Astrophysics and Mass Extinctions
Brian Thomas (March 28, 2019)
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A variety of astrophysical events may have affected life on Earth during the Phanerozoic. While most of these events with intensity and proximity great enough to have major impacts are relatively rare, over 100s of millions of years they become likely. Such events include supernovae, gamma-ray bursts, extreme solar activity, and possibly outbursts of the Galaxyâ€™s central supermassive black hole. Impacts on life can be direct, through direct radiation exposure, or indirect, through modification of a planetâ€™s atmosphere. Much work has focused on radiation-induced destruction of stratospheric ozone, leading to increased Solar ultraviolet (UV) radiation at Earthâ€™s surface. Studies of the subsequent biological effect of this increased UV have yielded mixed results, with primary productivity of marine phytoplankton less drastically affected than originally assumed. More work is needed, however, to evaluate both survivability under long-term depletion, as well as the ecological impacts of UV damage. Recent work for the case of supernovae has found that the more direct effects of cosmic ray air-shower secondaries (muons) are likely significant even in the case of less severe ozone depletion. Atmospheric ionization (common to all radiation events) may have other effects as well, including climate change (through changes in cloud cover, however this assertion is controversial), and possibly an increase in the global lightning rate, which may lead to increased wildfire and thereby ecosystem changes. I will review the various types of astrophysical events that may be important, their likely rates and terrestrial effects, and possible connections to mass extinctions.