Smiley face on Mars is a telltale sign of its past

Mars didn't lose all its water without a fight.The planet, today 1,000 times drier than the driest desert on Earth, gradually lost its insulating atmosphere. The Red Planet's once great Martian lakes and rivers evaporated some 3 billion years ago, and the rocky world plunged into a global freeze. But some pools of water with high levels of salt persisted, dramatically lowering the freezing point to some -40 degrees Fahrenheit (-40 degrees Celsius). Salt, as you probably know from swimming in the