The study of craters is important because no samples have been taken back from Mars for study here on earth. The impact of craters can give us an estimate on the age of the surface according to how much time the terrain has been exposed to incoming projectiles. The process of dating surfaces by looking at them is called stratigraphy, and the only tools available to us today are the photographs taken by un-manned vehicles.
Small craters (5 km in diameter) are bowl-shaped with slightly flat floors and raised rims. Large craters (50 to 70 km in diameter) look like shallow plains ringed by a circle of hills with their rims often worn down. The craters are distinguished by variations of their ejecta - or the material thrown out from the crater when there was impact. The ejecta around the Martian craters look like flows deposited in sheets. The odd patterns formed suggest that a layer of ice or water existed in the upper crust when the craters were made.
Most of southern hemisphere and some of northern has heavily cratered highlands situated 1 to 3 km above Martian sea level. Erosion processes usually happen everywhere at once and would not explain why only the northern hemisphere was affected. The only way to explain the relative smoothness of the northern hemisphere is through volcanic activity: lava flows buried the craters in the area. The lava flows that filled in the craters are not necessarily from visible volcanoes, but can come from fissures that are later filled in with the flow that flooded the surrounding areas. Because the southern hemisphere has so many craters, we know that it has an older surface than that of the northern hemisphere. Large craters were formed 3.8 billion years ago, and then the bombardment tapered off. Another theory to excuse the difference between the two hemispheres is that a huge meteor hit Mars early on and wiped the surface clean.
The most heavily cratered plains were formed more than 3.5 billion years ago, after the bombardment had slowed down. The sparsely cratered plains were formed less than 500 million years ago. The plains along the equator are larger than any seen on Earth and are made of lava and ash flows. Other plains were probably shaped by volcanoes, wind, and ice.