Mars' Third Order of Relief: Regions Differentiated by Age

• Crater-counting basis for eons (called periods on Mars):

• Noachian oldest
• very heavily cratered (often to saturation!)
• great diversity in crater sizes
• Hesperian
• noticeably fewer craters
• less diversity in crater sizes
• Amazonian youngest
• very few craters → pretty smooth surfaces
• almost all are pretty small