If you are being serious it's because... Whilst Mars does have water that is frozen at the poles, it isn't enough to cover the planet entirely. It had an atmosphere early in its life but due to it being small as planets go, the gravity wasn't strong enough and couldn't keep hold of most of its atmosphere which floated off into space, leaving a very thin layer left. This meant the majority of the water that was left either froze or fizzled off into space as well. It is cold there, but that's not what would kill you. It's the thin atmosphere that would. I'm pretty sure that's all there. If you were taking the Mickey, make sure you add the #satire tag.