# How long could the Moon retain an atmospherere? **URL:** \ **Category:** Factual Questions\ **Created:** [July 19, 2015, 6:17pm UTC](https://boards.straightdope.com/t/how-long-could-the-moon-retain-an-atmospherere/725625 "2015-07-19T18:17:13Z")\ **Posts on this page:** 15\ **Page:** 1
**Author:** ![Grim\_Render](https://avatars.discourse-cdn.com/v4/letter/g/a87d85/32.png) [@Grim\_Render](https://boards.straightdope.com/u/Grim_Render)\ **Post date:** [July 19, 2015, 6:17pm UTC](https://boards.straightdope.com/t/how-long-could-the-moon-retain-an-atmospherere/725625/1 "2015-07-19T18:17:13Z")
Assuming Alien Space Bats decide to terraform the Moon, including spinning up its core to generate a protective magnetosphere, spinning up the entire body to give it a 24-hour day/night cycle and carving out ocean basins to fill with seas on the surface. How long would the Moon retain an earth-density atmosphere before its lower gravity and tidal tugs allowed the escape of the air? At what point would the pressure drop below say, the equivalent to the South American Altiplano? Bonus question: Assuming the Moon could be given an earth-strength magnetosphere, would the two interact in any way? Auroras etc? ---
**Author:** ![Hail\_Ants](https://avatars.discourse-cdn.com/v4/letter/h/dc4da7/32.png) [@Hail\_Ants](https://boards.straightdope.com/u/Hail_Ants)\ **Post date:** [July 20, 2015, 12:00am UTC](https://boards.straightdope.com/t/how-long-could-the-moon-retain-an-atmospherere/725625/2 "2015-07-20T00:00:23Z")
Not exactly sure how to answer. Assuming you’re a god-like Q being from Star Trek and could ‘will’ a dense atmosphere into existence around the Moon, it would instantly expand out into space as the Moon’s gravity is far to weak to counter the dense air’s molecular pressure against itself. ---
**Author:** ![OldGuy](https://avatars.discourse-cdn.com/v4/letter/o/3bc359/32.png) [@OldGuy](https://boards.straightdope.com/u/OldGuy)\ **Post date:** [July 20, 2015, 12:08am UTC](https://boards.straightdope.com/t/how-long-could-the-moon-retain-an-atmospherere/725625/3 "2015-07-20T00:08:47Z")
I don’t think that’s correct. It certainly wouldn’t instantly vanish. Gasses escape the atmosphere when the molecules have a speed higher than the planet’s escape velocity. The velocity of the molecules will have a range of velocities that can be described statistically. The average depends on the temperature, so can Q make it real cold, too? ---
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [July 20, 2015, 12:12am UTC](https://boards.straightdope.com/t/how-long-could-the-moon-retain-an-atmospherere/725625/4 "2015-07-20T00:12:02Z")
Q could alter the gravitation constant … ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [July 20, 2015, 12:17am UTC](https://boards.straightdope.com/t/how-long-could-the-moon-retain-an-atmospherere/725625/5 "2015-07-20T00:17:34Z")
[Is There an Atmosphere on the Moon?](https://www.nasa.gov/mission_pages/LADEE/news/lunar-atmosphere.html#.Vaw9xounlnI) ---
**Author:** ![XT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/xt/32/456_2.png) [@XT](https://boards.straightdope.com/u/XT)\ **Post date:** [July 20, 2015, 12:27am UTC](https://boards.straightdope.com/t/how-long-could-the-moon-retain-an-atmospherere/725625/6 "2015-07-20T00:27:46Z")
> [@Grim\_Render](#): > > Assuming Alien Space Bats decide to terraform the Moon, including spinning up its core to generate a protective magnetosphere, spinning up the entire body to give it a 24-hour day/night cycle and carving out ocean basins to fill with seas on the surface. > > How long would the Moon retain an earth-density atmosphere before its lower gravity and tidal tugs allowed the escape of the air? At what point would the pressure drop below say, the equivalent to the South American Altiplano? > > Bonus question: Assuming the Moon could be given an earth-strength magnetosphere, would the two interact in any way? Auroras etc? I don’t think there is much of a core TOO spin up, since it’s comprised mostly of Earth mantle rock IIRC. I seem to recall someone talking about this once, and the consensus (from my vague memories of the discussion) was that you COULD put an atmosphere on the Moon (crash a few large icy asteroids into it and there you go, water vapor atmosphere), but that it would have to be constantly maintained (which would take a hell of a lot of energy, since you won’t be able to keep crashing icy bodies into it if you want to live there), would never get up to the pressures needed to walk around in shirt sleeves regardless, and would basically be blown away by solar winds in a few centuries at most, so it wouldn’t be worth even trying it. Mars is another matter, but even there from what I understand you’d only get an atmosphere that would last a few hundred thousand or million years at most before it was gone again. ---
**Author:** ![Carl\_Pham](https://avatars.discourse-cdn.com/v4/letter/c/e9a140/32.png) [@Carl\_Pham](https://boards.straightdope.com/u/Carl_Pham)\ **Post date:** [July 20, 2015, 8:50am UTC](https://boards.straightdope.com/t/how-long-could-the-moon-retain-an-atmospherere/725625/7 "2015-07-20T08:50:02Z")
Probably quite a while. The typical velocity of an N2 molecule at 300K, what we enjoy down here on the Earth’s surface, is about 500 m/s, which is significantly lower than the Earth’s escape velocity of 11.2 km/s. More importantly, it is still significantly lower than the Moon’s escape velocity, which is about 2.4 km/s. So if you covered the Moon in a nice thick nitrogen-oxygen atmosphere at 300K, so you have a shirt-sleeve environment, the typical molecule will stay put, right there, since it is moving about 1/4 as fast as it would need to escape the Moon’s gravity. However…you have two problems. First, not all of the molecules will have the same velocity. There is a distribution (called the Maxwell-Boltzmann distribution) of velocities. Some will have lower, some higher, and some much higher – indeed, above escape velocity, so they can indeed escape the Moon’s gravity. That will be a very small percentage, since they’d need to have a velocity 4x the average, and as anyone knows from taking exams, it’s hard to get _way_ above the average. Nevertheless, this represents a tiny steady loss to space. It’s not too hard to calculate the fraction that is at risk of being lost to space, but to calculate the actual loss rate you have to know how fast the gas is transported. You could probably do it with some crude assumptions, maybe just assuming above the critical temperature it does a free expansion and is immediately replaced. More importantly, quite possibly, is that molecules that reach high enough above the Moon will interact with the Sun, which will ionize them, heat them considerably, and whack them with the Solar Wind. All of these things will tend to blow away the atmosphere, just as it does with comets and such. This would be much harder to estimate, I think, but then that could be because I don’t know as much solar physics. Either process strikes me as something that would take a fairly short time on the cosmic or geological time scale, but quite long on the human – say, in the millions of years. ---
**Author:** ![AK84](https://avatars.discourse-cdn.com/v4/letter/a/85e7bf/32.png) [@AK84](https://boards.straightdope.com/u/AK84)\ **Post date:** [July 20, 2015, 12:18pm UTC](https://boards.straightdope.com/t/how-long-could-the-moon-retain-an-atmospherere/725625/8 "2015-07-20T12:18:57Z")
I thought that Luna **did** have an extremely thin atmosphere, which complicated Apollo landing and lift offs? ---
**Author:** ![Hermitian](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hermitian/32/470_2.png) [@Hermitian](https://boards.straightdope.com/u/Hermitian)\ **Post date:** [July 20, 2015, 1:56pm UTC](https://boards.straightdope.com/t/how-long-could-the-moon-retain-an-atmospherere/725625/9 "2015-07-20T13:56:51Z")
> [@Carl\_Pham](#): > > More importantly, quite possibly, is that molecules that reach high enough above the Moon will interact with the Sun, which will ionize them, heat them considerably, and whack them with the Solar Wind. All of these things will tend to blow away the atmosphere, just as it does with comets and such. This would be much harder to estimate, I think, but then that could be because I don’t know as much solar physics. One interesting thing I saw this morning on [Ars](http://arstechnica.com/science/2015/07/first-pluto-data-reveals-lots-of-terrain-that-is-not-easy-to-explain/) is that they think that Pluto looses 500 tons of Nitrogen _an hour_ due to solar wind. Considering how really really far Pluto is away from the Sun, I found that a bit surprising. I assume what the moon would loose would be far higher. However, looking at [Wikipedia](https://en.wikipedia.org/wiki/Atmospheric_escape), things may not be so simple. ---
**Author:** ![Ludovic](https://avatars.discourse-cdn.com/v4/letter/l/7ab992/32.png) [@Ludovic](https://boards.straightdope.com/u/Ludovic)\ **Post date:** [July 20, 2015, 2:11pm UTC](https://boards.straightdope.com/t/how-long-could-the-moon-retain-an-atmospherere/725625/10 "2015-07-20T14:11:06Z")
> [@Hermitian](#): > > Pluto looses 500 tons of Nitrogen _an hour_ due to solar wind. Considering how really really far Pluto is away from the Sun, I found that a bit surprising. Definitely. I wonder what the mechanism is by which Pluto actively looses the gas into outer space. I bet the amount would be lower if it were only stripped by the solar wind. ---
**Author:** ![alphaboi867](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/alphaboi867/32/3898_2.png) [@alphaboi867](https://boards.straightdope.com/u/alphaboi867)\ **Post date:** [July 20, 2015, 11:30pm UTC](https://boards.straightdope.com/t/how-long-could-the-moon-retain-an-atmospherere/725625/11 "2015-07-20T23:30:15Z")
IIRC this happened in an episode of **Space:1999** ; mysterious aliens gave the Moon a breathable atmosphere, increased it’s gravity, & even started a rain cycle. Then after a few days everything went back to normal within a few days after the Moon leaves orbit. I don’t remember if their reasons were explained, but remember thinking it was really bizarre that Moonbase Alpha was built with windows that could be opened. ---
**Author:** ![Quartz](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/quartz/32/267_2.png) [@Quartz](https://boards.straightdope.com/u/Quartz)\ **Post date:** [July 21, 2015, 12:09am UTC](https://boards.straightdope.com/t/how-long-could-the-moon-retain-an-atmospherere/725625/12 "2015-07-21T00:09:10Z")
> [@Hermitian](#): > > One interesting thing I saw this morning on [Ars](http://arstechnica.com/science/2015/07/first-pluto-data-reveals-lots-of-terrain-that-is-not-easy-to-explain/) is that they think that Pluto looses 500 tons of Nitrogen _an hour_ due to solar wind. Considering how really really far Pluto is away from the Sun, I found that a bit surprising. Likewise, although the article says _up to_ 500 tons / 450 tonnes. Assume 10,000 tonnes per day, so 3,650,000 tonnes per year - call it 3.5MT. 4.5 billion years, that’s 15.75 x 10^15 Tonnes. Now, the mass of Pluto is currently thought to be 1.3x 10^19 Tonnes, so that’s approx 0.1% of its total mass over the ages - quite significant. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [July 21, 2015, 1:02am UTC](https://boards.straightdope.com/t/how-long-could-the-moon-retain-an-atmospherere/725625/13 "2015-07-21T01:02:48Z")
> [@AK84](#): > > I thought that Luna **did** have an extremely thin atmosphere, which complicated Apollo landing and lift offs? The Moon has an extremely thin atmosphere, but it’s mainly atoms (mostly helium) from the solar wind that strike the surface and are sufficiently slowed that they hang around for a little while. Way too thin to influence the Apollo landings/lift-offs Come to think of it, the Apollo landings did change the lunar atmosphere. Before they made a moonwalk, they had to depressurize the lunar lander. There were no vacuum pumps on the landers; they just released the air to the outside. So every time they did this, it gave the Moon a temporary albeit very thin atmosphere. > [@Quartz](#): > > Assume 10,000 tonnes per day, so 3,650,000 tonnes per year - call it 3.5MT. 4.5 billion years, that’s 15.75 x 10^15 Tonnes. Now, the mass of Pluto is currently thought to be 1.3x 10^19 Tonnes, so that’s approx 0.1% of its total mass over the ages - quite significant. Pluto’s loss is probably not linear. I expect it loses more when it’s near perihelion (as now) than when it’s further from the Sun. ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [July 21, 2015, 2:08pm UTC](https://boards.straightdope.com/t/how-long-could-the-moon-retain-an-atmospherere/725625/14 "2015-07-21T14:08:03Z")
> [@Carl\_Pham](#): > > Probably quite a while. The typical velocity of an N2 molecule at 300K, what we enjoy down here on the Earth’s surface, is about 500 m/s, which is significantly lower than the Earth’s escape velocity of 11.2 km/s. More importantly, it is still significantly lower than the Moon’s escape velocity, which is about 2.4 km/s. So if you covered the Moon in a nice thick nitrogen-oxygen atmosphere at 300K, so you have a shirt-sleeve environment, the typical molecule will stay put, right there, since it is moving about 1/4 as fast as it would need to escape the Moon’s gravity. > > However…you have two problems. First, not all of the molecules will have the same velocity. There is a distribution (called the Maxwell-Boltzmann distribution) of velocities. Some will have lower, some higher, and some much higher – indeed, above escape velocity, so they can indeed escape the Moon’s gravity. . All true. Here is how to get an estimate of loss rate of atmosphere. Look at that distribution. Look at the area under the curve that is faster than the escape velocity for Mar’s gravity. Look at the area under the curve that is faster than the escape velocity of the moon. Compare those two areas. Lets say one is 10 times the area of the other (just making this number up, somebody else will have to figure out the right value). Now, throw out a number for how long Mars kept a decent atmosphere. From recent random Mars stuff read here and there, it sounds like it was on the order of hundreds of millions of years. Take that number and divide by the first number. Yeah, its crude, but it is a start. ---
**Author:** ![Ethilrist](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ethilrist/32/4604_2.png) [@Ethilrist](https://boards.straightdope.com/u/Ethilrist)\ **Post date:** [July 21, 2015, 2:31pm UTC](https://boards.straightdope.com/t/how-long-could-the-moon-retain-an-atmospherere/725625/15 "2015-07-21T14:31:51Z")
> [@alphaboi867](#): > > IIRC this happened in an episode of **Space:1999** ; mysterious aliens gave the Moon a breathable atmosphere, increased it’s gravity, & even started a rain cycle. Then after a few days everything went back to normal within a few days after the Moon leaves orbit. I don’t remember if their reasons were explained, but remember thinking **it was really bizarre that Moonbase Alpha was built with windows that could be opened.** That’s the point where the series got bizarre? Really? You mean, after a nuclear waste dump detonated and accelerated the moon to escape Earth orbit, you thought “Huh… those windows open. Weird…”