Crater size-frequency distributions and a revised Martian relative chronology

A revised Martian relative chronology is determined which dates geologic units with respect to the end of the period of heavy bombardment. This analysis differs from previous studies by using Viking 1:2M photomosaics to map all 25,826 craters ≥8km diameter which postdate the lithologic units making up the Martian surface. The relative plotting technique is used because it shows changes in the shape and density of the crater size-frequency distribution curves more clearly than cumulative plots. The changes in shapes of the distribution curves are interpreted to be due to two different impacting populations, one dominating during the heavy bombardment period and the other emplaced during the post heavy bombardment period. The 8-km-diameter limit was chosen to exclude the majority of secondary craters and to minimize obliteration effects acting over the age of the planet. The results obtained are average ages valid for units with typical thicknesses between about 1 and 3 km and extending over an area of about 10⁵ km². The shapes and densities of the crater distribution curves indicate that approximately 60% of the Martian lithologic units in which these craters occur formed during the period of heavy bombardment. Many of the small volcanoes (those generally less than 200 km diameter), including most in the Tharsis and Elysium regions, date from this period. The Hellas and Argyre impacts bracket the average age of the intercrater plains in the highlands. The ridged plains distribution curves indicate formation during the period of heavy bombardment, but their lower crater density indicates that they are the last geologic unit to retain the scars from this population of impactors. They therefore likely formed near the end of heavy bombardment. The remaining 40% of Martian terrain units show distribution curves indicative of emplacement since the end of heavy bombardment. The wide range in crater densities among different regions of plains indicate a range in formation ages. Although the general view of Martian geologic evolution does not differ significantly from previous studies, the present analysis explicitly states the relationship of numerous geologic units to the period of heavy bombardment and has revised the relative ages of localized regions across Mars.