Terrestrial impactors at geological boundary events: comets or asteroids?

Abstract

Evidence has been presented1,2 for a 26–28-Myr periodicity in both the terrestrial extinction record and the age of large well-dated impact craters on the Earth. A cometary source controlled by perturbations associated with either the Solar System's galactic z-motion, which has a 33-Myr half-period, or an unseen solar companion star in a 26–28-Myr orbit has been suggested3–5 for the impacts. Identifications of meteoritic material in the impact melts of the craters6 used to demonstrate the periodicity, however, are indicative of highly differentiated impactors, which are more representative of asteroids than of comets. Clustering of crater ages at some dates may be indicative of random comet showers, some associated with extinctions and some not. The source of asteroidal impactors with a ˜28-Myr period remains unknown and casts doubt on the entire periodicity argument.