Pressurized Person -> Unpressurized Environment -> Bang!
The pressure inside the person literally blows out. Why do you think astronauts wear spacesuits and not just oxygen tanks?
Actually, when in a de-pressurized environment, you're more likely to freeze than to explode. However, Mars would have a lot more sun all the time than hard vacuum, and matter would hold the heat longer, so... perhaps. But I think people would explode pretty fast in a vacuum, and not take nearly as long as they did... plus, don't you think having your head expand and your eyeballs swell up that big would cause brain damage at the very least?
Right now, there is 13.7 psi (pounds per square inch) holding your body together because you are in the Earth's atmosphere. In space there is atmosphere and therefore no pressure, therefore you will explode like an overfilled balloon.
Pressurization determines the boiling point of the matter within the body. In a vacuum water vaporizes, a large percentage of the human body "boils off" even in extreme cold. While these escaping gasses exert a great deal of force on the human body, it's also porous. In a vacuum gasses escape from the body through those pores, not by blowing the body apart. You end up freeze-dried rather than chunked.
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