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Mark Watney, in the movie The Martian, says that,
If the HAB breaches, I'm just gonna, kind of... implode.
The corresponding novel, by Andy Weir, says he will explode (as pointed out by @MikaelSundberg).
I think he will neither explode, nor implode, but simply die of cold and asphyxiation.
Can anyone scientifically explain what will happen?
PS: The HAB is a NASA designed habitat for humans on Mars.
The Martian atmosphere is effectively vacuum. He would be unconscious in less than 20 seconds and the he will end up freeze dried.
The Martian atmospheric pressure is approximately equal to $0.6\%$ of Earth's mean, at sea level, mostly consisted of $CO_2$ ($98\%$).
This is equivalent of pressure at altitude of approximately $17 km$ in the Earth atmosphere, with boiling point $30 ^oC$.
The pilots use oxygen masks at altitudes $> 4km$, so even if Mars's atmosphere was entirely of $O_2$, people wouldn't survive.
Regarding the temperatures on Mars, they vary from $-150 ^oC$ to $+20 ^o C$.
Consequently, sure death by asphyxiation and depending on the location and season, instant or later freezing, which will be accelerated by the low boiling point, especially if there is no suit.
It has appears in may movies and books, the clear point is that, in the BEST scenario you have 40 seconds to get back to normal atmosphere or you are dead. In this case, you see what is happening and you force all the air out of your body but not before oxygenating your blood though apnea rapid breathing just before been exposed.
worst case scenario, your decide to hold your breath, the 100 times of less pressure of the atmosphere, make the air of your lungs expand further till the tissue rupture, not sure if would be an explosion but if you have your ear in his chest you will hear the crack of ribs and diaphragm.
now back to the best case, 40 seconds... you are moving towards to airlock. The low atmosphere now is trying to pull out the air in your veins, your body swell, maybe your eyes too, (1990 total recall movie). I read somewhere that the skin is elastic but it is also strong, and will hold your body together, blood pressure will not drop enough for you to loose consciousness. In your veins, the atmospheric pressure has drop but not near Martian levels.
Second, supposing you are in a calm day at -60C with no wind. I think you dont freeze, I am almost sure you don't. just to be clear, in order to freeze water, it need to lose a lot of energy, and there is not enough air density to take away quickly that amount on energy.
If you fall to the ground which has much much more density than are and is already at -60C, that's something else. I think if your naked skin touches a -60C surfaces, will stick like a tinge sticks to a frozen pole.
so back to the point. Imagine that is your helmet that has been removed but your body still protected by the suit.. (at least you still keep your boots to be able to run without your skin touching the cold surface you will have this 40 sec maximum to walk or 20 if you run to reach safety without problems or consequences.
But I reiterate I dont think freezing will be a consequence of your exposure always you don't touch any surface with your naked skin. 1/100 the atmospheric pressure of earth I think would conduct temperature 100 times less than air.
However, 41 seconds later, once you die and fall to the cold powder iron oxide sand, I think your body quickly will freeze starting by the skin in contact and propagating the cold quickly from there to the rest of your body.
Note: Vacuum is a very very very good insulator and this is more close to vacuum than to earth conditions, so I dont think you will ever feel the cold, not in 40 seconds, not in 30 minutes. (again I am supposing that there is no wind)
