Title: Gamma-Ray Bursts as a Threat to Life on Earth
ArXiv ID: 0903.4710
Date: 2009-08-26
Authors: Researchers from original ArXiv paper
📝 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 ultraviolet radiation reaching the surface. This radiation is harmful to life, damaging DNA and causing sunburn. In addition, NO2 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 NO2 opacity, and deposition of nitrates through rainout of HNO3. In this paper I give a concise review of this work and discuss current and future work on extending and improving our estimates of the terrestrial impact of a GRB.
💡 Deep Analysis
Deep Dive into Gamma-Ray Bursts as a Threat to Life on Earth.
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 ultraviolet radiation reaching the surface. This radiation is harmful to life, damaging DNA and causing sunburn. In addition, NO2 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 t
📄 Full Content
Gamma-ray bursts (GRBs) have been recognized as the most powerful explosions in the universe (see e.g. Meszaros 2001;Piran 2005;Bloom et al. 2009). There appear to be two classes of GRB, based on duration of the event, which is also correlated with spectral hardness -short duration bursts tend to have harder spectra but less overall energy (Kouveliotou et al. 1993;Zhang and Choi 2008). There may also be different populations within these classes, based on luminosity (Chapman et al. 2007;Virgili et al. 2009;Berger et al. 2007;Cenko et al. 2008;Chapman et al. 2008). While still under study, long bursts are associated with core-collapse supernovae (Meszaros 2001;Piran 2005;Campana et al. 2008), while short bursts may be the result of mergers between compact objects such as neutron stars and black holes (O'Brien & Willingale 2007;Levan 2008).
Starting in the 1990’s it was recognized that GRBs, like supernovae and other astrophysical sources of ionizing radiation, could pose a threat to life on Earth over long time scales (Thorsett 1995;Scalo & Wheeler 2002;Melott et al. 2004). The radiation from a GRB is highly beamed, and therefore the Earth must fall within that beam in order to be impacted. It has been estimated that the nearest likely long duration burst pointed at the Earth in the last billion years would be on the order of 1 kpc distant (Melott 2004;Dermer & Holmes 2005;Thomas & Melott 2006).
The author and collaborators (see http://kusmos.phsx.ku.edu/~melott/Astrobiology.htm
) have performed extensive simulations of the likely impact on the Earth’s atmosphere and biosphere by a “typical” long duration GRB. Here I will briefly review this work, describe some recent efforts and discuss some outstanding issues in more accurately quantifying the impact of GRBs on life on Earth. Full details of past work can be found in Thomas et al. (2005a,b), Melott et al. (2005), Thomas & Melott (2006), Thomas & Honeyman (2008) and Melott & Thomas (2009). In addition, we have explored a wider range of event duration and spectral parameters in order to draw more general conclusions about the impact of astrophysical sources of ionizing radiation (Ejzak et al. 2007).
Our simulations have been performed using the Goddard Space Flight Center 2-dimensional atmospheric chemistry and dynamics model, developed by Charles Jackman and others. This code has been used extensively to study ozone changes due to a variety of effects, including supernovae (Geherls et al. 2003). The model has 18 latitude bands and 58 altitude bands in log pressure. Further details of the model may be found in Thomas et al. (2005b).
Effects discussed below will primarily focus on results obtained for a “standard” long GRB with power 5 x 10 44 W, with duration 10 seconds, at a distance of 2 kpc, delivering a fluence of 100 kJ m -2 . We have used the Band spectrum (Band et al. 1993), with E 0 = 187.5 keV, to compute the atmospheric ionization, which is then used as source of nitrogen oxides in the atmospheric model. In Ejzak et al. (2007) we have investigated a broader range of event duration and spectra. These results will be discussed in a general way below.
A GRB within a few parsecs that is directed at the Earth will impact one hemisphere of the planet with a short, but intense blast of high-energy photons. Gamma-rays and X-rays are highly attenuated by the Earth’s atmosphere. Therefore, the ground-level effects are primarily indirect. A small fraction of the incident energy reaches the ground as dangerous ultraviolet (UV) radiation (Smith et al. 2004), but this is limited in time to the duration of the event, which is at most 10’s of seconds for a long burst, and is less than a seconds for a short burst. While it is possible that this flash would affect some organisms, it seems unlikely that a biological catastrophe would result from this effect alone. Of course, for planets with thinner atmospheres the energy deposited at the ground would be greater and more serious effects may be expected (Smith et al. 2004;Galante & Horvath 2007). We are concerned here with effects on life on Earth and so will concentrate on the longer-term impacts.
There are three potentially harmful long-term effects of a GRB that follow from changes in atmospheric chemistry (Reid & McAfee 1978). High-energy photons cause dissociation, ionization and ionizing dissociations of N 2 and O 2 in the atmosphere. Subsequent reactions lead to the formation of nitrogen oxides, most importantly NO and NO 2 . These compounds catalytically deplete ozone (O 3 ) in the stratosphere, leading to increases in surface-level solar UV over long time periods (years). Secondly, NO 2 itself is a brown gas that absorbs strongly in the visible. This may potentially have a climatic effect by reducing solar insolation at the ground, thereby leading to cooling. Third, the atmosphere returns to normal via the removal of nitrogen oxides by way of precipitation of nitric acid (HNO 3 ).
