The secondary crater population of Mars

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.