Mars' Third Order of Relief: Regions Differentiated by Age

• Large regions (terræ, plana, and planitiæ)

• Different ages reflected in different crater counts
• the older a region, the greater the number of impacts it has accumulated
• the older a region, the greater the diversity in crater sizes
• lots of junk in the early solar system
• soon drawn together into planets
• first billion years or so, other junk careened into planets
• some really big ones in earliest 100 million years
• a new surge of larger objects stirred up by changes in the orbits of the gas/ice giants: Late Heavy Bombardment (~4.1-3.7 billion years ago)
• small stuff continues to smack all planets