AAS 201st Meeting, January, 2003
Session 99. Supernovae, SNRs, and Our Atmosphere
Oral, Wednesday, January 8, 2003, 10:00-11:30am, 618-619

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[99.05] Ozone Depletion from Nearby Supernovae

N. Gehrels, C.H. Jackman, J.K. Cannizzo, B.J. Mattson (NASA Goddard Space Flight Center), C. M. Laird (University of Kansas)

We study the effect on the Earth's atmosphere of irradiation by cosmic and gamma radiation from a nearby supernova (SN). As first proposed by Ruderman in 1974, enhanced irradiation of the upper atmosphere catalyzes nitrogen oxides which react with and reduce the ozone levels in the stratosphere. We improve on previous work by using the latest Goddard two-dimensional atmospheric photochemical transport code and gamma-data from SN 1987A. Two contributions to the irradiation of the upper atmosphere are considered: a short-lived gamma-ray blast (300 d) and a longer-lived cosmic-ray component (20 yr). The total energies in these components are ~ 2 x 1047 ergs and ~ 4 x 1049 ergs, respectively. We find that, for a SN to be strong enough to roughly halve the nominal ozone column density, it must occur within about 8 pc. Recent work by Madronich et al. shows that a halving of the ozone column would roughly double the "biologically active" UV radiation reaching the surface and produce a significant perturbation in a representative sampling of life forms. The latest SN data indicates a galactic-averaged rate of such nearby SNe to be about 1.5 per Gyr. When combined with the fact that multicellular life on Earth has existed for the past ~0.5 Gyr, we conclude that this particular pathway to extinction may be less important than previously thought.

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