Mars did not dry up all at once, but fluctuated between dry spells and wetter times before becoming the arid planet it is today, a new study finds.

Previous studies havefound that Mars was once wet enough to cover its entire surface with an ocean containing about half as much water as Earth’s Atlantic Ocean. However, the Red Planet is now an extraordinarily arid planet, a thousand times drier than the driest parts of the Atacama Desert in Chile, the driest place on Earth, according to NASA.