If it were possible to magically transport a cup of water from Earth to the surface of Mars, the liquid would instantly vaporize. Mars’s atmosphere is so vacuous (it’s less than 1% as dense as Earth’s) that liquid water simply can’t exist for very long on the Red Planet. That’s a puzzle to planetary scientists, because Mars’s surface is littered with signs of liquid water. Dried up valley networks, sedimentary deposits, and chaotic flood plains hint that billions of years ago Martian water flowed freely and that the atmosphere there must have been substantially thicker than it is now. But where did it all that Martian air go?