The role of secondary craters on Martian crater chronology

The influence of secondary craters on the accuracy of crater chronology has been heavily debated. We review the production of secondaries on Mars and their cumulative effect on the global crater size-frequency distribution (SFD). We characterize the SFD of secondaries produced by four large primaries (∼50–220 km) and develop a model for secondary accumulation with time, accounting for spatial clustering. The number of km-scale secondaries produced globally may exceed primaries by an order of magnitude on Ga timescales. However, most secondaries are clustered around their parent primary, and regions far from large primaries have lower concentrations of secondaries. We estimate that the crossover diameter between primaries and secondaries on a median surface exceeds 1 km after ∼1–2 Ga, though subsequent crater erasure has significantly influenced the number of secondaries visible today. Because of the high spatial variability of secondaries, care should be taken when using small craters for crater counts.