, and that even oxygen is closer to the "very quickly" than to the "almost indefinitely" side of things. Moon would do a worse job of holding on to oxygen than Earth does with regards to Hydrogen.

Conclusion: The Moon's gravity isn't quite weak enough to rule out an Earth-like atmosphere altogether, but it'd leak like a sieve and thus would need constant replenishment. As terraforming projects go, this one would be truly "sisyphean".

 

[jwbales]

An atmosphere for the moon would have to be something heavier than the nitrogen/oxygen atmosphere of the Earth. Something like sulfur hexaflouride which is six times heavier than air. It's also non-toxic and non-flammable. If enough of it could be generated on the moon to give 2 psi then humans could breathe pure oxygen from tanks and go about outdoors without pressure suits. Question is, being a molecule rather than atomic, how stable would it be under bombardment by solar particles or reaction with lunar soil?

 

[Czcibor]

According to wikipedia sulfur hexaflouride is terribly innert and quite stable - "estimated atmospheric lifetime of 800–3200 years". Not bad, (very nice out of box idea) but I still doubt it would be workable - it should be as hard to maintain as helium in Earth atmosphere.

 

[-Callisto-]

Let's see if I can remember how to do this...

Excellent post, exactly what I was looking for, can't thank you enough.

An atmosphere for the moon would have to be something heavier than the nitrogen/oxygen atmosphere of the Earth. Something like sulfur hexaflouride which is six times heavier than air. It's also non-toxic and non-flammable. If enough of it could be generated on the moon to give 2 psi then humans could breathe pure oxygen from tanks and go about outdoors without pressure suits. Question is, being a molecule rather than atomic, how stable would it be under bombardment by solar particles or reaction with lunar soil?

According to wikipedia sulfur hexaflouride is terribly innert and quite stable - "estimated atmospheric lifetime of 800–3200 years". Not bad, (very nice out of box idea) but I still doubt it would be workable - it should be as hard to maintain as helium in Earth atmosphere.

Thanks for your input guys, extremely interesting ideas. Very much appreciated. :)