The Noachian period of Martian history, from 4.1 to 3.7 billion years ago, is the poster child for a wet Mars. There is abundant evidence from orbital images of valley networks and mineral maps ...

In the 1970s, images from the NASA Mariner 9 orbiter revealed water-sculpted surfaces on Mars. Other findings over the years include ... and all these things had to be active for some extended period ...