The geologic history of Mars has been divided into three broad time periods, or Epochs. From oldest to youngest, these are Noachian, Hesperian, and Amazonian (named after places on Mars). These Epochs are defined by the number of impact craters on the ground surface; older surfaces have more craters. The Noachian Epoch extends from the birth of Mars to between 3. 8 and 3. 5 billion years ago. The actual timing of the Epochs is not known because we have different models of the rates of meteorite falls on Mars.