You Really Don't Want To Go There
Mars sucks. It's a terrible place for humans to visit, much less colonize. Mars is a desiccated irradiated hell-hole that promises to slaughter anyone foolish enough to go there. If the planet doesn't kill you outright, just the stress of surviving its horrors will likely mentally scar you for life. Our under-rated Moon suffers from its own share of faults. It's no paradise. But compared to colonizing Mars, it's an amble on the beach. We should focus on it, not Mars.
Let's list the many failings of Mars and then compare those to our Moon.
The Voyage and Some Initial Ways To Die
The first way Mars sucks is the health impact of simply getting there. The trip will take about 7 months, during which the isolated crew will float around in a metal tube hurtling through space, drinking recycled urine and trying not to get on each other's nerves. The psychological pressures will be enormous. The floating part will suck too, as the hazards of long-duration weightlessness are well-documented: loss of muscle mass, bone demineralization, diminished cardiac function, immune system depression, and neurological impacts. No matter how much you exercise or how much freeze-dried broccoli you choke down, your body will slowly degenerate in this alien environment. Stay weightless long enough and you'll become as weak as a kitten. That's why astronauts who return from long-duration stays in orbit are such a mess. They often have difficulty walking, much less conquering new planets. Afterward they're susceptible to problems with balance, fatigue and bone fractures. Sometimes it takes several years for full function to return.
Those are the lucky ones, as upon returning to Earth top medical teams help them recover from their disabilities. In contrast, Mars sorely lacks medical infrastructure. You're on your own upon landing and will just have to suck it up.
Then there's the radiation. Our planet's magnetic field shields us from solar radiation, but once in interplanetary space Mother Earth can't help you anymore. Your resulting radiation dose would equal about 80 full-body CT scans. Not immediately deadly, but if you start sprouting tumors don't be surprised. If you're unlucky you might also get hit with a deadly solar flare, which would be about as healthy as tunneling into a reactor core and licking one of the fuel rods. If that happens, you and your fellow colonists will need to retreat to a special shelter to ride out the storm and hope for the best. Don't forget a deck of cards and some of Elon's weed. You're going to need it.
But suppose nothing breaks, nothing goes wrong, and you actually make it to Mars. A fiery re-entry, a gut-punch of a landing, and then the feeling of gravity overwhelming your decrepit body. You're weak, irradiated, and probably eager to kill your fellow crewmates at the first opportunity, but meanwhile here you are - on Mars. Success!
The Arrival and Some More Ways To Die
Welcome and take a hard look at the creepy howling wilderness that's now your home. No elite medical teams await you, no nurses serving proper meals, not even a hot shower. Instead, after months in space you're now expected to explore hell. You'll be in poor shape for such a task. The best approach would be to recover in the craft for some weeks, as any trip to the surface will be a surreal challenge. At most you'll be able to slither around while throwing up - or worse - into your spacesuit.
About that spacesuit: never leave home without it, because the atmosphere of Mars would like nothing better than to kill you. It's 95% carbon dioxide, with the rest being a mix of nitrogen, argon and methane. None of those are remotely breathable. If you tried you'd immediately hyperventilate as your body struggled for oxygen. Your heart rate would soar. In a few seconds you'd be struck down by nausea and then convulsions, followed in a couple of minutes by coma and death.
The Martian atmosphere isn't done with you yet. Not only is it lethal but there also isn't enough of it - the average pressure on the surface is a scant 0.095 psi. That's less than 1% the pressure on Earth's surface, equivalent to climbing a mountain that's 5 times the height of Everest. Your body requires pressure. Without it seriously bad things happen. If there's the slightest crack in your habitation or space suit ... well then, adios colonista. So always be ultra-careful with sharp objects and never run with scissors, as you're always just one misstep away from depressurized oblivion.
The effects of depressurization are quite undesirable. First off, your blood would literally start boiling. Your eyes would bulge out and turn red from bursting capillaries. You'd scream from the agony of it, a frothing and writhing red-eyed demon, until you collapsed into an oozing corpse. This would take about 60 seconds, thus safely beating out C02 as your cause of death.
Now on the plus side and contrary to popular belief, your head wouldn't explode. That's because your skull would confine your bubbling brain tissue safely inside its case, more or less. Thus you would die like a hero with your head still whole and proudly on your shoulders, albeit with your eyeballs dangling and perhaps some minor cerebral leakage around the ears. So Mars isn't all bad.
What Happens To All The Dead People?
Have you ever wondered what happens to the dead on Mars? Well, it ain't pretty. The average temperature on Mars is -60C and humidity hovers microscopically just above zero. The surface is basically a vacuum (remember: 1% the pressure of Earth), which means exposed bodies quickly sublimate all available moisture. Furthermore, Mars lacks the self-respect to maintain even a minimal magnetic field. That means that much of the radiation you felt in space follows you right to the surface, scouring dead carcasses as they dehydrate. The result is a Martian mummy, drier than beef jerky and just about as edible. It would look like a supersized prune and hardly recognizable as human, aside from the teeth grinning inside the blackened skull. Photos of that will look great in recruitment brochures. Come to Mars! I did!
Depending on how many take up this offer, Mars might eventually be littered with such mummies. The surface will become an enormous public graveyard, the province of the dead and a monument to just how spectacularly Mars sucks. The irradiated freeze-dried environment wlll preserve them forever, an eternal reminder of hubris and the perils of not thinking things through.
Yet More Ways To Die
Due to its lack of a magnetic field, the surface of Mars isn't safe. To escape the radiation explorers will have to build massively thick domes or move underground. As a result, people won't live on Mars, they'll live inside of it. That doesn't sound so romantic, does it? The colonists will become human mole-rats, scurrying around in the subterranean Martian darkness and praying that their filthy little rat habitat never springs a leak and adds their bodies to the growing mummy collection on the surface.
While on the subject of the Martian surface, be aware that it also wants to kill you. Percholates - a class of toxic salts used in rocket fuel - infuse the soil and are abundant throughout this sorry planet. Unsurprisingly, rocket fuel is poisonous not only to humans but to just about every other organism as well, including bacteria and plants. Even cockroaches say "no thanks" to percholates. Adding to the chemical misery, percholates are water-soluble and so likely have contaminated whatever ice and fluids are found on the planet. So forget drinking Martian water without extensive purification, unless you want to launch parts of your body into orbit. Martian dust is even more toxic, as it contains both percholates and extremely fine silicates. These can cause a range of respiratory problems and other health impacts.
Lastly, let's turn our attention to where Mars truly and massively sucks: it has the worst supply chain imaginable. Due to orbital mechanics, there's just a short window every 26 months where transit between Mars and Earth is possible. Combined with the 7-month voyage itself, that means that any colonists are truly on their own. Missing a part? Need a screw to fix that leaky door that threatens to depressurize your rat tunnel? Well, you'd better figure it out yourself, because the next supply run is years away. What happens if someone has a medical emergency or even a toothache? Just forget about getting any help and break out the emergency medical kit.
On Earth we are so use to our support networks that they're invisible to us. But on Mars the lack of support will become dramatically visible and quite likely lead to mass death. Just one failed component could translate to a dire struggle for survival. It's easy to imagine colonists eventually envying the serene mummies piling up on the surface, shortly before joining them.
To The Rescue: Our Lovely Moon
Now let's compare this to the Moon.
First off, getting to the Moon is relatively easy. A 3-day trip and you're there. With minimal health impacts due to weightlessness and radiation, you'll arrive hale and hearty and ready to rip. Next, the Moon doesn't have an atmosphere - and that's actually a very good thing. The lack of air plus the lower gravity means no need for a fiery re-entry, making transportation far safer and cheaper. In addition, compared to Mars, the surface itself is much more welcoming. The Moon has pure drinkable water ice and it's been shown that plants can grow in its soil.
True, there are some negatives. For one, the Moon's dust is probably just as bad as Martian dust. It's very fine and sharp and therefore probably hazardous to breathe. It also smells like gunpowder. Therefore filters will continually need to remove dust from inside habitations. Radiation is also a problem on the Moon and underground living will be necessary. But this is no worse than Mars and the Moon has no pretensions about being a habitable place to begin with. It's just an honest barren planetoid, so unlike its romanticized competitor.
But the best thing about the Moon is that it would have a functioning supply chain from Day One. There are no launch windows limiting travel and a rocket can reach the Moon on any given day. Therefore, it's possible for colonists to quickly get supplies and spare parts, or send back to Earth anyone who has an emergency of some sort. The Moon would be connected to Earth, and it's impossible to overstate how critically important that is. It will radically improve the likelihood that colonists can survive and prosper, particularly in the early days when there will be so many unknowns.
These are historic times. Humans are getting ready to leave our planet and began an amazing journey. But let's be practical about our first targets. Mars sucks and the Moon is so much better. Let's go to the Moon.