Up: Proofs for an extraterrestrial impact

It has been suggested that all mass extinctions were due to meteorite impacts. The impacts would occur periodically, due to the existence of a companion star to the sun ('Nemesis') perturbing the trajectories of comets and sometimes sending them to a new trajectory crossing that of the earth. A study of the fossil extinctions seems to reveal a period of 26 million years. The dates of the few well-dated craters on the earth seem compatible with a period of about 28 million years. However, the statistical significance of the fossil and crater data is subject to caution. The companion star has not yet been detected.

Whether periodical or not, it is possible that other mass extinctions were due to impacts. For instance, shocked quartz has been found for the Triassic-Jurassic transition, together with a Canadian crater whose age is far from certain. Yet this transition seems to be much more complex than the K-T: there seem to had been several extinctions, with distinct patterns on different places, and the herbivorous species are less affected than expected for an impact extinction (see scheme of the extinction). For the (smoother) Eocene-Oligocene extinction, both iridium and shocked quartz have been found, and there were probably several minor impacts.

However, other, sometimes large, craters have been found on the earth, corresponding to periods when no extinction took place, for example in the Pliocene. Conversely, an important geological breach together with shocked quartz was found in the Devonian, and thought to have caused a mass extinction, but has been shown to have been a bit older than the extinction.

As regards the Permo-Triassic extinction, the most important one in the history of life, no traces of an impact have been found.


Up: Proofs for an extraterrestrial impact