Martian ages

The subjects of this paper are a discussion of the methodology of relative age determination by impact crater statistics, a comparison of currently proposed Martian impact chronologies for the determination of absolute ages from crater frequencies, a report on our work of dating Martian volcanoes and erosional features by impact crater statistics, and an attempt to understand the main features of Martian history through a synthesis of our crater frequency data and those published by other authors. Two cratering chronology models are presented and used for inference of absolute ages from crater frequency data: model I, with nearly equal Martian and lunar cratering rates around (ca.) 4‐ to 10‐km crater sizes, and model II, equivalent to model I for ages > 3.5 · 10 9 years but with a factor of 2 higher Martian cratering rate at ages <3 · 10 9 years. Those model cratering chronologies are applied to the data. The interpretation of all crater frequency data available and tractable by our methodology leads to a global Martian geological history that is characterized essentially by two epochs of activity. The division between the two epochs is measured at a cumulative crater frequency value for 1‐km craters (crater retention age) of N (1) = 8 · 10 −4 (km −2 ) corresponding to an absolute age of ca. 3 · 10 9 years (applying model I cratering chronology) and of ca. 1.5 · 10 9 years (applying model II cratering chronology). In the ancient epoch all major events like emplacement of the plains lavas, the piling up of most volcanic constructs, and large‐scale erosion of channels and mensae (highland/northern lowland boundary) have taken place. During the younger epoch, only the big Tharsis shield volcanoes were active, and some minor erosion took place. This means that Mars is not a youthful planet but an ancient one with respect to most of its surface features.