NASA scientists made their first global map of Mars in 1978, using data from Mariner 9. Teams have made a few other maps since, as NASA sent more missions to the Red Planet. Compared to previous geologic maps, this new map finds a greater percentage of Mars' crust originated from the first of its three epochs, the Noachian Epoch. The scientists who made this map also dated all the impact craters that are at least 150 kilometers (93 miles) in diameter. Mars' biggest craters formed mostly early in the planet's history—more than 80 of them formed in the Noachian Epoch, 4.5 billion to 3.7 billion years ago, but fewer than 10 formed after that.