paper

Importance of Biologically Active Aurora-like Ultraviolet Emission: Stochastic Irradiation of Earth and Mars by Flares and Explosions

arXiv:astro-ph/0307543 · doi:10.1023/B:ORIG.0000043120.28077.c9

Abstract

(Abridged) We show that sizeable fractions of incident ionizing radiation from stochastic astrophysical sources can be redistributed to biologically and chemically important UV wavelengths, a significant fraction of which can reach the surface. This redistribution is mediated by secondary electrons, resulting from Compton scattering and X-ray photoabsorption, with energies low enough to excite atmospheric molecules and atoms, resulting in a rich aurora-like spectrum. We calculate the fraction of energy redistributed into biologically and chemically important wavelength regions for spectra characteristic of stellar flares and supernovae using a Monte-Carlo transport code written for this problem and then estimate the fraction of this energy that is transmitted from the atmospheric altitudes of redistribution to the surface for a few illustrative cases. Redistributed fractions are found to be of order 1%, even in the presence of an ozone shield. This result implies that planetary organisms will be subject to mutationally significant, if intermittent, fluences of UV-B and harder radiation even in the presence of a narrow-band UV shield like ozone. We also calculate the surficial transmitted fraction of ionizing radiation and redistributed ultraviolet radiation for two illustrative evolving Mars atmospheres whose initial surface pressures were 1 bar. Our results suggest that coding organisms on planets orbiting low-mass stars (and on the early Earth) may evolve very differently than on contemporary Earth, with diversity and evolutionary rate controlled by a stochastically varying mutation rate and frequent hypermutation episodes.

21 pages, 2 figures, accepted for publication in Origins of Life and Evolution of the Biosphere

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