Last post I mentioned that Mars sucked as a place to put down a colony. I figured it'd make a good post to elaborate on that comment. First, I'd like to preface that when we're looking at doing anything within the solar system, all options suck. If it were easy or a good choice, we'd have done it already. Chances are good that there's no good option anywhere in the space around us either, but we're not entirely sure yet. We're not able to accurately look at planetary systems just yet. I plan on fudging the numbers a bit to allow for a good number of potential colonization spots, if only to make things interesting.
What would I define as the perfect place to colonize? Anywhere that a caveman could live, or to be a bit more accurate; anywhere a human doesn't need any tools or knowledge about the environment in order to survive. Any aspect of life that isn't handled naturally by the environment must be handled by the humans on the colony. Thus, the more tools and knowledge needed for anyone to survive, the worse an area the place is for colonization.
For an example, think of where you live and see what you'd need to live. Here in New Hampshire, I'd need at least fire for warmth, probably shelter as well, and a little bit of knowledge on what to eat and what to avoid eating. That's a small bit of tools compared to requiring an artificial habitat and importing nearly everything that a potential Mars colony would require.
Here are the major points I'd pay attention to when determining good locations:
Radiation. For anyone that isn't familiar with this, the radiation I'm talking about is roughly any particle that is relatively small and has a relatively high amount of energy. Light is radiation, but not really relevant due to it being relatively low energy, while x-rays are energetic enough to be worrying. On Earth, the main protection we have against radiation from space is our atmosphere and the Earth's magnetic field. Unfortunately for Mars, it has neither, and unfortunately for us, we know of no way to restart it. (At least, as far as I know)
Luckily, as far as I can tell the magnetic field is not terribly important for protecting living things from radiation. The main problem would come from vastly increasing the loss of water (along with some other molecules, but water is the big one).
Atmosphere. The big factors that we care about in regards to atmosphere are: we want it breathable, we want it safe, we want it to protect against radiation, and we want it to keep the planet warm enough. For the atmosphere to be breathable it needs to contain enough oxygen to support life, and at a proper ratio at the given pressure. Too little oxygen and we can't pull enough out to support us; too high and the oxygen becomes poisonous, along with increased fire risks. For an atmosphere to be safe, breathing can't be harmful from toxins or harmful organisms. To give some examples, the air in my home is safe and breathable, the air at the top of Mt. Everest is safe but not breathable, the air in a burning building is breathable but not safe. The atmosphere on Mars is both unsafe and not breathable, along with not being dense enough for protection against radiation or for providing a proper temperature.
Fixing Mars' atmosphere is something we know how to do on a technical level. Spewing gases into the atmosphere isn't exactly something new for us. The primary problem we would have is one of scale. I'm sure we're all well used to hearing all about carbon dioxide, but the amount we put out is very insignificant compared to the entire atmosphere. Suddenly exporting all our carbon dioxide production to Mars would still take millions of years to get a more reasonable atmosphere. Though ideally it'd be more similar to a nitrogen/oxygen split, as carbon dioxide is harmful in large quantities.
According to estimates we can make, Mars should be able to hold onto an atmosphere dense enough for breathing and to keep a comfortable temperature. The only real unfortunate problem is Mars isn't big enough to hold onto the smaller molecules at all easily, which is only made worse by the lack of a magnetic field. For our purposes, the steady loss of water would be unfortunate, but occurring over millenia. Certainly replenishable by an interested space-faring race. Any caveman stuck on Mars would thus have a clock to rediscover space flight before all of the water on the planet ran out. No pressure.
Biosphere. The plants and animals and other living things that reside on a planet make up the biosphere of that planet. As far as colonization is concerned, it's useful for providing the food and oxygen we need. Also likely to be a major source of problems with any foreign planet that has developed it's own biosphere independent of Earth, but on Mars that problem should be minimal. All of Mars' biosphere would come from Earth, and unlikely to cause problems. The difficulty comes with getting it built quick. Given a long stretch of time it'd be fairly straightforward to bring on, but there's plenty of problems, like the lack of soil, water, and oxygen that is hard to build incrementally.
Gravity. Mars is on the small side of what is likely to work well for humanity, and is possibly too low for long-term viability. Mars' gravity is only 37% that of Earth, which is pretty low. Unfortunately, we don't exactly have much research on the subject, as we understandably don't have many options when testing biological beings at gravities smaller than that of Earth. We don't even have very much research on how we're affected in free fall, especially as in regards to fetuses. (Highly important if you're looking to live in space is whether or not you can have kids)
Hope that helps outline all the things that Mars is missing. Remember that for everything here, missing something means that the colony must provide it in some other manner. Comparably, popping a colony down in the antarctic is easier and requires less resources than one on Mars. Far less cool though, suck it Antarctica. We're going to Mars.