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Session 119 - Gamma Ray Bursts.
Oral session, Saturday, January 10
International Ballroom Center,
Gamma-ray bursts now appear to be primarily of extragalactic origin. Statistically, assuming isotropic emission, the observed event rates and fluxes imply that one event occurs per 10^4\ - 10^6\ years per galaxy, with about 10^51\ - 10^53\ ergs in gamma-rays emitted per event. Unless the Milky Way is unusual, a gamma-ray burst should occur within 10^2\ - 10^3\ pc of the Sun in a time span of order 10^8\ years. Independent of the underlying cause of the event, it would irradiate the solar system with a brief flash of MeV gamma-rays with a fluence as large as 10^9 - 10^11\ erg cm^-2. What is the effect of such an event on the Earth and objects nearby? Ruderman (\underbarScience, 184, 1079, 1974) and subsequent authors have considered a number of effects of a flash of gamma-rays from a nearby supernova explosion on the Earth's atmosphere, and on its biota. However, with regard to the demise of the dinosaurs, it appears that there was a marked increase in the deposition rate of the rare earth iridium coincident with their extinction. For this reason, an asteroid-Earth impact has been considered the leading contender for the death of the dinosaurs. Here we consider a new mechanism for mass biological extinctions, caused by small comets nudged into the inner solar system by nearby gamma-ray bursts. If comets populate the Oort cloud with a wide distribution of masses, radii and orbital eccentricities, we find that small (< 1 km), low density (10^-2\ gm cm^-3) objects in highly eccentric orbits can be injected into the inner solar system by a nearby gamma-ray burst. For a relatively brief period of time, the near Earth comet population would increase dramatically. The consequent increased probability of comet-Earth impacts of appropriate energy and material content could account for many of the characteristics of the Cretaceous-Tertiary or other terrestrial mass biological extinctions.
