Wikipedia has a number of interesting images on its terraforming page showing what various worlds (Mars, Venus and the Moon) would look like at partial and full stages of terraforming, and the image to the right there is the one showing what the Moon would look like if fully terraformed.



The problem with the Moon though is that its gravity is so low that the solar wind would be able to soon blow the atmosphere back out into space. What the word soon means though is hard to say - apparently an atmosphere created there would get blown away in a few thousand years, which is a blink of an eye in geological terms but quite long from a practical point of view. Still, the Moon is still huge and terraforming it is simply beyond our capabilities.



That's where the idea of paraterraforming comes in. This is basically the building of a greenhouse-like environment that humans could live in from the start, which could then be expanded bit by bit as a colony grows. Eventually an environment could be large enough that those living on the Moon could have a great amount of freedom to do the things we take for granted on Earth such as going for long walks, driving, getting lost, whatever. And in fact, even the capital of the Maldives (Malé) is a similar environment to this, with an area of only 5.798 km² for a population of 103,693. It looks like this:



(larger image here)



This shows just how many people can live in a small area. On the Moon the first places that we will need to colonize will be the peaks of eternal light, where we will not to deal with a 14-day night. In these locations it will be possible to collect solar energy in an environment with no darkness whatsoever, except when an eclipse occurs and the Earth blocks the Sun for a short time.



In fact, one of the reasons for paraterraforming is that any colonization really is a type of paraterraforming, considering that humans can only survive in earthlike environments, and a continually growing colony will simply be a small version of it. Also, complete terraforming is an extremely long-term project, and it is very unlikely that humans on the surface will be interested in staying in a tiny area while working towards a goal that they will never live to see; no, they will be much more interested in building a new greenhouse, constructing an indoor park, getting more spacious quarters, and so paraterraforming really is the only way to go.



However, there is one problem: the rest of the Moon will have no atmosphere at all. This is no problem in living within and expanding on a colony, but the lack of atmosphere means that at any time a tiny meteor could impact the surface, and protection against this might not be an easy task. And even if the colony is fortified against meteorites there will always be trips made outside the colony to explore parts of the Moon and gather resources, and it's possible that a person could be struck by an undetectable meteorite just a few millimetres in diameter, and that could be fatal.



So why not adopt a middle of the road approach? The creation of an Earth-like atmosphere may be impossible in the beginning, but what about an extremely thin one? Even a very thin atmosphere would provide protection against the smallest of meteorites, which are the most dangerous since they are nearly impossible to detect. An atmosphere just thick enough to create weather patterns would also help to alleviate the problem of moon dust, which is extremely sharp due to the lack of wind. Dust on Earth is constantly being pushed around, and this constant pummeling acts like a kind of rock grinder to smooth it out. Considering the colossal size of the Moon, it would probably be in our best interests to create a small atmosphere of this sort that could move the moon dust around without us needing to do anything, and this would likely remove the problem in a short time. It would also moderate the temperature extremes on the Moon to a certain extent, which would make it easier to work outside the peaks of eternal light as the nights would be a tiny bit less extreme. Instead of simply having light areas that are hot and dark areas that are cold, the winds would cool down the light areas next to the dark ones, and warm up the dark areas next to the light.



So how much atmosphere would be needed? This would depend on two factors for the above needs: 1) the minimum amount needed to provide protection against the smallest of micrometeorites, and 2) the minimum amount needed to create simple weather patterns. Mars has a surface area 3.83 times that of the Moon and an atmosphere 25 teratons in mass, so for an atmosphere with the same thickness as Mars it would have a total mass of 6.5 teratons, compared to the 5148 teratons for our atmosphere. That's still far beyond our capacity.



However, Neptune's moon Triton provides an example of a location with a much thinner atmosphere that still has weather patterns: the atmosphere extends to 800 km above the surface, there are clouds a few km above the surface, high altitude prevailing winds are located 8 km above the surface, and are capable of moving material of over a micrometre in size. It has a pressure of only 14 microbars, compared to Mars with 6 millibars, so even an atmosphere 430 times thinner than that of Mars is capable of creating weather, and that's good news. That lowers the amount required to 15.1 gigatons (15.1 billion tons). Compare that to the total amount of CO2 emissions emitted by the US over a year: 6 billion tons. So if the total CO2 emissions from the United States could be shipped directly to the Moon, within a few years we would start to see some simple weather patterns. It looks like the creation of an extremely thin atmosphere on the Moon would be a possibility then, though certainly not for quite some time.



Also keep in mind that with continued colonization of the Moon this creation of a thin atmosphere is probably a bit of an inevitability, due to the fact that human settlement and industry results in gases being created. So perhaps it's just a question of when, not if. The creation of a thin atmosphere sounds quite far off when imagining a colony with only a dozen or so members, but when you expand it to 100,000 suddenly it doesn't seem like such a far-fetched idea.



Finally, one other interesting point: though the Moon is the brightest object in the sky it only has an albedo of 0.12, meaning that it's actually quite a dark object. Compare that to Venus which has the highest albedo at 0.76, which is due to the extensive cloud cover above the surface. Earth has an albedo of 0.39. Any clouds present on the Moon would result in a higher albedo, so it would look a bit different in the sky if there was enough cloud cover to accomplish this.





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Edit: one timely article to read here. Apparently there is going to be a press conference later today to report on India's Chandrayaan-1 probe, and the press conference is supposed to announce that a great deal of water has been confirmed on the Moon. We'll find out soon enough.