The Noachian epoch is the oldest of the three broad time periods into which the geologic history of Mars has been divided. The Noachian epoch extends from the birth of Mars to between 3.8 and 3.5 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.