Gravity + atmosphere of the Moon

mithridates · Aug 20, 2006

Ask the astronomer might be an okay place to ask this question but I'm also interested in discussion about the topic as well - how much more mass would the Moon need to hold its own atmosphere, and would that be enough to cause any serious changes in tides over here? Just a theoretical question. I've always thought it was too bad that the Moon never got to be quite big enough to hold its own atmosphere even though its so large compared to most other moons around their planets. Were we to have moons like Phobos and Deimos at only 6000 km and 24000 km away for example (some 15 to 60 times closer than our moon is) we probably would have had colonies there already just because they would be so effortless to get to. <div class="Discussion_UserSignature"> ----- http://mithridates.blogspot.com </div>

3488 · Aug 20, 2006

Not only is this a matter of surface gravity, but of surface temperature. If the moon was orbiting Saturn, Uranus or Neptune, it would be able to hold on to a dense atmosphere (colder gassess are denser & there fore heavier). Titan orbiting Saturn has a surface gravity of about 14% that of Earth (slightly weaker than the moon's 16.7 %), dense atmosphere (1.6 Bars average). Mercury surface gravity 38% (Same as Mars), close to the Sun, no atmosphere to speak off. The same would apply to our moon. Andrew Brown. <div class="Discussion_UserSignature"> "I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before". Linda Morabito on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.http://www.launchphotography.com/ http://anthmartian.googlepages.com/thisislandearthhttp://web.me.com/meridianijournal </div>

harmonicaman · Aug 20, 2006

3488 - That's a great point! If the Moon were a free-floating object in outer space and not associated with a Solar System, it would likely have gathered a substantial atmosphere. Note that the atmosphere of the moon is so immaterial that it is postulated that the Apollo moon missions doubled the existing atmosphere of the moon.

mithridates · Aug 20, 2006

It's not temperature but distance from the Sun and the strength of the solar wind which knocks the atmosphere out into space. Your point is correct about Mercury vs. Titan but it's not temperature which does this. What I'm wondering though is exactly how much more gravity the moon would require in its present orbit to be able to keep an atmosphere. <div class="Discussion_UserSignature"> ----- http://mithridates.blogspot.com </div>

mithridates · Aug 20, 2006

Oh, looks like there's a similar thread here: http://uplink.space.com/showflat.php?Cat=&Board=businesstech&Number=510310&page=2&view=collapsed&sb=5&o=0&fpart= It's not exactly what I'm looking for though. I'm trying to find an exact number. Say for example, would the moon have an atmosphere if it were out at the distance Mars is at? And on the other hand would Mars have an atmosphere if it orbited at 1 AU? <div class="Discussion_UserSignature"> ----- http://mithridates.blogspot.com </div>

Saiph · Aug 20, 2006

actually, temperature is a key component in atmosphere retention. The reason is not that cold gases are denser, thus heavier (which isn't the case, the individual atoms are the same weight, the collective gas is the same weight, it just occupies a smaller volume). A higher temperature means a higher kinetic energy of the atoms in a gas (that is what temperature measures!) A higher kinetic energy means a higher velocity, as the mass of the atoms does not change. This higher velocity allows more of the gas to reach escape velocity, and escape the planet. Distance from the sun does drop the temperature, or at least the energy from the sun put into the system. Solar wind helps strip atmospheres away, but it isn't the main/only cause. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

mithridates · Aug 20, 2006

I stand corrected. <div class="Discussion_UserSignature"> ----- http://mithridates.blogspot.com </div>

spacester · Aug 20, 2006

. . . the Apollo moon missions doubled the existing atmosphere of the moon. I've been wondering for some time if that is or is not going to become an important 'environmental impact' in the future. IOW, if we set up shop on the moon to take advantage of the vacuum environment, how long before we degrade that environment and lose it? That's right folks, I'm talking about a Lunar Environmental Impact Policy. Sorry about that, but someone had to bring this up sooner or later. <img src="/images/icons/wink.gif" /> If you want to use the vacuum, then you want to conduct some process "in the open", right? And this process is going to involve what, casting metals? Crystal growth? Glassmaking? Various chemical reactions and industrial processes, right? So if you're doing these activities, how much 'out-gassing' (of whatever form) will we be able to get away with before we turn around and realize that the moon no longer has a 'perfect' (no such thing) vacuum? At what point have we polluted the vacuum so much that it no longer does what we want? Will there be lunar 'Vacuum Shed' policies similar to 'Air Shed' policies where the right to pollute is bought and sold? Will certain processes be 'banned'? How would that be enforced? Should NASA be leading the way in figuring those things out? I tried Googling 'Lunar Environmental Impact Policy ', I'll check if this post shows up in a few days or so, ain't the internet a hoot? . . . but oh look, right here at sdc we have a recent story, Space the Fragile Frontier by Mark Williamson, I had missed! (Im glas I found this before posting - this guy is way ahead of me and he's talking about irreversible effects. Was there a thread I missed as well?) Well then, one more set of not-so-rhetorical questions, for this forum: At what rate can we pollute the m <div class="Discussion_UserSignature"> </div>

3488 · Aug 21, 2006

I think that I could have worded my post better (I knew what I meant anyway). Of course the atoms making up the gassess would be the same 'weight' if at +200 C or -200 C, but of course at the higher temperature, the atoms would be moving much faster, thus occupying a larger space. I still stand by the statement that surface temperature plays a big part. The analogy of Titan vs Mercury is still good (Titan average surface temp -180 C, Mercury +407 C dayside, -186 C nightside, the greatest temperature extreme of the major planets). It seems strange that the night time temps on Mercury & the average on Titan are remarkably similar!! The moon has average daytime of +117 C, average night time -167 C, polar craters -210 C. Because the high daytime temperatures on both Mercury & the Moon (Surface gravities of 38% & 16.7% respectively), neither object can hold onto a substantial atmosphere at their current distances from the sun. Venus is different with its much higher surface gravity (89% that of Earth's) that despite the high temperatures (average +457 C) that Venus can keep a dense atmosphere of Carbon Dioxide (94 atmospheres) at mean ground level. Of course the history of the objects play a part. Why does Ganymede & Callisto not hold on to dense atmospheres. They are cold enough (-146 C) & have enough surface gravity at this temperature (Ganymede 17.8% of Earth's, Callisto 12.4 % of Earth's yet Titan, 14% of Earth's does)? Maybe Titan is geologically active & the atmosphere is being replenished, but Ganymede & Callisto are not, plus any substantial atmosphere they may have had in the past has been stripped away by Jupiter's magnetosphere (yet Ganymede has its own internally driven magnetosphere, the only moon in the solar system to have one, so blows away that second theory). Andrew Brown. <div class="Discussion_UserSignature"> "I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before". Linda Morabito on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.http://www.launchphotography.com/ http://anthmartian.googlepages.com/thisislandearthhttp://web.me.com/meridianijournal </div>

mithridates · Aug 21, 2006

I've seen terraforming proposals using a solar shield in between Venus and the sun at their Lagrange points; would a shield in between the Moon and the Sun then have an effect on the atmosphere? If the surface temperature were to suddenly become an even -167C or so, would enough time at that temperature allow it to eventually have an atmosphere considering the gases that emerge from it? I'm wondering if some sort of thin cloudy atmosphere would work; cloudy enough to keep out a lot of the solar energy but thick enough that the temperature stays much more uniform than it is right now. There must be a few proposals here and there on trying to create an atmosphere on the Moon. The best part about Titan is that with its low surface gravity and high atmospheric pressure, even a human with an artificial pair of wings can take off and fly just by flapping. <div class="Discussion_UserSignature"> ----- http://mithridates.blogspot.com </div>

mithridates · Aug 21, 2006

Hm, I just found something interesting: We often read in books that the Moon is an airless world. This is in fact not true at all. The Moon's atmosphere is a very delicate balance between various processes and may already have been altered by human action beyond all hope of recovery. Apollos 14-17 carried very sensitive experiments to measure the ambient lunar atmosphere at the surface, other experiments were carried in orbit in the SIMBAY of the Service Module. The surface experiments, consisting of Cold Ion Gauges in Apollos 14-16 and a Mass Spectrometer in Apollo 17, were so sensitive that leaks from the space suits of the astronauts saturated them every time that they approached to within a few metres. These experiments revealed that there is a very tenuous lunar atmosphere, which has a total mass of only about 10 000Kg (10 tonnes). Most of the atmosphere is made up of Hydrogen, Helium and Neon captured from the solar wind. There is also a component of gases from radioactive decay in the surface rocks: 10% of the Helium appears to come from alpha decay and 90% of the Argon (at 10000 atoms per cubic centimetre it is one of the major components of the atmosphere) comes from this source. Argon actually presents a problem because, given the known amount of radioactive Potassium 40 in the rocks, it requires all the Argon 40 produced from the decay of Potassium 40 in the top 5km of the lunar crust to be released very efficiently to the atmosphere to explain the amount of gas which is detected. Smaller amounts of Carbon Dioxide and Methane are seen, but the water vapour content of the atmosphere is negligible: around 0.5 molecules per cubic centimetre and that due to reactions of solar wind particles in the soil. Totalled over the entire lunar surface there is only around half a gram of water in the atmosphere. Obviously, as water was one of the main gases released by the landings, the problems of contamination are very severe when <div class="Discussion_UserSignature"> ----- http://mithridates.blogspot.com </div>

3488 · Aug 21, 2006

How about this? I had read somewhere that if the current lunar 'atmosphere' was condensed to a volume equal to a room 10 metres x 10 metres x 10 metres (One Cubic Kilometre) it would equal the average density of Earth's. Shows how tenuous it is. I had heard that the Apollo landers had thickened it, along with the seven pre Apollo Surveyor landers (1,3,5,6,7 were successful) had also contributed a little. I had not heard about the micro meteoroid record. I knew that was one of the goals of Apollo, but had not seen any results. How long ago do scientists reckon that the moon's atmosphere was 10 million times denser than now? Perhaps when the moon had active volcanoes? If so about 3.8 billion years ago, perhaps? What was its likely composition & what happened to it? Andrew Brown. <div class="Discussion_UserSignature"> "I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before". Linda Morabito on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.http://www.launchphotography.com/ http://anthmartian.googlepages.com/thisislandearthhttp://web.me.com/meridianijournal </div>

spacester · Aug 21, 2006

<img src="/images/icons/cool.gif" /> Thanks for all the info, gents! Keep it coming if you are so inclined! <div class="Discussion_UserSignature"> </div>

ittiz · Aug 21, 2006

According to my rough estimates the Moon could hold an atmosphere of SF6 (a stable, inert gas) at one bar of pressure for an indefinite period of time. That is if it's protected from solar wind. Light gasses like N2 or O2 would float off even without solar wind. If you wanted to terraform the moon you could put a atmosphere of SF6 to make the pressure and temp comfortable. Then continuously replenish the O2. Some mechanism based on genetic engineering of microbes or maybe chemical methods could be used to keep the O2 from flying away. Either way solar wind would still be an issue.

yevaud · Aug 21, 2006

http://sirius.bu.edu/planetary/moon.html (Mike Mendillo was one of my Professors, lo those many moons ago) <div class="Discussion_UserSignature"> Differential Diagnosis: "I am both amused and annoyed that you think I should be less stubborn than you are." </div>

vogon13 · Aug 21, 2006

Yoiks! 14.7 pounds of sulfur and flourine (in 1/6th e lunar gravity!) per square inch (!!!!!!!!) of lunar surface. That's a big job, even if you don't lose any gas at all in the process of setting it up. Big, big job! Egad, that's a big job. My goodness gracious, how could anyone ever hope to move/process that much material? Big, big, big, big job . . . . . . . <div class="Discussion_UserSignature"> TPTB went to Dallas and all I got was Plucked !!So many people, so few recipes !!Let's clean up this stinkhole !! </div>

ittiz · Aug 21, 2006

Not as big as terraforming Venus. Also the gas should stay fairly well put (about as well put as N2 on Titan) during the Lunar day in vacuum conditions. The job would be huge and would probably require Von Neuman (self replicating) machines to complete in a practical way. Also you probably wouldn't want a full bar of pressure. SF6 is the best insulating gas known to man so you would probably need much less than a bar to make the surface comfortable. If a full bar was used the moon surface temp would probably turn into venus like conditions.

silylene old · Aug 22, 2006

Well, if we are going to conjecture a fanatsy world of a stable atmosphered-moon, I think UF6 would also be an interesting gas to create our future lunar atmosphere. In fact, if we could raise the UF6 atmospheric pressure to about 2.5 atm, then we could make lunar lakes of liquid UF6 when it is sunlit. We would need to fill in those pesky forever-dark polar lunar craters, so they don't become suck up too much of our newly condensed lunar atmosphere. I wonder if the natural radioactivity of the U will generate enough heat to keep those lakes thawed even during the lunar nights.... below is the phase diagram for UF6: <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center">- - - - - - - - - - - - - - - - - - - - - - </div><div class="Discussion_UserSignature" align="center">I really, really, really miss the "first unread post" function. </div> </div>

mithridates · Aug 22, 2006

Yeah, and it wouldn't require a full bar of pressure right away - the atmospheric pressure at the top of Mt. Everest for example is only one third that of the surface (boiling point there is 69C). If I remember correctly the biggest danger with low pressure is that one's blood starts to boil, so as long as the boiling point is kept at least 50 C then I *think* people should be able to go outside just with breathable air in tanks, but without pressure suits. I think a fifth of the air pressure on Earth should be enough. <div class="Discussion_UserSignature"> ----- http://mithridates.blogspot.com </div>

3488 · Aug 22, 2006

I think that giving the moon a dense atmosphere with current processes is a non starter, much like terraforming Mars. The moon & Mars are the way they are for several different reasons. Human intervention will not change the underlying reasons. The moon's artificial atmosphere & a terraformed Mars would have to be maintained. However it is an interesting excercise in planetary chemistry. I would be interested to know, when it was thought that the moon's atmosphere was 10 million times denser than now? Also what happened to it (was it driven away when the sun slowly brightened, or blasted away during large impacts)? <div class="Discussion_UserSignature"> "I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before". Linda Morabito on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.http://www.launchphotography.com/ http://anthmartian.googlepages.com/thisislandearthhttp://web.me.com/meridianijournal </div>

ittiz · Aug 22, 2006

I considered UF6 but I discounted it because of radioactivity. Also it reacts with water to make hydrofluoric acid and free uranium. I looked at a bunch of different gasses and SF6 seems to be the best choice. There are other gasses that are heavier and inert but break down in a few months or years. Another gas almost as heavy (but worth it's weight in gold) is Xenon, but it has some unexplained effects on human physiology kind of like nitrous oxide. Using SF6 would also be good for the lunar night because it retains heat so well. The forever dark craters probably wouldn't be a problem. Firstly light would be scattered by the atmosphere into them, also because of the moon's slow rotation there would only be one major hadley cell in each hemisphere so the whole moon would have a fairly temperate climate like the middle latitudes of earth. Because of it's heat retention abilities it might even be possible to use a thicker atmosphere of it to terraform Jupiter's moons. Also because of it's mass the dense gas would block radiation from Jupiter's radiation belts much better than other gasses. 3488: The reason why these bodies don't contain gasses like SF6 and CFCs is because they aren't natural gasses. The serious issue with a body retaining a gas is solar wind stripping the upper layers off. That is probably why Mars has no N2. It appears Mars might have retained it's thicker atmosphere for a billion years or so. Plenty of time in human terms and it might be possible to engineer a biological system that would replenish the atmosphere. Something would be needed for SF6 (at least for a long term fix) because lightning breaks it down to F2 and SF4 (both highely toxic). A biological system could probably be engineered to oxidize SF4 with F2 to make SF6.

silylene old · Aug 22, 2006

Also it reacts with water to make hydrofluoric acid and free uranium. Actually, UF6 + H20 - /> HF + UO2F2 (uranyl flouride) I will leave it to the reader to balance the equation. There isn't any water on the moon, so UF6 should be stable to hydrolysis. I wasn't considering terraforming, rather I was considering what would be interesting from a hypothetical point of view. UF6 is of course highly toxic. The possibility of UF6 lakes intrigued me. For the matter of SF6, it is reactive to high energy light and to plasmas, making S2F10. S2F10 is highly dangerous neurotoxin. I would worry that S2F10 would be created by interaction with the solar winds and solar far UV irradiation. So I don't think SF6 would be a good terraforming gas, unless you had some type of S2F10 scrubber. <div class="Discussion_UserSignature"> <div class="Discussion_UserSignature" align="center">- - - - - - - - - - - - - - - - - - - - - - </div><div class="Discussion_UserSignature" align="center">I really, really, really miss the "first unread post" function. </div> </div>

ittiz · Aug 22, 2006

"Actually, UF6 + H20 -> HF + UO2F2 (uranyl flouride)" Oh sorry, I Assumed it was free uranium based of the health hazard stuff I read about it. Some kind of system would have to be engineered to remove both the break down products (SF4 and F2) and reaction product (S2F10) of SF6. For a few reasons I prefer the idea of using biological systems to do that.

mithridates · Sep 6, 2006

I have two more thoughts on the subject: Moon dust is really sharp, as there is pretty much no atmosphere and so it just sits there without rubbing against other dust particles and softening up like on Earth. How much atmosphere would be necessary to bring about this process, and would it be worth it? Apparently Moon dust can be a bit of a hazard if it gets into cracks or parts of ones suit, and it might be nice to find a way to reduce that. How much atmosphere would be needed for that? Also, as it has been written that the Apollo missions had effectively increased the atmosphere by a factor of ten due to exhaust, how much more would it increase assuming a permanent colony of some 25 people or so for a decade? Assuming there's no concerted effort to pump air from a colony to make an atmosphere, but just regular expelling of gases through driving rovers across the surface, other regular activities, etc. <div class="Discussion_UserSignature"> ----- http://mithridates.blogspot.com </div>

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