Based on the geology of Mars's northern plains, the new study suggests that bodies of water formed as groundwater slowly seeped through cracks in the crust. This process would have made oceans and lakes quickly—within just a few years—but also could have sustained the bodies of water over millennia.

(Related: "Ancient Mars Had Vast Ocean, New Evidence Shows.")

However, even when Mars was supposedly wet, the planet likely didn't have a very thick atmosphere. Many scientists therefore think that if life as we know it evolved on Mars, the best places to look for it would be where liquid water would have been protected from extreme temperature changes and damaging ultraviolet radiation