Mars is a well-mined subject here on SciShow Space, whether we're talking about the challenges of future human exploration there or following the amazing things Curiosity is doing right now, but here's one question we have yet to answer: how long could you just survive on the surface of Mars without a spacesuit? The good news is that you'd last longer than you would on Venus, which is probably the most inhospitable place on the surface of any planet. The bad news is that you're still going to pass out in less than 30 seconds and be dead in a minute (maybe 90 seconds if you're lucky).

Now, yes, Mars and Earth do have some very basic things in common. Like Earth, Mercury, and Venus, it's a rocky planet, so it actually has a surface that you can stand on, which is nice. But, because it's just over half as big as Earth and much less dense, Mars has only 38% of Earth's gravity; so, as you're fumbling for your keys to get back in your space ship, or whatever, your movements might be kind of jerky and sudden and weird, but that is definitely the least of your problems right now.

It's also very cold, for one thing, thanks both to its thin atmosphere and its greater distance from the sun. With not much atmosphere covering the planet's surface to retain heat, the average temperature on Mars hovers around -60° Celsius, though the extremes range from -125 at the poles to a balmy 20 degrees at the equator. 20 degrees, that's perfectly livable. That's, like, Earth-like. 

And then there's the radiation problem. The atmosphere is way too thin to absorb ultraviolet light from the sun the way the Earth's does. It also doesn't have a magnetic field the way that the Earth does, so all that radiation is just hitting the ground pretty much in full strength. And it won't kill you right away, but should you survive your jaunt on the Martian surface, problems will come up later as that radiation starts to cause mutations in your cells.

But your biggest problem is the atmosphere itself. The surface of Mars is not technically a vacuum, but it's about as close as you can get without actually being in outer space. What atmosphere there is on Mars is composed almost entirely of carbon dioxide with trace amounts of nitrogen, argon, and oxygen. That's enough of an atmosphere to support some clouds and wind, but the surface pressure on Mars is about 1 one hundredth that of what we have on Earth, and the human body does not do well when suddenly exposed to extremely low atmospheric pressure.

Contrary to what you might have heard, exposure to vacuum-like conditions will not cause your blood to boil or your eyes to pop out of their sockets. But, with so little air pressure many of your bodily fluids will start to vaporize. That means your sweat, mucus, saliva, and tears are going to evaporate within a few seconds, which is going to be uncomfortable.

Also, all that water in your body is about to turn into water vapor. Thanks to your strong and elastic skin, you're not going to explode, but you will become bloated before you've had a chance to take in the view. The release of all the gasses in your blood and other fluids will basically give you a very quick and very severe form of the bends, the decompression sickness that affects divers who return to the surface too fast.

So, if you do become part of that generation of explorers that makes it to Mars, and I really hope that you do, for the love of Pete, don't forget to wear your spacesuit.

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