Cosmic rays and and ancient catastrophes

Abstract

A NUMBER of suggestions have been made^1–3 that encounters of the solar system with extra-solar phenomena in earlier epochs led to climatic and biological catastrophes. Most recently Whitten et al.⁴ and Clark et al.⁵ have drawn attention to the role of nearby supernovae (within ≃ 10 pc) from which catastrophic effects are expected (primarily the depletion of the atmospheric ozone layer and consequent transmission of damaging solar ultraviolet.) Whitten et al. estimate the probability of an encounter within a period of 10⁸ yr to be 1–5% and Clark et al. calculate 50% for the same quantity. In this letter, we argue that it is likely that rare and very big solar flares, and their attendant cosmic-ray fluxes, will give even more dramatic effects. This idea is not new, in that Reid et al.⁶ have drawn attention to the effect of solar flares on the evolution of life, but here we endeavour to estimate the likely frequency of solar flares of sufficient strength to have significant effects. Furthermore, we draw attention to the ‘explosion’ at the galactic centre postulated by some⁷ to have occurred about 10⁷ yr ago; if this actually took place then its effect too would have been important and exceeded likely supernova phenomena.