Impact cratering over time
With an age of up to four billion years, Noachis Terra is one of Mars’ oldest regions. Countless impact craters of different sizes were formed and later eroded here over the course of billions of years. Through these erosion processes, impact craters that had once been trough-shaped were worn away over time by wind, water or ice into shallow depressions devoid of relief. The lack of ejected material, the low and partially non-existent crater rim, the flat crater floor and the numerous impact craters within it indicate that Greeley Crater is very old. A closer look reveals a myriad of tiny craters here and there. Known as secondary craters, these are a result of the impact of material ejected when other objects hit the planet’s surface. Small, narrow gullies cross some of the larger inner craters, indicating that water once flowed within the crater. Other wider channels suggest that masses of ice loaded with sediment once crept down the slopes. The different colouration of the material on the crater floor is due to the differences in composition – the lighter material was weathered through water and the darker surfaces covered by volcanic sands.