Adrian melott
Department of Physics & astronomy University of Kansas Email:    melott@ku.edu www.ucmp.berkeley.edu/ordovician/ordovician.html

Gamma-ray bursts are the most powerful explosions known in the Universe. A GRB within our galaxy could have catastrophic consequences for the Earth.  Extrapolations from the global rate suggest an average interval of a few hundred million years for events in which the Earth is irradiated from an event on our side of the Galaxy.  The initial burst would irradiate the surface of the earth with UV about ten times more intense than seen today.  The atmosphere would become heavily ionized, resulting in major destruction of the ozone layer, possibly also darkened skies and nitric acid rain.

Both the prompt UV and the solar UV resulting from long-term loss of the ozone layer are destructive to living organisms.  The attenuation length of UV in water is tens of meters.  There is a strong candidate for a GRB based mass extinction in the late Ordovician, 440 My ago. Planktonic organisms and those animals living in shallow water seem to have been particularly hard hit during this mass extinction.