Given that deep craters on Mercury's polar regions could provide protection from the searing heat and radiation of the closer Sun, a colony on Mercury could be possible. For any colony to be sustainable, it would need to be as self sufficient as possible.
So, would there be enough natural resources on Mercury itself to sustain the needs of a human colony?
The MESSENGER mission is giving us a very tantalizing picture of Mercury's resources. No surprise is that much of the surface is basaltic minerals containing about 42% Oxygen on average by weight. Iron is notably diminished at about 1.5% - odd considering the planet's overall density is 5.4 gm/cm3 - while Sulfur is at around 2.3%.
Colonizing any planet has to include an idea of the returns versus the expense. Is it worth the cost to develop the technology to cope with Mercury's heat and cold, radiation, toxic regolith and resource issues? My answer is an emphatic YES!!
Mercury's orbital position is its greatest asset. Being able to launch spacecraft to any other planet three or four times a year suggest a very aggressive program of exploration can be based there. If solar sails are used to fly probes to other planets, the 'chemical cost' of launching from Mercury is limited to 4.2 Km/sec. This being the velocity needed to get off Mercury's surface to a near-escape orbit for rendezvous with a sail. In contrast, launching from Earth dooms us to at least 11.2 km/sec.
As an astronomical platform, Mercury can support all types of instruments in all wavelengths. So can the Moon. The difference is that, on Mercury, a given instrument has 88 days of ultimate darkness to do full-sky surveys and 44 days to basically stare at any specific object. The Moon, at best, offers only 14 days.
For those interested in colonizing Mars, Mercury can produce and transport mega-tonnages to Mars in much less time than any Earth/Moon-based program as Mercury has seven times the launch opportunity. This is due to Mercury's synodic period with Mars being about 101 days, as compared to the 780-day Earth-Mars period. The fact that Mercury can bring 20 times as much solar power to bear on production than facilities on Mars doesn't hurt Mercury's potential either.
I could go on. . .
The environmental challenge is more about learning the details of what is there. We are doing that with MESSENGER now, and hopefully with a lander mission of some kind as a follow-up.
One last thing: If I'm asked what is the single greatest challenge to colonizing Mercury, it will be the kinetic energy effect on manned vehicle's going to Mercury. The only answer available right now is a very large, high-impulse propulsion with a very light payload.
Well, one large issue is that on Mercury we can't make plastics. Petrochemicals are needed as a raw material, and as far as I can tell, there were no dinosaurs on Mercury.
Bioplastics exist, but we would need a large plantation to be able to sustainably make plastics. We already would need large plantations for food and for sustaining the atmosphere, so this might get infeasible due to space considerations. Also, we need to consider the recycling of organic matter. Plastics are a dead end in the organic matter chain, they cannot be easily degraded to manure even if they are created from organic matter. On a planet where the building blocks must be shipped from the Earth, this can be a problem.
So, no plastic.
Mercury seems to have tons of iron though. That's good.
The question to ask is not whether Mercury has the resources we need, but what we can do with the resources Mercury has.
Given a large enough initial colonization payload, in theory, any colony could continue almost indefinitely without additional input of matter. If we perfect the technology, we could colonize the space between Earth and Venus. Take this planet for example: We started with a, uh, REALLY big colonization payload, and we just keep going using the same matter, mostly. If we manage to destroy ourselves with some disaster like global warming, that will be a matter of management, not lack of resources.
To sustain a colony at or below its initial population, the only resource that is absolutely required is energy, which Mercurial colonists would get from the sun just like we do, in much greater abundance, if they can harvest it. If we want the colony to be able to grow beyond its initial size, it will need the materials of bio-matter, like hydrogen, carbon, oxygen, and nitrogen. As far as I can tell, we don't have enough information to assess those resources on Mercury yet. For example, although there is no significant atmospheric oxygen, we will probably find metal oxides in sufficient quantity on the surface. If not, the colony can only grow at the rate that we can throw comets at it. In any case, the main challenge is not lack of resources at all, but of technology.
Given only technology that already exists today, I don't think those colonists would have a snowball's chance on the day-time surface of Mercury of surviving the first Mercurial year. But once we have probed Mercury enough to have a strong sense of the variety of materials available, which will take decades or longer at our current rate, we need to determine if we can use those materials to build atmospheric containment structures, light reflection, dispersion and filtration materials, atmospheric replacement, soils, etc.
If we remain stuck on the idea that we need plastic or glass domes to contain our atmosphere and let in light, then a lack of silicon and petroleum will present a serious problem, but if we find a way to make a strong translucent material out of mostly hydrogen, sulfur, and magnesium, which we already know to be plentiful on Mercury, or out of something else we happen to find with our probes in sufficient quantity, then we're in business. We have barely scraped the surface of what is possible in materials sciences. We have only explored a handful of options that we have found to be interesting and useful for the challenges we face on Earth.
Edit in response to paul23:
I am not a materials scientist or chemist, but just off the cuff, I can see that water (H2O) is transparent as a gas, liquid or crystalline solid. Sulfur has the same outer shell as Oxygen, so what about H2S? There is plenty of Hydrogen and Sulfur on the surface of Mercury. So I decided to Google the properties of that chemical. It is most familiar to us on Earth as something that smells like rotten eggs, and readily dissolves in water to form a powerful acid. But I eventually found some interesting data here about it's chemical properties when isolated. In the absence of water, it is pH neutral and is transparent as a gas and a liquid. I can't immediately find any information about its opacity as a crystalline solid, but I know it can form a crystalline structure because it is a relatively high-temperature superconductor, which if anything makes it a lot more interesting as a potential resource for a Mercurial colony. Of course, the fact that it is a gas at low pressure and human-livable temperatures is an immediate engineering challenge. But look, I'm a layman giving it a few minutes of creative thought and doing a quick search on the internet. My goal is only to point out that you do not know what kinds of material properties researchers may discover until you look, and I suspect that we haven't even spent our first million on Mercurial materials science yet.
In any case, I'm certainly not buying the idea that sulfur just "won't be capable of creating" the right kind of crystal roster. We haven't done the science to find out yet! Some things, perhaps this thing even, may not have an easy solution. But other things may have extraordinarily awesome solutions. I mean, maybe a Mercurial colony would have to live deep under ground where they use their H2S superconductive technology to operate at high enough efficiency to reduce the burden of harvesting solar energy at the surface. There is a lot of research to do before we know what the pivotal challenges will be. I don't think it's time yet to throw up our hands and say those materials cannot possibly work.
This is an old question, but since it recently received a new answer, I'll add my own.
Given today's technology, the answer is "no way." Given our knowledge of what might be possible, the current answer is "who knows?"
Right now, humans haven't the foggiest idea regarding how to make a sustainable human colony in space. Sending people into space is not anything like the 16th century age of discovery where a sailing ship could be sent around the world and expect to find air, water, and food wherever the ship went.
The one thing we do know is that we don't know how to do it. We can't make a sustainable human colony in Arizona, let alone on the Moon, Mars, or Mercury.
Too many scifi fans get hung up on the physics of space exploration and completely ignore the biological aspects of the problem. The physics part is comparatively easy. Biology? We just don't know how to do it. Making a self-sustaining space colony for now is a perpetually TRL 1 concept. In other words, it's science fiction for now, and I suspect it will be for a long time.
