Life - at its simplest - has a tendency to terraform. On Earth, life made oxygen abundant in the atmosphere.
If there were abundant oxygen in the Martian atmosphere, it would be a lot easier for us to colonise. But Mars is smaller than Earth, with less gravity. Would Mars even be able to retain an oxygen-rich atmosphere? I don't know.
Mars has a negligible atmosphere. It is, off the top of my head, about 0.3% the density of Earth's atmosphere.
The reason for this starts with the inverse square law. Mars is approximately half the diameter of Earth. It's composed of roughly the same material ( lots of iron ) and formed about the same time in the emergence of the solar system. Being smaller though, it has lost it's core heat faster, cooling enough that its crust is considerably thicker than Earth's. This has one consequence that Mars has a single solid surface, without tectonic plates - so there are no earthquakes on Mars.
The effect on atmosphere is that this means Mars has a weak magnetosphere, which in Earth is caused by the rotation of the molten iron core ( and why magnetic north is not exactly the same as axial north ). This has robbed Mars of protection from the solar wind. On Earth, it is the magnetosphere that draws the magnetically charged solar wind towards the poles, causing it to burn brightly as it falls through the atmosphere - the aurora. On Mars, the solar wind has been free to strip the atmosphere over geological time, leaving it with barely a wisp today. It also means solar radiation is a bigger problem on the surface of Mars because it has neither a strong magnetosphere nor thick atmosphere to protect it.
What little atmosphere Mars has is more than 95% carbon dioxide, with oxyegen being less that 1%.
The problem with terraforming any planet is that whatever natural phenomena exist to have caused the planet to be in the state it is currently will persist against any effort to change the surfae conditions. There is no feasible way to increase Mars' mass or endow it with a magnetosphere. Even a major ( really major ) long-term engineering project to manufacture a thicker, never mind more earth-like, atmosphere would be constantly up against the very forces that stripped the original atmosphere away in the first place.
And even if it were possible to manufacture an Earth-like atmosphere, because Martian gravity is only about one third of Earth's, the atmosphere would need to be substantially more massive ( I haven't done the math ) so that it resulted in the same Earth-like pressure at the surface.
Overall, not really looking feasible.
I remember seeing a Horizon documentary back in the 80s or early 90s about terraforming Mars, and one engineer advocated a 'best compromise' solution of creating a gigantic Eden Project type enclosed space, covering a substantial portion of the planet, as the only realistic goal.
The recent photo of a cavemouth on Luna prompted some articles to mention the idea of using Martian caves for colonisation, at least in the short term.
And anyway, it's all pretty much moot. If we were going to contaminate Mars, we already have. Starting with Viking.
Not true. NASA has made special effort to prevent the contamination of Mars by even the slightest biological material from earth, because it might irrevocably destroy any chance of answering the life-on-mars question conclusively.
Here is an essay about exactly that, relating to the Viking and other missions:
http://search.nap.edu/nap-cgi/skimchap.cgi?recid=11381&chap=11-21
Preventing The Forward Contamination Of Mars said:
which affirms that the search for extraterrestrial life is an important objective of space research, that the planet of Mars may offer the only feasible opportunity to conduct this search during the foreseeable future, that contamination of this planet would make such a search far more difficult and possibly even prevent for all time an unequivocal result, that all practical steps should be taken to ensure that Mars be not biologically contaminated until such time as this search can have been satisfactorily carried out, and that cooperation in proper scheduling of experiments and use of adequate spacecraft sterilization techniques is required on the part of all deep space probe launching authorities to avoid such contamination.
Landing a sterilised object made of plastic and metal does not contaminate Mars. It merely litters it
