WHERE TO LOOK FOR SIGNS OF A TERRESTRIAL IMPACT FROM COSMIC IONIZING RADIATION EVENTS

Supernovae and gamma ray bursts are among the most powerful events in the universe, emitting a tremendous amount of energy as the host star is destroyed. There has been speculation that such events may cause an impact on life on Earth; even that nearby events such as these may cause mass extinctions. While this is possible, it is not likely because of the rapid attenuation of radiation intensity with distance, and because of the relative hardiness of living organisms exposed to ionizing radiation. In particular, the distance at which ionizing radiation from a supernova or gamma ray burst could affect life on Earth is so close that the average interval between such events is comparable to the age of the Earth. As such, it is unlikely that supernovae or gamma ray bursts are directly responsible for mass extinctions. However, supernovae and gamma ray bursts are also capable of completely destroying the Earth's ozone layer, with concomitant effects on organisms exposed to sunlight. In fact, cosmic events are more likely to lead to long-term mild elevations in background radiation levels, with potential impacts on mutations rates and rates of evolution. In this presentation we will discuss the expected magnitude and biological effects of cosmic ionizing radiation on terrestrial life. We will also discuss the manner in which such events might manifest themselves in the geologic and fossil records. For example, greatly increased levels of UV radiation would be unable to directly affect most benthic or subterranean organisms – worms and clams can be expected to be unaffected, while plants and plankton might be severely impacted. Seeing evidence of such events in the fossil record, in conjunction with the presence of supernova debris would confirm a cosmic origin for these events.