Observational constraints on the cometary impact flux on Mars and Earth

Abstract

While dynamical models indicate that both cometary and asteroid collisions likely contribute to the population of bodies that impact the inner solar system, the distinction between the craters produced by the two populations remains uncertain. Although cometary impacts larger than 10 km in diameter are rare, dynamical studies cannot confidently eliminate their possible contribution to the total impact population at levels lower than ~10%. Due to the speed and volatile content of comets, their impact should couple more energy with a planetary atmosphere and result in observable signatures. The tenuous atmosphere of Mars makes the planet an ideal location to recognize such signatures. Here we use computational simulations and laboratory experiments to model both asteroid and cometary impacts on Mars. We explore the relationship between such impacts and enigmatic radial streaks that form around certain impact craters due to high-speed, long-duration winds. While lower speed asteroid impacts into thick (500 m) surface ices also can produce intense outward winds, the global distribution of craters with impact-generated wind streaks limits this mode of origin to just a few candidates; the rest are likely cometary. Based on the occurrence and relatively young age of these craters, we conclude that ~1% of the impactors on Mars over the last 2 Ga resulted from Long-Period cometary impacts, which should be about 6 times higher on Earth. Inclusion of Jupiter-Family and Haley-type comets (but not including cometary asteroids) then predicts that ~ 16% of all terrestrial impacts, with important implications for future impact hazards on Earth.