First, we’d have to deal with the immediate shock to our biology. Mars has about 38% of Earth's gravity. While bounding around like an astronaut sounds fun, long-term exposure to low gravity wreaks havoc on the human body. Our bones would lose density, our muscles would atrophy, and our cardiovascular systems would become "lazy" because they no longer have to pump blood against strong gravity. Over a few generations, "Martian humans" would likely grow taller but have much more fragile skeletal structures than Earthlings.

• The Air: Mars has an atmosphere that is 95% carbon dioxide and is less than 1% as thick as Earth's. Step outside without a pressure suit, and your blood would literally boil.