Near-Earth object velocity distributions and consequences for the Chicxulub impactor

An Opik-based geometric algorithm is used to compute impact probabilities a nd velocity distributions for various near-Earth object (NEO) populations. The resulting crater size distributions for the Earth and Moon are calculat ed by combining these distributions with assumed NEO size distributions and a selection of crater scaling laws. This crater probability distribution i ndicates, that the largest craters on both the Earth and the Moon are domin ated by comets. However, from a calculation of the fractional probabilities of iridium deposition, and the velocity distributions at impact of each NE O population, the only realistic possibilities, for the Chicxulub impactor are a short-period comet (possibly inactive) or a near-Earth asteroid. For these classes of object, sufficiently large impacts have mean intervals of 100 and 300 Myr respectively, slightly favouring the cometary hypothesis.