The Hesperian epoch is the middle of three broad time periods into which the geologic history of Mars has been divided. The Hesperian epoch extends from about 3.5 to about 1.8 billion years ago. From oldest to youngest the martian epochs are the Noachian, Hesperian, and Amazonian (named after places on Mars). They are defined by the number of impact craters on the ground – older surfaces having more craters. The precise timing of the epochs is not known because different models exist of the rates of meteorite falls on Mars.