The Mars “atmosphere” is so thin that the moisture lining your lungs would boil. It is thin enough so that it would count as a laboratory vacuum. So, no, there is no way that you could survive on Mars without a spacesuit similar to the ones the Apollo astronauts used on the Moon.

Science fiction writers write about turning Mars into a habitable planet like Earth in a few centuries. On the Mars Society website you will read that we could turn it into a planet with an Earth pressure CO2 atmosphere and grow trees there in a thousand years.

A CO2 atmosphere would be poisonous to humans even with an oxygen mask, requiring full cycle scuba gear (CO2 is harmful to us above 1% so you’d need to be careful not to let it leak in). You would also need to use artificial greenhouse gases or large mirrors in space to keep the planet warm, because it gets half the sunlight of Earth.

However, that depends on assuming that there is enough CO2 locked up in the Martian regolith to create an Earth pressure atmosphere. Nobody knows how much is there, but the upper limit of modern estimates is about 10% of Earth’s atmosphere with others saying it is more like just a few percent. This, if it exists, is dry ice that’s well below the surface and would take a long time to warm up. The dry ice on the surface amounts to around 1% of Earth’s atmosphere and the current Mars atmosphere is around 1%. So there’s lots of room for skepticism about whether there is enough CO2 there.

Some have suggested that after warming up Mars we could use special microbes to digest carbonates to convert it into CO2, but that’s highly speculative. It would then be 100,000 years to sequester the carbon out of the atmosphere to make it oxygen rich, and then you need nitrogen as a buffer gas as oxygen is so flammable, and where do we get that? And how do you keep your atmosphere stable once you get that far?

And do we know what our descendants a thousand years from now will want on Mars? And when has any administration carried out a large scale expensive project that takes a thousand years to completion? Some medieval cathedrals took over a century, e.g. one of the ones we have good records for, Santiago cathedral used a workforce of 50 skilled labourers from 1075 to 1211. A decade or two is a challenge sometimes in modern times!

What we can do over shorter timescales is to make city domes, lava tube cave colonies on the Moon, Stanford Torus type habitats in free space etc. But we don’t need a planet to do that - we can start with the Moon, much more convenient, close at hand, safer too. If that’s promising we can move on to the asteroids, Venus clouds, Mercury, Callisto, many other places.

The big issue with Mars is that the thing that makes Mars more interesting than the Moon is that it might have had life there in the past and even more exciting, it may have life there right now. Recent discoveries of possible present day seeps of briny water and other potential habitats for life on Mars even on or near the surface have got some scientists excited about the possibility of present day life on Mars.

If you send humans to Mars there is a good chance their spaceship will crash and once you have bodies in tiny fragments and food, air, water etc scattered over the surface and dust blown in the dust storms, then from then on, any readings of life signs - dead or alive - over the whole of Mars would be suspect, as probably coming from that crash. That would make it very hard indeed to investigate Mars for signs of present day life especially. Earth life could also make Mars life extinct, especially if the Mars life is some early less developed form of life as is quite possible.

Meanwhile the Moon is a very interesting place and I’m sure many people would jump at the chance to set up home on the Moon too, if it was offered to them.

But I think it is rather early days to think of colonizing these places unless there is a very strong economic reason. That’s like Shackleton saying “Oh, we managed to survive a winter here, huddled under a boat and hunting seals, amazing, let’s colonize Antarctica”.

Those remaining on Elephant Island in Antarctica waving farewell to Shackleton and his five crew as he set off in the James Caird boat to find rescue in South Gorgia. They managed to survive a winter in Antarctica, but they didn’t say “Oh great, let’s colonize Antarctica :) “ Ernest Shackleton and the Endurance expedition, The voyage of the James Caird, Elephant Island

(they weren’t first to overwinter, that honour goes to the Southern Cross Expedition)

Norway's most significant historic site in Antarctica Southern Cross expedition - first to over winter in Antarctica Southern Cross Expedition

Mars may look more habitable than Antarctica but that’s mainly because it is so dry. If it did have enough water, the whole planet would be covered in a thick sheet of ice. It is very very cold there, especially at night when the air gets so cold that some of it starts to freeze out as dry ice, even in equatorial regions, for many nights of the year forming the ice / CO2 frosts photographed by Viking.

Actually Antarctica is far the more habitable of the two, it has a breathable atmosphere, hard to beat that, no radiation problems, and you don’t have to hold in the air with an outwards pressure of several tons per square meter which is why all space habitats like the ISS have to be rather massive feats of engineering, and usually with few and tiny windows, because it is a big challenge to make a transparent pane that can withstand several tons per square meter of outwards pressure .

(It’s ten tons per square meter outwards pressure if you use Earth sea level pressure as for the ISS, which makes it nearly a ton pressing outwards on a tiny window 30 cms by 30 cms).

Even the summit of Mount Everest is far far more habitable than Mars.

Compared to the surface of Mars, the summit of Mt.Everest is a paradise and would be a wonderful place to grow your tomatoes compared to Mars. You can almost breathe the air, with only supplemental oxygen, you don’t have to wear a spacesuit, your lungs won’t be damaged irreversibly with the water lining them boiling, you just need to warm it up and supply a bit of extra oxygen, easy peasy :). Well not really but compared to Mars it is.

See also my Case For Moon First

and my answer to: How can Mars be "warmed up" as Elon Musk implied during his September 2016 presentation?