I often find myself mildly frustrated that there are, broadly speaking, two consensuses in popsci articles on colonizing and terraforming Mars (often conflating the two), and they're both wrong.
On one hand, we have the "it's impossible to live on Mars, radiation will kill you and the soil is poison" consensus. This one is very frustrating because it broadly fails to pay any note to the fact that these problems do in fact have solutions - some which are trivial and some which are more troublesome, but none of which are true showstoppers in my opinion.
Take, for example, the doomsaying over detection of high perchlorate concentrations in the soil. This neglects to note that the perchlorate salts in the martian regolith (the toxin of note) are water soluble. Habitats will indeed need to rinse soils before using them for agriculture or landscaping, but "acquiring large amounts of water" is already a necessity for maintaining a human settlement of any appreciable size, and is by no means impossible - deep radar soundings from modern Mars orbiters show plenty of regions with shallow-buried glaciers in the mid-high latitudes. If we ever do fully terraform Mars, the resultant lakes and seas will at first be extremely briny as the new rains wash the perchlorate into them, but remediation of perchlorate-rich waters for human-use and ecological health purposes is actually already something we do here on Earth. It'll be a vast task, but any reasonable plan for terraforming already implies the application of self-replicating industrial automation on a fairly massive scale - whether this is a showstopper depends on whether you think automation can get significantly better (I would certainly like to think so)!
Radiation is a problem, but not one we can't take on. Galactic cosmic rays (GCR) are likely to be fairly bad for you, although we have yet to derive any real long-term data on this - the vast majority of (that is to say all, as far as I know) chronic dose exposure estimates are approximated by applying the long-term dose to research animals in short intervals, which may not represent the full picture in the same way the effects of having a ten watt LED shining on you for a year are not entirely the same as having a three-hundred megawatt laser shining on you for one second. The flares of proton and x-ray radiation from solar storms are troublesome if you happen to be out on the surface in a spacesuit, but are actually fairly easy to shield against, being relatively slow light atoms and evil photons - anything that knocks GCR doses to anything like earth-background levels will totally knock out solar storm radiation, and much less (a tank of water ~thirty centimeters thick) will suffice for temporary duck-and-cover shelter on moving vehicles. The important thing to remember in my opinion is that radiation is not some implacable aura of death. There's an extremely simple and well tested way of stopping the galactic cosmic rays that are troublesome to Mars colonization and it's called putting stuff between you and them. Any stuff, really - sandbags, rammed-earth walls, bricks, plastics, water, so long as you have a hefty (a few tonnes per square metre) roof over your head you're basically fine. You may be thinking "but a few tonnes per square metre is huge!". This is true. But! Owing to Mars' extremely thin atmosphere, a pressurized hab will actually be strengthened against compression by the air inside pushing against its walls. You can pile on dirt by the ton and still have the structure be predominantly in tension against its internal atmosphere!
Gravity is a wild-card, but tentative results from experiments on centrifuged mice on the ISS suggest that even lunar gravity may be fairly benign to terrestrial life.
On the other hand, of course, is the "it's actually unbelievably easy, just plant some ice moss and heat the poles enough to release their dry ice" people. This is, tragically, extremely unlikely to be true unless some frankly miraculous composition assumptions are made about the clathrate content of the polar ice caps - modern data from the MAVEN and MRO missions show that the total dry ice available from the poles is just 15 millibars: 12 mbar from buried deposits at the south pole and 3 mbar from cold-trapped seasonal frosts. There's some talk about clathrates (trapped carbon in the polar water ice) possibly contributing up to 150 mbar if the water ice caps are completely intermixed with the stuff, but we crucially have yet to see any real evidence for clathrates - a strong indication that a relatively small fraction of the polar ice harbors them, if they're there at all. Volatiles adsorbed into the regolith will contribute some outgassing if we warm the planet with mirrors, but due to heat taking a long time to penetrate through the ground this is a long-term prospect short of directly burning out deep new melt-canyons with big lenses in space - which I personally think is viable, mind, but it's certainly a significant step above simply warming the poles with some statites!
If it turns out there's actually huge deposits of clathrate in the poles I will gladly take the egg on my face there. I just feel it's unlikely to yield more than ten millibars or so extra, or we plausibly would see significantly more evidence of dramatic climate change in the relatively recent past; Mars axial tilt chronically wobbles up to a bit over twice its current value, driving its polar caps to migrate entirely towards a band around the equator - and presumably to give up their trapped CO2 in the process, at least temporarily.
Now, fifteen millibars isn't nothing: it'd more than double the shielding effect of the present-day Martian atmosphere (from an average of seventeen kilos-per-square-meter equivalent to forty-two), and plausibly allow seasonal liquid water to flow in some lowland areas. But it's not even close to enough for a "shirtsleeve" environment - one where people can survive without pressurized habs or suits - even if you can seed the planet with some engineered flora that photosynthesize it all into oxygen rapidly, and keep the sequestered carbon from reentering the atmosphere. Absolute barely-minimally viable breathable atmospheres for humans consist of almost pure oxygen at around 70 mbar pressure (Earth's atmosphere has a total pressure of 1,013.3 millibars, 209.5 of which are oxygen at sea level), and liberating all the easy polar CO2 and converting it into oxygen and stable carbon (in the form of desiccated plants or minecraft cubes of graphite or whatever) would yield a pressure at the bottom of Hellas (the deepest crater basin) of just 20 mbar. Without significant deployment of "soletta" mirrors in space to provide extra heat, complete conversion of the atmosphere to oxygen would also have the unfortunate side effect of negating any increase in greenhouse warming caused by the liberated polar CO2.
As an aside here, it's actually notable that most of Mars ancient atmosphere did not escape to space, probably. The majority of it is thought to have been sequestered in carbonate rocks deep underground by as yet poorly understood hydrothermal processes.
So, cliffnotes version: breathable atmosphere on Mars, not as easy to do as we might have hoped!
But! As I noted, even that relatively thin fifteen-millibar atmosphere, the kind of thing we could plausibly achieve in an unextended human lifetime, could do a lot of good for putative mars settlements. There's a lot to be said for even a partial terraforming effort - figure a way to get the atmospheric pressure above 60-70 millibars and you'll be able to step out onto the surface with just a positive pressure oxygen mask and some warm clothes, even if the atmosphere isn't actually breathable. Get it above about 100 millibars and you've effectively obviated the need for any shielding at all for your habitats - background doses standing naked out on the desert plains would be significantly lower than that found in the highest background-radiation areas on Earth (in Ramsar, Iran).
The thing that makes all this especially annoying to me is that it's a false dichotomy of "it's impossible" vs "it's easy". Like a lot of worthwhile tasks, it's difficult but plausibly achievable if we set our minds to it. And even if it does turn out that we can't achieve a complete shirtsleeve terraforming, that doesn't write off Mars as a place people could live at all - we could build quite nice habitats on the surface of Mars in its present-day state with sufficient industry, cities full of airy arcades and kilometers of tented parkland. The issue is getting the industry there - no easy task to be sure, but not a physical impossibility either.
The issues with getting to a breathable atmosphere are not actually insurmountable either. To start getting round them you really need to start figuring out an extremely scalable in-space industrial base, but one can hope we'll do that someday anyway. God knows exponentially scalable industry would do us a lot of good here on Earth too - a self-replicating carbon sequestration plant that can ctrl-c ctrl-v itself across the Sahara would help a lot with our terrestrial troubles, and you can imagine plenty of other applications like desalination plants, solar cell printers, CANDUs, etc etc. Skimming ice-rich asteroids through Mars' atmosphere would go a long way to thickening it up - impact-driven heating is in fact thought to have been the cause of the episodic warm and wet periods that carved river valleys into the Noachian tablelands billions of years ago. Massive foil lenses parked in solar-sailing "statite" pseudo-orbits could burn hundreds of millibars of oxygen and carbon out of iron oxides in rocks. Nitrogen is troublesome, but maybe not entirely necessary - lower convection in the lower gravity might make fire risk less troublesome even in higher oxygen concentrations. Worst case, there's teratonnes of the stuff out in the kuiper belt, waiting for a whisper of thrust to send them down the long road to the realm of the terrestrial planets...
Another hangup I see often is whether a terraformed Mars would be self-sustaining. I think this is broadly silly. It doesn't have to be, is the thing - already on Earth the ecology of plenty of places is de facto controlled by human action, both intentional and not. This is not necessarily a bad thing, either - the slash-and-burn savannahs of the indigenous Americans were a grand achievement in engineered ecology, but they fell apart into seemingly virgin forest within centuries of the extirpation of the cultures that made them. Would we then say that their creation wasn't a worthwhile endeavor? Our Mars will be a slightly clumsy slash-and-burn ecology too, almost certainly - creating something as messily interlinked and full of delicate local optima as Earth's ecology in anything shorter than geological timescales is likely beyond us. But building something that can support life in many forms with our help is almost certainly not. That it won't survive without us seems kind of like a pointless concern - so what? Planning for posterity can be as simple as intending to be around to keep things ticking over for a long, long time.
These are enormous challenges, but I think they're within the capabilities of a plausible-midfuture (let's define this as 'within the next five hundred years') humanity. I also believe that the undertaking of such challenging tasks is likely to have spinoff applications that could be really spectacularly good for a lot of people on Earth - see my digression about carbon sequestration and other tasks earlier. It is frankly depressing that the will to imagine huge positive applications of technology seems to have become a fairly rare thing, and often coded as right-wing at that - you don't just see this in space exploration and settlement, but in other realms like transhumanism, nuclear power, and advanced robotics and AI. Ceding the tools likely to prove to be fulcrums of future industry to your ideological enemies seems, ultimately, unwise.
Further reading:
A nice little review of the literature on Martian palaeoclimate. Plenty of links to further further reading.
(Yes, that is in fact an Orion's Arm link.) Kind of extremely out there, a lot of beautifully breezy "and then we simply,"s about engineering projects on a scale rarely even envisioned. Some wonky assumptions (understatement). But the thing is, I kind of love it. I think the general concept of the soletta and aerial lens is sound - you may have seen it before in fiction in Kim Stanley Robinson's Mars Trilogy.
"Inventory of CO2 available for terraforming Mars"
Only pays note to available CO2, and assumes extreme excavatory projects like Birch's soletta aren't viable. Still of note - the "terraforming is really trivially easy" crowd also assume these things. It also makes the baseless assumption that one whole bar of CO2 would be not only desirable but necessary - even if you wanted a 1 bar exactly-like-Earth atmosphere you would only want to liberate ~290 mbar of CO2, such that it can be converted into the 200 mbar of O2 we enjoy on Earth today.
"Mars in ice ages for ~25% of post-Noachian history"
More Martian palaeoclimate - relevant to the bit about clathrates.
Other blogs on here to check out.
The writing of a friend of mine who convinced me to start putting my thoughts here.
I think a lot of the right-wing interested in space projects will pass. Elon will only live on so long, and this is a project that lasts well beyond the scope of any human lifetime. Elon is temporary but Mars is forever. Or closer to forever, at least.