In many ways, Mars bears remarkable similarities to Earth, but in some ways it is drastically different. Scientists often use Earth as an example, or analog, to help us to understand the geologic history of the Red Planet.
As we continue to study Mars, it is vitally important to remember in what ways it differs from Earth. One very apparent way, readily observed from orbit, has to do with its preservation of numerous craters of all sizes, which are densest in its Southern hemisphere. Earth has comparatively little preserved craters—about 1,000 to 1,500 times fewer—due to very active geologic processes, especially involving water. When it comes to impact craters, there are some things that can no longer be observed on Earth, but can be observed on Mars.