The secondary crater population of Mars

•D≥1 km craters classified as secondary or primary on Mars.•Diameter at which secondaries outnumber primaries is estimated.•Maps of secondary crater contamination produced and presented.•Noachian and Hesperian terrain transition is ∼3/4 km, Amazonian ∼1/2 km.

Impact craters (“craters”) are ubiquitous across most solid surfaces in the Solar System. The most common use of crater populations (populations as defined by diameter- or “size-” frequency) is to estimate relative and absolute model surface ages based on two assumptions: Craters will form spatially randomly across the planetary body, and craters will form following a random distribution around a known or assumed temporal flux. Secondary craters - craters that form from the ejecta of a crater formed by an extraplanetary-sourced impactor - belie both of these assumptions and so will affect crater-based ages if not removed from crater counts. A question unanswered with observational data to this point has been, what is the population of primary versus secondary craters on a given planet? We have answered this question for Mars for craters larger than 1 km in diameter by using a recently published global crater database, classifying craters as primary or secondary, and creating maps of the population statistics. Our approach was to err on the side of a crater being primary by default and hence our work is a conservative measurement. We show that, globally, secondary craters are at least 24% as numerous as primary craters (comprising 19% of the total population) for diameters D≥1 km. However, there are many “hot spots” across the globe where secondary craters are more numerous than primary craters for diameters as large as 9 km. This is the first time such a study has been conducted globally for any body and it shows that, not only are secondary craters numerous, but they can significantly affect crater populations in a non-uniform way across a planetary surface.