Gamma‐Ray Bursts and the Earth: Exploration of Atmospheric, Biological, Climatic, and Biogeochemical Effects

Abstract

Gamma-ray bursts (GRBs) are likely to have made a number of significant impacts on the Earth during the last billion years. The gamma radiation from a burst within a few kiloparsecs would quickly deplete much of the Earth's protective ozone layer, allowing an increase in solar UVB radiation reaching the surface. This radiation is harmful to life, damaging DNA and causing sunburn. In addition, NO 2 produced in the atmosphere would cause a decrease in visible sunlight reaching the surface and could cause global cooling. Nitric acid rain could stress portions of the biosphere, but the increased nitrate deposition could be helpful to land plants. We have used a two-dimensional atmospheric model to investigate the effects on the Earth's atmosphere of GRBs delivering a range of fluences, at various latitudes, at the equinoxes and solstices, and at different times of day. We have estimated DNA damage levels caused by increased solar UVB radiation, reduction in solar visible light due to NO 2 opacity, and deposition of nitrates through rainout of HNO 3 . For the ''typical'' nearest burst in the last billion years, we find globally averaged ozone depletion up to 38%. Localized depletion reaches as much as 74%. Significant global depletion (at least 10%) persists up to about 7 yr after the burst. Our results depend strongly on time of year and latitude over which the burst occurs. The impact scales with the total fluence of the GRB at the Earth but is insensitive to the time of day of the burst and its duration (1-1000 s). We find DNA damage of up to 16 times the normal annual global average, well above lethal levels for simple life forms such as phytoplankton. The greatest damage occurs at mid-to low latitudes. We find reductions in visible sunlight of a few percent, primarily in the polar regions. Nitrate deposition similar to or slightly greater than that currently caused by lightning is also observed, lasting several years. We discuss how these results support the hypothesis that the Late Ordovician mass extinction may have been initiated by a GRB.

Did high-energy astrophysical sources contribute to Martian atmospheric loss?

“…Photon energy (eV) = -0.8, β = -2.3, spread over 1 keV -10 MeV range (Thomas et al 2005). We obtain 3.96×10 18 eV cm −2 deposited in the top 1 g cm −2 of the atmosphere.…”

Biological effects of gamma-ray bursts: distances for severe damage on the biota

“…On the other hand, the residual nitrates of this process may make the soil more fertile after the end of this GRB winter, allowing lands to be populated by vegetation, as suggested by Thomas et al (2005b).…”

“…Other consequences are expected as well, as pointed by Thorsett (1995) and Thomas et al (2005b): the rise of the NO x concentration on the atmosphere may have a global cooling effect, blocking visible sunlight and making photosynthesis inefficient. On the other hand, the residual nitrates of this process may make the soil more fertile after the end of this GRB winter, allowing lands to be populated by vegetation, as suggested by Thomas et al (2005b).…”

Ionizing Radiation and Life

“…Thomas and Honeyman (2008) looked specifically at the consequences of a GRB on amphibian populations, as previous studies have found that a combination of higher UVB flux, elevated nitrate concentrations, and lower pH had a synergistic deleterious effect on such organisms. They considered a worst-case scenario of a GRB hitting the North Pole at autumnal equinox (as in Thomas et al, 2005b). Although the gamma photon fluence was short (10 s), the subsequent rain-out of HNO 3 from the atmosphere was calculated to last several years.…”