If we terraformed MUN (or the real moon)

[kerbiloid]

6 hours ago, Bill Phil said:

14 hours ago, Nathair said:

If the moon could hold onto an atmosphere it would have one.

It technically does...

Every moon has such atmosphere which it deserves.

9 hours ago, MichaelPoole said:

Physics do not work that way.

_{If I had a 1 money unit every time when they tell me so...}

(Silently watches at a close binary star picture...)  Of course, the upper gas layers disturbed by the Earth gravity would mostly dissipate aside, not fall down to the Earth. 
But anyway, lunar orbital speed at its upper atmosphere height will be comparable to the heat speed of molecula.

Edited January 26, 2017 by kerbiloid

[MichaelPoole]

11 hours ago, KerikBalm said:

We had a forums thread like this before.

The rate at which the atmosphere would last is about a million times less than that....

I have never seen such low estimates. The lowest ones I have seen are in thousands of years which is long for a human. Usual ones are around 10k-100k years, top one I have seen was 100 million.

3 hours ago, kerbiloid said:

Every moon has such atmosphere which it deserves.

_{If I had a 1 money unit every time when they tell me so...}

(Silently watches at a close binary star picture...)  Of course, the upper gas layers disturbed by the Earth gravity would mostly dissipate aside, not fall down to the Earth. 
But anyway, lunar orbital speed at its upper atmosphere height will be comparable to the heat speed of molecula.

Binary stars and the Earth-Moon system are not really comparable,

[kerbiloid]

Ok. Earth atmosphere temperature ~1000 K (for the upper atmosphere, as we need the fastest molecula to escape). 
Heat speed = sqrt(3 * 8.31441 * 1000 / 0.029) ~= 900 m/s.

Earth exosphere height ~ 1000 km. Moon equivalent = 1000 * 6 = 6000 km. Radius = 1735+6000 ~= 8000 km.
Escape velocity = sqrt(2 * 1.62 * 1735000² / 8000000) = 1100 m/s.

So, on the upper bound of a dense lunar atmosphere the heat speed is almost equal to the escape speed (900 vs 1100 m/s), which means the gas will flow away as from a fan.

Moon orbital speed ~ 1 km/s. So, most of the gas escaped the Moon gravity (losing its potential energy) will stay in the Earth gravity.

[78stonewobble]

19 hours ago, Nathair said:

If the moon could hold onto an atmosphere it would have one.

Over hundreds of millions of years to billions of years? Yes...

But we could possibly settle for tens to hundreds of thousands of years of settlement.

...

Even if it is practically impossible to do and/or just a silly investment of ressources as compaired to orbital habitats / mars / whatever.

 

 

Edited January 26, 2017 by 78stonewobble

[kerbiloid]

Earth to Moon surface ratio = (6.371 / 1.735)2 = 13.5

Uniform atmosphere height (see above) 50 km : 8 km. Ratio 6:1

So, total uniform volume of the Moon atmosphere to keep Earth-like conditions = 6/13.5 ~= 0.5 Earth atmosphere uniform voulme.

This means: a human-friendly Moon atmosphere must have mass ~= 0.5 Earth atmosphere mass.

Total Moon surface to be used (minus artificial ocean) somewhat less than just Africa.

Edited January 26, 2017 by kerbiloid

[Nathair]

30 minutes ago, 78stonewobble said:

Over hundreds of millions of years to billions of years? Yes...

But we could possibly settle for tens to hundreds of thousands of years of settlement.

 

Even if it would take about a thousand years to go from an Earth style atmosphere to the current Lunar exosphere, that's not the problem. Even though we would find the atmosphere useless to our purposes long before it reached current conditions, that's not the problem. (Half an atmosphere isn't going to help us much.) Even though the rate at which the atmosphere bled away would be highest at the start and so the breaking point for our purposes would be quite early in that curve rather than dozens or hundreds of years down the road, that is still not the problem. Even though the elements of the atmosphere would vanish at different rates, changing the composition as it went, that's still not the biggest problem. The problem is that if you installed, somehow, an Earth style atmosphere on the Moon it would take (without some sort of roof to hold it in) a matter of minutes (or seconds) for it to inflate to the point that the surface conditions were comparable to standing at the top of Everest. So although it might take a hundred years to blow off enough atmosphere to make it completely useless for our purposes the fairly rapid loss of atmosphere would not be as important as the almost instant rarefaction of the atmosphere.

[78stonewobble]

23 minutes ago, Nathair said:

Even if it would take about a thousand years to go from an Earth style atmosphere to the current Lunar exosphere, that's not the problem. Even though we would find the atmosphere useless to our purposes long before it reached current conditions, that's not the problem. (Half an atmosphere isn't going to help us much.) Even though the rate at which the atmosphere bled away would be highest at the start and so the breaking point for our purposes would be quite early in that curve rather than dozens or hundreds of years down the road, that is still not the problem. Even though the elements of the atmosphere would vanish at different rates, changing the composition as it went, that's still not the biggest problem. The problem is that if you installed, somehow, an Earth style atmosphere on the Moon it would take (without some sort of roof to hold it in) a matter of minutes (or seconds) for it to inflate to the point that the surface conditions were comparable to standing at the top of Everest. So although it might take a hundred years to blow off enough atmosphere to make it completely useless for our purposes the fairly rapid loss of atmosphere would not be as important as the almost instant rarefaction of the atmosphere.

Which is basically what kerbiloid calculated... But is it actually impossible to add enough atmosphere to keep surface livable for eg. thousands, tens of thousands or hundreds of thousands of years?

Even if actually adding so much atmoshere and/or continiously replenishing it is practically impossible.

[Kerbart]

Mass is not your only problem, the lack of a magnetic field is another. That seems to be the major reason Mars doesn't have much of an atmosphere anymore.

[Nathair]

29 minutes ago, 78stonewobble said:

is it actually impossible to add enough atmosphere to keep surface livable for eg. thousands, tens of thousands or hundreds of thousands of years?

 

Without a roof, yes. As I said the dense-enough atmosphere would expand into a huge "bubble" of rarefied gas pretty much instantly. It would then rapidly blow away, but the instant rarefaction makes it useless for our purposes immediately.

Since roofing the planet is an impractical notion (to say the least) we would just roof the areas in which we actually need to have an atmosphere. It's not like we'd colonize the Moon or Mars and instantly need to climate control every square centimetre of the surface, just enough of it to live and work in. There's really no escaping (escaping, get it?) the fact that it's gravity that holds the air down and Moon or Mun there just isn't enough G to do the job.

[78stonewobble]

26 minutes ago, Nathair said:

Without a roof, yes. As I said the dense-enough atmosphere would expand into a huge "bubble" of rarefied gas pretty much instantly. It would then rapidly blow away, but the instant rarefaction makes it useless for our purposes immediately.

Since roofing the planet is an impractical notion (to say the least) we would just roof the areas in which we actually need to have an atmosphere. It's not like we'd colonize the Moon or Mars and instantly need to climate control every square centimetre of the surface, just enough of it to live and work in. There's really no escaping (escaping, get it?) the fact that it's gravity that holds the air down and Moon or Mun there just isn't enough G to do the job.

Well, if it's that quick... then yeah.

[KerikBalm]

10 hours ago, MichaelPoole said:

I have never seen such low estimates. The lowest ones I have seen are in thousands of years which is long for a human. Usual ones are around 10k-100k years, top one I have seen was 100 million.

Found the thread from last year:

 

So basically... if we assume a temperature profile at the exosphere as on earth (same gas composition, and same distance from the sun, so seems valid), the exosphere is lost instantaneously. The layers below it become the new exosphere... and so on. A significant fraction of the atmosphere is lost each hour... 

But then cantab rightfully pointed out that this evaporation of the atmosphere takes energy away and cools the outer layers, and the proper way to look at this is in terms of energy balance.

Quote

The Moon receives about 10¹⁶ W of power from the Sun. (Solar constant times Moon's cross-sectional area).

To warm 1 kilo of oxygen gas from ~300 to ~1000K takes around 10⁶ J of energy.

Therefore solar irradiance limits the Moon to losing around 10¹⁰ kg of gas per second. There just isn't the heat available to go faster.

Now if our terraformed Moon has an atmosphere like Earth's scaled according to its surface area, that's around 10¹⁷ kg. Which means it can't escape quicker than 10¹⁰ seconds.

And that's 300 years.

Now in this particular case, there was a pure O2 atmosphere assumption. If we also allow for water vapor and N2, those molecules have a MW of 18 and 28 respectively, compared to 32 for O2. They escape much faster.

Water gets lost pretty fast. No N2 means no nitrogen fixation, and not much plant growth... not very terraformed.

Of course as pointed out on this thread, cantab's assumption was that the moon's atmosphere mass was scaled relative to its surface area - this results in lower pressure, but in that thread there was an assumption that 0.2 atmospheres was fine if it was pure O2 - the terraforming definition was just that you could EVA without a spacesuit and not die.

Now the surface area of the moon is ~7.5% of Earths. But as pointed out, the mass of the atmosphere for 1 atm would need to actually be ~50% of Earth's.  So that 300 years to escape increases to ~2,000 years....

but wait.... thats far too long, because you'd never want a 100% O2 atmosphere(extreme combustion hazard). So you either start from 20% (can get out and breath, not really terraformed) - 500 years, or you use other lower MW gasses like N2, which also escape much faster. Lets say complete loss in about 1,000 years

And as also pointed out... just because the atmosphere hasn't disappeared completely, doesn't mean it hasn't changed enough that its a major problem. Even 10%, and you'd be having major ecosystem collapse. 50%, and you'd be having it become uninhabitable for people.

Atmosphere maintenance would be a problem for each generation.

This is not a problem that plays out over the course of hundreds of millions of years, as was said earlier in this thread... its a problem that leads to complete catastrophe on the order of hundreds of years, and needs maintenance on the order of decades.

 

[kerbiloid]

Or they should cover the Moon with a multistorey layer of megabubbles filled with air.

Spoiler