Changes in the solar neighbourhood due to the motion of the sun in the Galaxy, solar evolution, and Galactic stellar evolution influence the terrestrial environment and expose life on the Earth to cosmic hazards. Such cosmic hazards include impact of near-Earth objects (NEOs), global climatic changes due to variations in solar activity and exposure of the Earth to very large fluxes of radiations and cosmic rays from Galactic supernova (SN) explosions and gamma-ray bursts (GRBs). Such cosmic hazards are of low probability, but their influence on the terrestrial environment and their catastrophic consequences, as evident from geological records, justify their detailed study, and the development of rational strategies, which may minimize their threat to life and to the survival of the human race on this planet. In this chapter I shall concentrate on threats to life from increased levels of radiation and cosmic ray (CR) flux that reach the atmosphere as a result of (1) changes in solar luminosity, (2) changes in the solar environment owing to the motion of the sun around the Galactic centre and in particular, owing to its passage through the spiral arms of the Galaxy, (3) the oscillatory displacement of the solar system perpendicular to the Galactic plane, (4) solar activity, (5) Galactic SN explosions, (6) GRBs, and (7) cosmic ray bursts (CRBs). The credibility of various cosmic threats will be tested by examining whether such events could have caused some of the major mass extinctions that took place on planet Earth and were documented relatively well in the geological records of the past 500 million years (Myr).
A credible claim of a global threat to life from a change in global irradiation must first demonstrate that the anticipated change is larger than the periodical changes in irradiation caused by the motions of the Earth, to which terrestrial life has adjusted itself. Most of the energy of the sun is radiated in the visible range. The atmosphere is highly transparent to this visible light but is very opaque to almost all other bands of the electromagnetic spectrum except radio waves, whose production by the sun is rather small. The atmosphere protects the biota at ground level from over-exposure to high fluxes of extraterrestrial gamma-rays, X-ray and UV light. Because of this atmospheric protection, life has not developed immunity to these radiations (except species that perhaps were exposed elsewhere during their evolution to different conditions, such as Deinoccocus radiodurance), but has adapted itself to the normal flux levels of radiations that penetrate the atmosphere. In particular, it has adapted itself to the ground level solar irradiance, whose latitudinal and seasonal redistribution undergoes long-term quasi-periodical changes, the so-called Milankovitch cycles, due to quasi-periodical variations in the motions and orientation of the Earth. These include variation in the Earth’s eccentricity, tilt of the Earth’s axis relative to the normal to the plane of the ecliptic and precession of the Earth’s axis. Milutin Milankovitch, the Serbian astronomer, is generally credited with calculating their magnitude and the times of increased or decreased solar radiation, which directly influence the Earth’s climate system, thus impacting the advance and retreat of the Earth’s glaciers. The climate change, and subsequent periods of glaciation resulting from these variables is not due to the total amount of solar energy reaching Earth. The three Milankovitch Cycles impact the seasonality and location of solar energy around the Earth, thus affecting contrasts between the seasons. These are important only because the Earth has an asymmetric distribution of land masses, with virtually all (except Antarctica) located in/near the Northern Hemisphere.
Fig. 12.1 The intense solar flare of 4 November 2003. A giant sun spot region lashed out with an intense solar flare followed by a large coronal mass ejection (CME) on 4 November 2003. The flare itself is seen here at the lower right in an extreme ultraviolet image from the sun-staring SOHO spacecraft’s camera. This giant flare was among the most powerfulever recorded since the 1970s, the third such historic blast from AR10486 within two weeks. The energetic particle radiation from the flare did cause substantial radio interference.
Credit: SOHO-EIT Consortium, ESA, NASA
Even when all of the orbital parameters favour glaciation, the increase in winter snowfall and decrease in summer melt would barely suffice to trigger glaciation. Snow and ice have a much larger albedo (i.e., the ratio of reflected to incident electromagnetic radiation) than ground and vegetation (if the Earth was covered in ice like a giant snowball, its albedo would be approximately 0.84). Snow cover and ice masses tend to reflect more radiation back into space, thus cooling the climate and allowing glaciers to expand. Likewise supernovae (SNe), GRBs, solar flares, and cosmic rays had large influence on the terrestrial environment.
Fig. 12.2 Comet Shoemaker-Levy 9 Collision with Jupiter. From 16 through 22 July 1994, pieces of the Comet Shoemaker-Levy 9 collided with Jupiter. The comet consisted of at least 21 discernable fragments with diameters estimated at up to 2 kilometres. The four frames show the impact of the first of the 20 odd fragments of Comet Shoemaker-Levy 9 into Jupiter. The upper left frame shows Jupiter just before impact. The bright object to the right is its closest satellite Io, and the fainter oval structure in the southern hemisphere is the Great Red Spot. The polar caps appear bright at the wavelength of the observations, 2.3 μm, which was selected to maximize contrast between the fireball and the jovian atmosphere. In the second frame, the fireball appears above the southeast (lower left) limb of the planet. The fireball flared to maximum brightness within a few minutes, at which time its flux surpassed that of 10. The final frame shows Jupiter approximately 20 minutes later when the impact zone had faded somewhat. Credit: Dr. David R. Williams, NASA Goddard Space Flight Center.
Fig. 12.3 The supernova remnant Cassiopeia A. Cas A is the 300-year-old remnant created by the SN explosion of a massive star. Each Great Observatory image highlights different characteristics of the remnant. Spitzer Space Telescope reveals warm dust in the outer shell with temperatures of about 10° C(50°F), and Hubble Space Telescope sees the delicate filamentary structures of warmer gases about 10,000°C. Chandra X-ray observatory shows hot gases at about 10 million degrees Celsius. This hot gas was created when ejected material from the SN smashed into surrounding gas and dust at speeds of about 10 million miles per hour.
Credit: NASA/CXC/MIT/UMass Amherst/M.D.Stage et al.
The 1912 Milankovitch theory of glaciation cycles is widely accepted since paleoclimatic archives contain strong spectral components that match the Milankovitch cycles. However, it was recently argued that high precision paleoclimatic data have revealed serious discrepancies with the Milankovitch model that fundamentally challenge its validity and reopen the question of what causes the glacial cycles. For instance, Kirkby et al. (2004) proposed that the ice ages are initially driven not by insolation cycles but by cosmic ray changes, probably through their effect on clouds. Even if the cause of the glacial cycles is still debated, changes in global irradiation of astronomical origin must be larger than the orbital modulation of the solar irradiation in order to pose a credible threat to terrestrial life.
Fig. 12.4 The after glow of the gamma-ray burst (GRB) 030329: Images of the fading optical after glow of the GRB 030329 that took place on 29 March 2003 taken by the very large telescope (VLT) of the European Southern Observatory (ESO) in Chile on 3 April 2003 and 1 May 2003. The image taken on 1 May is dominated by an underlying supernova that produced the GRB. The discovery of the underlying supernova SN203dh convinced the majority of the astrophysicists community that ‘long-duration’ GRBs are produced by highly relativistic jets as long advocated by the Cannon ball model of GRBs. The underlying supernova was first discovered spectroscopically in the fading after glow of GRB 0302329 10 days after the GRB took place, as predicted by Dado et al. (2003) from their study of the early after glow of GRB 030329.
Credit: European Southern Observatory.
Solar flares are the most energetic explosions in the solar system. They occur in the solar atmosphere. The first solar flare recorded in astronomical literature, by the British astronomer Richard C. Carrington, occurred on 1 September 1859. Solar flares lead to the emission of electromagnetic radiation, energetic electrons, protons and atomic nuclei (solar cosmic rays) and a magnetized plasma from a localized region on the sun. A solar flare occurs when magnetic energy that has built up in the solar atmosphere is suddenly released. The emitted electromagnetic radiation is spread across the entire electromagnetic spectrum, from radio waves at the long wavelength end, through optical emission to X-rays and gamma-rays at the short wavelength end. The energies of solar cosmic rays reach a few giga electron volts = 109ev [1 ev = 1 6021753(14).10−13 J]. The frequency of solar flares varies, from several per day when the sun is particularly active to less than one per week when the sun is quiet. Solar flares may take several hours or even days to build up, but the actual flare takes only a matter of minutes to release its energy.
The total energy released during a flare is typically of the order 1027 erg s−1. Large flares can emit up to 1032 erg. This energy is less than one-tenth of the total energy emitted by the sun every second (I. = 3.84 × 1033 erg s−1). In the unlikely event that all the magnetic field energy in the solar atmosphere is radiated in a single solar flare, the solar flare energy cannot exceed ~ B2R3/12 ~ 1.4 × 1033 erg where B ~ 50 Gauss is the strength of the sun’s dipole surface magnetic field and R. MSS for symbol.] = 7x 1010 cm is the solar radius. Even this energy is only approximately one-third of the total energy emitted by the sun every second. Thus, individual solar flares are not energetic enough to cause global catastrophes on planet Earth. However, solar flares and associated coronal mass ejections strongly influence our local space weather. They produce streams of highly energetic particles in the solar wind and the Earth’s magnetosphere that can present radiation hazards to spacecraft and astronauts. The soft X-ray flux from solar flares increases the ionization of the upper atmosphere, which can interfere with short-wave radio communication, and can increase the drag on low orbiting satellites, leading to orbital decay. Cosmic rays that pass through living bodies do biochemical damage. The large number of solar cosmic rays and the magnetic storms that are produced by large solar flares are hazardous to unprotected astronauts in interplanetary space. The Earth’s atmosphere and magnetosphere protect people on the ground.
Global temperature has increased over the twentieth century by approximately 0.75°C relative to the period 1860–1900. Land and sea measurements independently show much the same warming since 1860. During this period, the concentration of CO2 in the Earth’s atmosphere has increased by approximately 27% from 290 to 370 parts per million (ppm). This level is considerably higher than at any time during the last 800,000 years, the period for which reliable data has been extracted from ice cores. This increase in the CO2 concentration in the atmosphere is widely believed to be anthropogenic in origin, that is, derived from human activities, mainly fossil fuel burning and deforestation. Recently, the Intergovernmental Panel on Climate Change (IPCC) concluded that ‘most of the observed increase in globally averaged temperatures since the mid-twentieth century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations’ via the greenhouse effect (the process in which the emission of infrared radiation by the atmosphere warms a planet’s surface, such as that of Earth, Mars and especially Venus, which was discovered by Joseph Fourier in 1829). Relying on climate models, scientists project that global surface temperatures are likely to increase by 1.1-6.4°C until 2100. An increase in global temperatures is expected to cause other changes, including sea level rise, increased intensity of extreme weather events, and changes in the amount and pattern of precipitation. Other effects of global warming include changes in agricultural yields, glacier retreat, species extinctions and increases in the ranges of disease vectors.
However, although scientists essentially agree that mankind should drastically reduce the emission of greenhouse gases and other pollutants, there are scientists who disagree with the IPCC conclusion that there is substantial evidence to prove that the increase of CO2 concentration in the atmosphere from anthropogenic sources and of other greenhouse gases are the primary cause for global warming. They point out that half of the increase in global temperature over the twentieth century took place in the beginning of the century, long before the bulk of the human influence took place. Moreover, the Earth has experienced pre-human large warming and cooling many times in the past as inferred from geological records and global temperature proxies (variables used to infer past global temperature), such as the concentration of heavy water molecules (D2O and H218O) in ice cores: The relative rate of evaporation of these molecules from seawater compared to light water molecules (H2O), increases with temperature. This increases the concentration of the heavy water molecules in precipitation, which solidify into ice over the north and south poles of the Earth. In particular, the ice cores of the Vostok and Epicaant arctic sites, which date back by 740,000 years, reveal eight previous glacial cycles with strong variation in temperature, up to a decrease by -8°C (relative to the present temperature) during the coldest ice ages and an increase by +3°C during the warmest periods. The change in the concentration of atmospheric CO2 has followed closely the change in global temperature. Supporters of the anthropogenic origin of global warming argue that dramatic increase in greenhouse gases from natural sources was responsible for past global warming, while others suggest that the release of large quantities of CO2 by the oceans was caused by the increase in their temperature by global warming. Unfortunately, so far no precise data have been presented that allow to determine which event preceded the other, the increase in atmospheric CO2 or the global warming.
Global warming remains an active field of research, although the scientific consensus is that greenhouse gases produced by human activity are responsible for it. However, a consensus is not a proper substitute for a scientific proof. Other hypotheses have been suggested to explain the observed increase in mean global temperature and should be examined scientifically. Perhaps, the one that looks most credible (to the author) is the hypothesis that the current global warming is largely the result of the reduction in the flux of cosmic rays that reach the atmosphere by an increased solar activity (e.g., Shaviv, 2005; Svens mark, 1998). This possibility is discussed shortly in Section 12.3.
The sun is about 4.5 billion years old. It will continue to shine for another 5 billion years. But, it will start to run out of hydrogen fuel in its core in less than 2 billion years from now. Then its core will contract and become hot enough for helium fusion to occur in it and hydrogen fusion in a shell around its growing helium core. Owing to the growing radiation pressure in the burning shell, the sun will begin expanding into a red giant. This fast phase will last less than 10 million years. When the sun becomes a red giant, Mercury and Venus will be swallowed up by the sun and perhaps the Earth will be too. Even if the Earth will not be swallowed up, conditions on its surface will make it impossible for life to exist. The sun’s increased luminosity will heat the Earth’s surface so much that the water of oceans and atmosphere will evaporate away. In fact, in only 1 or 2 billion years, prior to the red giant phase, the energy output of the sun will increase to a point where the Earth will probably become too hot to support life.
The most violent events likely to have occurred in the solar neighbourhood during geologic and biological history are SN explosions. Such explosions are the violent death of either massive stars following gravitational collapse of their core (core-collapse supernovae) or white dwarfs in binary systems whose mass increases by accretion beyond the Chandrasekhar mass limit (thermonuclear SN).
Core-collapse supernova takes place when the nuclear fuel in the core of a massive star of more than eight solar masses (M > 8M) has been exhausted and can no longer produce the thermal pressure that balances the gravitational pressure of the outlying layers. Then the core collapses into a neutron star or a stellar black hole and releases a huge amount of gravitational energy (~3 × 1053 erg), most of which is converted to neutrinos and only a few percent into kinetic energy of the ejected stellar envelope, which contains radio isotopes whose decay powers most of its radiation.
Thermonuclear supernovae involve the thermonuclear explosion of a white dwarf star in a binary star system. A white dwarf is the end point in the evolution for stars with mass less than eight solar masses (M < 8M). It is usually made of carbon or oxygen. Its mass cannot exceed 1.4 times the mass of the sun. A white dwarf in a binary star system can accrete material off its companion star if they are close to each other because of its strong gravitational pull. The in-falling matter from the companion star causes the white dwarf to cross the 1.4 solar-mass limit (a mass called the Chandrasekhar limit after its discoverer) and collapse gravitationally. The gravitational energy release increases the temperature to a level where the carbon and oxygen nuclei fuse uncontrollably. This results in a thermonuclear explosion that disrupts the entire star.
If a supernova explosion occurred sufficiently close to the Earth it could have dramatic effects on the biosphere. Potential implications of a nearby SN explosion for the Earth’s biosphere have been considered by a number of authors (Ellis and Schramm, 1995; Ellis et al., 1996; Ruderman, 1979) and later work has suggested that the most important effects might be induced by their cosmic rays. In particular, their possible role in destroying the Earth’s ozone layer and opening the biosphere to the extent of irradiation by solar UV radiation has been emphasized (Ellis and Schramm, 1995; Ellis et al., 1996). We shall first consider the direct radiation threats from SN explosions.
Among the new elements produced in core-collapse and thermonuclear SN explosions is radioactive nickel, which liberates huge amounts of energy inside the debris. Most of this energy is absorbed within the debris and radiated as visible light. However, the SN light does not constitute a high risk hazard. The brightest supernovae reach a peak luminosity of approximately 1043 erg s−1 within a couple of weeks after the explosion, which then declines roughly exponentially with a half-life time of 77 days (the half-life time of the radioactive cobalt that is produced by the decay of nickel). Such a luminosity at a distance of 5 parsecs from the Earth over a couple of weeks adds approximately 1% to the solar radiation that reaches the Earth and has no catastrophic consequences whatsoever. Moreover, the mean rate of Galactic SN explosions is approximately 1 in 50 years (van den Bergh and Tammann, 1991). Most of the SN explosions occur at distances from the Galactic centre much smaller than the radius of the solar orbit. Using the observed distribution of Galactic SN remnants, and the mean rate of SN explosions, the chance probability that during the next 2 billion years (before the red giant phase of the sun) the solar system in its Galactic motion will pass within 15 light years (LY) from an SN explosion is less than 10−2.
The direct threats to life on the Earth from the UV, X-ray and gamma-ray emission from SN explosions and their remnants are even smaller because the atmosphere is opaque to these radiations. The only significant threat is from the possible stripping of the Earth’s ozone layer followed by the penetration of UV radiation and absorption of visible sunlight by NO2 in the atmosphere. However, the threat from supernovae more distant than 30 LY is not larger than that from solar flares. The ozone layer has been frequently damaged by large solar flares and apparently has recovered in relatively short times.
Gamma-ray bursts are short-duration flares of MeV gamma-rays that occur in the observable universe at a mean rate of approximately 2–3 per day (e.g., Meegan and Fishman, 1995). They divide into two distinct classes. Nearly 75% are long-duration soft spectrum bursts which last more than 2 seconds; the rest are short-duration hard-spectrum bursts (SHBs) that last less than 2 seconds. There is mounting evidence from observations of the optical after-glows of long-duration GRBs that long bursts are produced by highly relativistic jets ejected during the death of massive stars in SN explosions (e.g., Dar, 2004 and references therein). The origin of SHBs is only partially known. They are not produced in SN explosion of any known type and their energy is typically three orders of magnitude smaller.
Thorsett (1995) was the first to discuss the potential effects on the atmosphere of the Earth and the damage to the biota from the hard X-rays and gamma-rays from a Galactic GRB pointing towards the Earth, while Dar et al. (1998) suggested that the main damage from Galactic GRBs arises from the cosmic rays accelerated by the jets that produce the GRBs (Shaviv and Dar, 1995). Whereas the fluxes of gamma-rays and X-rays from Galactic GRBs that illuminate the Earth and their frequency can be reliably estimated from GRB observations and their association with SN explosions, this is not the case for cosmic rays whose radiation must be estimated from debatable models. Consequently, although the effects of cosmic ray illumination may be much more devastating than those of the gamma-rays and X-rays from the same event, other authors (e.g., Galante and Horvath, 2005; Melott et al., 2004; Scalo and Wheeler, 2002; Smith et al., 2004; Thomas et al., 2005) preferred to concentrate mainly on the effects of gamma-ray and X-ray illumination.
The Galactic distribution of SN explosions is known from the distribution of their remnants. Most of these SN explosions take place in the Galactic disk at Galactocentric distances much shorter than the distance of the Earth from the Galactic centre. Their mean distance from the Earth is roughly 25,000 LY. From the measured energy fluence of GRBs (energy that reaches Earth per unit area) of known red shift it was found that the mean radiation energy emitted in long GRBs is approximately 5 × 1053/A£2/4;r erg, where Aí2 is the solid angle illuminated by the GRB (the beaming angle). The radiation energy per solid angle of short GRBs is smaller by approximately two to three orders of magnitude.
If GRBs in our Galaxy and in external galaxies are not different, then the ratio of their fluences scale like the inverse of their distance ratio squared. Should a typical Galactic GRB at a distance of d = 25,000 LY point in the direction of the Earth, its hemisphere facing the GRB will be illuminated by gamma-rays with a total fluence Fy ~ 5 × 1053/4n d2 ~ 4 × 107 erg s−1 within typically 30s. The gamma-rays’ energy and momentum will be deposited within typically 70 gcm−2 of the upper atmosphere (the total column density of the atmosphere at sea level is ~1000 gcm−2). Such fluxes would destroy the ozone layer and create enormous shocks going through the atmosphere, provoke giant global storms, and ignite huge fires. Smith et al. (2004) estimated that a fraction between 2 × 10−3 and 4 × 10−2of the Gamma-ray fluence will be converted in the atmosphere to an UV fluence at ground level. The UV radiation is mainly harmful to DNA and RNA molecules that absorb this radiation. The lethal dose for a UV exposure, approximately 104 erg cm−2, makes a GRB at 25,000 LY potentially highly lethal (e.g., Galante and Horvath, 2005) to the hemisphere illuminated by the GRB. But, protection can be provided by habitat (underwater, underground, under-roof, shaded areas) or by effective skin covers such as furs of animals or clothing of human beings. The short duration of GRBs and the lack of a nearly warning signal, however, make protection by moving into the shade or a shelter, or by covering up quickly, unrealistic for most species.
It should be noted that the MeV gamma-ray emission from GRBs may be accompanied by a short burst of very high energy gamma-rays, which, so far, could not be detected, either by the gamma-ray and X-ray satellites (CGRO, BeppoSAX, HETE, Chandra, XMMNewton, Integral, SWIFT and the interplanetary network), or by ground-level high energy gamma-ray telescopes such as HESS and Magic (due to timelag in response). Such bursts of GeV and TeV gamma-rays, if produced by GRBs, might be detected by the Gamma-ray Large Area Space Telescope (GLAST), which will be launched into space in 16.V.2008. GeV-TeV gamma-rays from relatively nearby Galactic GRBs may produce lethal doses of atmospheric muons.
The mean energy density of Galactic cosmic rays is similar to that of star light, the cosmic microwave background radiation and the Galactic magnetic field, which all happen to be of the order of approximately 1 eV cm−3. This energy density is approximately eight orders of magnitude smaller than that of solar light at a distance of one astronomical unit, that is, that of the Earth, from the sun. Moreover, cosmic rays interact at the top of the atmosphere and their energy is converted to atmospheric showers. Most of the particles and gamma-rays in the atmospheric showers are stopped in the atmosphere before reaching ground level, and almost only secondary muons and neutrinos, which carry a small fraction of their energy, reach the ground. Thus, at first it appears that Galactic cosmic rays cannot affect life on the Earth significantly. However, this is not the case. There is accumulating evidence that even moderate variations in the flux of cosmic rays that reach the atmosphere have significant climatic effects, despite their low energy density. The evidence comes mainly from two sources:
1. The interaction of cosmic rays with nuclei in the upper atmosphere generates showers of secondary particles, some of which produce the radioisotopes14C and10Be that reach the surface of the Earth either via the carbon cycle (14CO2) or in rain and snow (10Be). Since this is the only terrestrial source, their concentration in tree rings, ice cores, and marine sediments provides a good record of the intensity of Galactic cosmic rays that have reached the atmosphere in the past. They show clear correlation between climate changes and variation of the cosmic ray flux in the Holocene era.
2. The ions produced in cosmic ray showers increase the production of low altitude clouds (e.g., Carslaw et al. 2002). In data collected in the past 20 years, by satellites and neutron monitors, there is a clear correlation between global cloud cover and cosmic ray flux above 10 GeV that penetrate the geomagnetic field. Cloud cover reduces ground level radiation by a global average of 30 Wm−2, which are 13% of the ground level solar irradiance. An increased flux of Galactic cosmic rays is associated with an increase in low cloud cover, which increases the reflectivity of the atmosphere and produces a cooler temperature.
Cosmic rays affect life in other ways:
1. Cosmic ray-produced atmospheric showers of ionized particles trigger lightening discharges in the atmosphere (Gurevich and Zybin, 2005). These showers produce NO and NO2 by direct ionization of molecules, which destroy ozone at a rate faster than its production in the discharges. The depletion of the ozone in the atmosphere leads to an increased UV flux at the surface.
2. The decay of secondary mesons produced in showers yields high energy penetrating muons, which reach the ground and penetrate deep underground and deep underwater. A small fraction of energetic protons and neutrons from the shower that increases with the energy of the primary cosmic ray particle also reaches the surface. Overall, the very penetrating secondary muons are responsible for approximately 85% of the total equivalent dose delivered by cosmic rays at ground level. Their interactions, and the interactions of their products, with electrons and nuclei in living cells, ionize atoms and break molecules and damage DNA and RNA by displacing electrons, atoms and nuclei from their sites. The total energy deposition dose from penetrating muons resulting in 50% mortality in 30 days is between 2.5 and 3.0 Gy (1 Gy = 104, erg g−1). A cosmic ray muon deposits approximately 4 Mev g−1 in living cells, and thus the lethal flux of cosmic ray muons is 5 × 109 cm−2 if delivered in a short time (less than a month). In order to deliver such a dose within a month, the normal cosmic ray flux has to increase by nearly a factor of a thousand during a whole month).
A large increase in cosmic ray flux over extended periods may produce global climatic catastrophes and expose life on the ground, underground, and underwater to hazardous levels of radiation, which result in cancer and leukaemia. However, the bulk of Galactic cosmic rays have energies below 10 GeV. Such cosmic rays that enter the heliosphere are deflected by the magnetic field of the solar wind before they reach the Earth’s neighbourhood and by its geomagnetic field before they reach the Earth’s atmosphere. Consequently, the Galactic cosmic ray flux that reaches the Earth’s atmosphere is modulated by variations in the solar wind and in the Earth’s magnetic field.
Life on the Earth has adjusted itself to the normal cosmic ray flux, which reaches its atmosphere. Perhaps, cosmic ray-induced mutations in living cells played a major role in the evolution and diversification of life from a single cell to the present millions of species. Any credible claim for a global threat to life from increasing fluxes of cosmic rays must demonstrate that the anticipated increase is larger than the periodical changes in the cosmic ray flux that reaches the Earth, which result from the periodic changes in solar activity, in the geomagnetic field and in the motions of the Earth, and to which terrestrial life has adjusted itself.
The Earth’s magnetic field reverses polarity every few hundred thousand years, and is almost non-existent for perhaps a century during the transition. The last reversal was 780 Ky ago, and the magnetic field’s strength decreased by 5% during the twentieth century! During the reversals, the ozone layer becomes unprotected from charged solar particles, which weakens its ability to protect humans from UV radiation. However, past reversals were not associated with any major extinction according to the fossil record, and thus are not likely to affect humanity in a catastrophic way.
Cosmic rays are the main physical mechanism controlling the amount of ionization in the troposphere (the lower 10 km or so of the atmosphere). The amount of ionization affects the formation of condensation nuclei required for the formation of clouds in clean marine environment. The solar wind – the outflow of energetic particles and entangled magnetic field from the sun – is stronger and reaches larger distances during strong magnetic solar activity. The magnetic field carried by the wind deflects the Galactic cosmic rays and prevents the bulk of them from reaching the Earth’s atmosphere. A more active sun therefore inhibits the formation of condensation nuclei, and the resulting low-altitude marine clouds have larger drops, which are less reflective and live shorter. This decrease in cloud coverage and in cloud reflectivity reduces the Earth’s albedo. Consequently, more solar light reaches the surface and warms it.
Cosmic ray collisions in the atmosphere produce14C, which is converted to14CO2 and incorporated into the tree rings as they form; the year of growth can be precisely determined from dendrochronology. Production of14C is high during periods of low solar magnetic activity and low during high magnetic activity. This has been used to reconstruct solar activity during the past 8000 years, after verifying that it correctly reproduces the number of sunspots during the past 400 years (Solanki et al., 2004). The number of such spots, which are areas of intense magnetic field in the solar photosphere, is proportional to the solar activity. Their construction demonstrates that the current episode of high sunspot number and very high average level of solar activity, which has lasted for the past 70 years, has been the most intense and has had the longest duration of any in the past 8000 years. Moreover, the solar activity correlates very well with palaeoclimate data, supporting a major solar activity effect on the global climate.
Using historic variations in climate and the cosmic ray flux, Shaviv (2005) could actually quantify empirically the relation between cosmic ray flux variations and global temperature change, and estimated that the solar contribution to the twentieth century warming has been 0.50 ± 0.20°C out of the observed increase of 0.75 ± 0.15°C, suggesting that approximately two-thirds of global warming resulted from solar activity while perhaps only approximately one-third came from the greenhouse effect. Moreover, it is quite possible that solar activity coupled with the emission of greenhouse gases is a stronger driver of global warming than just the sum of these two climatic drivers.
Radio emission from the Galactic spiral arms provides evidence for their enhanced CR density. Diffuse radio emission from the interstellar medium is mainly synchrotron radiation emitted by CR electrons moving in the interstellar magnetic fields. High contrasts in radio emission are observed between the spiral arms and the discs of external spiral galaxies. Assuming equipartition between the CR energy density and the magnetic field density, as observed in many astrophysical systems, CR energy density in the spiral arms should be higher than in the disk by a factor of a few. Indeed, there is mounting evidence from radio and X-ray observations that low energy CRs are accelerated by the debris of core-collapse SNe. Most of the supernovae in spiral galaxies like our own are core-collapse SNe. They predominantly occur in spiral arms where most massive stars are born and shortly thereafter die. Thus, Shaviv (2002) has proposed that when the solar system passes through the Galactic spiral arms, the heliosphere is exposed to a much higher cosmic ray flux, which increases the average low-altitude cloud cover and reduces the average global temperature. Coupled with the periodic variations in the geomagnetic field and in the motion of the Earth around the sun, this may have caused the extended periods of glaciations and ice ages, which, in the Phanerozoicera, have typically lasted approximately 30 Myr with a mean separation of approximately 140 Myr. Indeed, Shaviv (2002) has presented supportive evidence from geological and meteoritic records for a correlation between the extended ice ages and the periods of an increased cosmic ray flux. Also, the duration of the extended ice ages is in agreement with the typical crossing time of spiral arms (typical width of 100 LY divided by a relative velocity of approximately 10 km s−1yields approximately 30 Myr crossing time). Note also that passage of the heliosphere through spiral arms, which contain a larger density of dust grains produced by SN explosions, can enter the heliosphere, reach the atmosphere, scatter away sunlight, reduce the surface temperature and cause an ice age.
The cosmic ray flux from a supernova remnant (SNR) has been estimated to produce a fluence F ~ 7.4 × 106 (30 LY/d)2 erg cm−2 at a distance d from the remnant. The active acceleration time of an SNR is roughly 104years. The ambient CR flux near the Earth is 9 × 104 erg cm−2 year−1. Thus, at a distance of 300 LY approximately, the ambient flux level would increase approximately by a negligible 0.1 % during a period of 104 years approximately. To have a significant effect, the supernova has to explode within 30 LY from the sun. Taking into consideration the estimated SN rate and distribution of Galactic SNRs, the rate of SN explosions within 30 LY from the sun is approximately 3 × 10−10 year−1. However, at such a distance, the SN debris can blow away the Earth’s atmosphere and produce a major mass extinction.
Radio, optical, and X-ray observations with high spatial resolution indicate that relativistic jets, which are fired by quasar and micro-quasars, are made of a sequence of plasmoids (cannonballs) of ordinary matter whose initial expansion (presumably with an expansion velocity similar to the speed of sound in a relativistic gas) stops shortly after launch (e.g., Dar and De Rujula, 2004 and references therein). The photometric and spectroscopic detection of SNe in the fading after glows of nearby GRBs and various other properties of GRBs and their after glows provide decisive evidence that long GRBs are produced by highly relativistic jets of plasmoids of ordinary matter ejected in SN explosions, as long advocated by the Cannonball (CB) Model of GRBs (see, for example, Dar, 2004, Dar &A. De Rujula 2004, “Magnetic field in galaxies, galaxy clusters, &intergalactic space in: Physical Review D 72, 123002123006; Dar and De Rujula, 2004). These jets of plasmoids (cannonballs) produce an arrow beam of high energy cosmic rays by magnetic scattering of the ionized particles of the interstellar medium (ISM) in front of them. Such CR beams from Galactic GRBs may reach large Galactic distances and can be much more lethal than their gamma-rays (Dar and De Rujula, 2001; Dar et al., 1998).
Let v = β3 c be the velocity of a highly relativistic CB and 
be its Lorentz factor. For long GRBs, typically, γ ~ 103 (v ~ 0.999999c!). Because of the highly relativistic motion of the CBs, the ISM particles that are swept by the CBs enter them with a Lorentz factor γ ~ 103 in the CBs’ rest frame. These particles are isotropized and accelerated by the turbulent magnetic fields in the CBs (by a mechanism proposed by Enrico Fermi) before they escape back into the ISM. The highly relativistic motion of the CBs, boosts further their energy by an average factor γ through the Doppler effect and collimates their isotropic distribution into an arrow conical beam of an opening angle θ ~ 1/γ around the direction of motion of the CB in the ISM. This relativistic beaming depends only on the CB’s Lorentz factor but not on the mass of the scattered particles or their energy.
The ambient interstellar gas is nearly transparent to the cosmic ray beam because the Coulomb and hadronic cross-sections are rather small with respect to typical Galactic column densities. The energetic CR beam follows a ballistic motion rather than being deflected by the ISM magnetic field whose typical value is B ~ 3 × 10−6 Gauss. This is because the magnetic field energy swept up by the collimated CR beam over typical distances to Galactic supernovae is much smaller than the kinetic energy of the beam. Thus, the CR beam sweeps away the magnetic field along its way and follows a straight ballistic trajectory through the interstellar medium. (The corresponding argument, when applied to the distant cosmological GRBs, leads to the opposite conclusion: no CR beams from distant GRBs accompany the arrival of their beamed gamma-rays.)
The fluence of the collimated beam of high energy cosmic rays at a distance from a GRB that is predicted by the CB model of GRBs is given approximately by F ~ Ekγ2/4π d2 ~ 1020 (LY/d2) erg cm−2, where the typical values of the kinetic energy of the jet of CBs, Ek ~ 1051 erg and γ ~ 103, were obtained from the CB analysis of the observational data on long GRBs. Observations of GRB after glows indicate that it typically takes a day or two for a CB to lose approximately 50% of its initial kinetic energy, that is, for its Lorentz factor to decrease to half its initial value. This energy is converted to CRs with a typical Lorentz factor γCR ~ γ2 whose arrival time from a Galactic distance is delayed relative to the arrival of the afterglow photons by a negligible time, Δt ~ d/cγCR. Consequently, the arrival of the bulk of the CR energy practically coincides with the arrival of the afterglow photons.
Thus, for a typical long GRB at a Galactic distance d = 25,000 LY, which is viewed at a typical angle θ ~ 1/γ ~ 10−3 radians, the energy deposition in the atmosphere by the CR beam is F ~ 1011 erg cm−2 while that deposited by gamma-rays is smaller by about 3 order of magnitude (the kinetic energy of the electrons in the jet is converted to a conical beam of gamma-rays while the bulk of the kinetic energy that is carried by protons is converted to a conical beam of CRs with approximately the same opening angle). The beam of energetic cosmic rays accompanying a Galactic GRB is deadly for life on Earthlike planets. When high energy CRs with energy Ep collide with the atmosphere at a zenith angle θZ, they produce high energy muons whose number is given approximately by Nμ (E > 25 GeV) ~ 9.14[Ep/TeV]0.757/cos θz (Drees et al., 1989). Consequently, a typical GRB produced by a jet with EK ~ 1051 erg at a Galactic distance of 25,000 LY, which is viewed at the typical viewing angle θ ~ 1/γ ~ 10−3, is followed by a muon fluence at ground level that is given by Fμ(E > 25 GeV) ~ 3 × 1011cm−2. Thus, the energy deposition rate at ground level in biological materials, due to exposure to atmospheric muons produced by an average GRB near the centre of the Galaxy, is 1.4 × 1012 MeVg−1. This is approximately 75 times the lethal dose for human beings. The lethal dosages for other vertebrates and insects can be a few times or as much as a factor 7 larger, respectively. Hence, CRs from galactic GRBs can produce a lethal dose of atmospheric muons for most animal species on the Earth. Because of the large range of muons (~4[Eμ/GeV]m) in water, their flux is lethal, even hundreds of metres under water and underground, for CRs arriving from well above the horizon. Thus, unlike other suggested extraterrestrial extinction mechanisms, the CRs of galactic GRBs canal so generate massive extinctions deep under water and underground. Although half of the planet is in the shade of the CR beam, its rotation exposes a larger fraction of its surface to the CRs, half of which will arrive within over approximately 2 days after the gamma-rays. Additional effects that will increase the lethality of the CRs over the whole planet include:
1. Evaporation of a significant fraction of the atmosphere by the CR energy deposition.
2. Global fires resulting from heating of the atmosphere and the shock waves produced by the CR energy deposition in the atmosphere.
3. Environmental pollution by radioactive nuclei, produced by spallation of atmospheric and ground nuclei by the particles of the CR-induced showers that reach the ground.
4. Depletion of stratospheric ozone, which reacts with the nitric oxide generated by the CR-produced electrons (massive destruction of stratospheric ozone has been observed during large solar flares, which generate energetic protons).
5. Extensive damage to the food chain by radioactive pollution and massive extinction of vegetation by ionizing radiation (the lethal radiation dosages for trees and plants are slightly higher than those for animals, but still less than the flux estimated above for all but the most resilient species).
In conclusion, the CR beam from a Galactic SN/GRB event pointing in our direction, which arrives promptly after the GRB, can kill, in a relatively short time (within months), the majority of the species alive on the planet.
Geological records testify that life on Earth has developed and adapted itself to its rather slowly changing conditions. However, good quality geological records, which extend up to approximately 500 Myr ago, indicate that the exponential diversification of marine and continental life on the Earth over that period was interrupted by many extinctions (e.g., Benton 1995; Erwin 1996, 1997; Raup and Sepkoski, 1986), with the major ones exterminating more than 50% of the species on land and sea, and occurring on average, once every 100 Myr. The five greatest events were those of the final Ordovician period (some 435 Myr ago), the late Devonian (357 Myr ago), the final Permian (251 Myr ago), the late Triassic (198 Myr ago) and the final Cretaceous (65 Myr ago). With, perhaps, the exception of the Cretaceous-Tertiary mass extinction, it is not well known what caused other mass extinctions. The leading hypotheses are:
• Meteoritic Impact : The impact of a sufficiently large asteroid or comet could create mega-tsunamis, global forest fires, and simulate nuclear winter from the dust it puts in the atmosphere, which perhaps are sufficiently severe as to disrupt the global ecosystem and cause mass extinctions. A large meteoritic impact was invoked (Alvarez et al., 1980) in order to explain their idium anomaly and the mass extinction that killed the dinosaurs and 47% of all species around the K/T boundary, 65 Myr ago. Indeed, a 180 km wide crater was later discovered, buried under 1 km of Cenozoic sediments, dated back 65 Myr ago and apparently created by the impact of a 10 km diameter meteorite or comet near Chicxulub, in the Yucatan (e.g., Hildebrand, 1990; Morgan et al., 1997; Sharpton and Marin, 1997). However, only for the End Cretaceous extinction is there compelling evidence of such an impact. Circumstantial evidence was also claimed for the End Permian, End Ordovician, End Jurassic and End Eocene extinctions.
• Volcanism: The huge Deccan basalt floods in India occurred around the K/T boundary 65 Myr ago when the dinosaurs were finally extinct. The Permian/Triassic (P/T) extinction, which killed between 80% and 95% of the species, is the largest known is the history of life; occurred 251 Myr ago, around the time of the gigantic Siberian basalt flood. The outflow of millions of cubic kilometres of lava in a short time could have poisoned the atmosphere and oceans in a way that may have caused mass extinctions. It has been suggested that huge volcanic eruptions caused the End Cretaceous, End Permian, End Triassic, and End Jurassic mass extinctions (e.g., Courtillot, 1988; Courtillot et al., 1990; Officer and Page, 1996; Officer et al., 1987).
• Drastic Climate Changes: Rapid transitions in climate may be capable of stressing the environment to the point of making life extinct, though geological evidence on the recent cycles of ice ages indicate they had only very mild impacts on biodiversity. Extinctions suggested to have this cause include: End Ordovician, End Permian, and Late Devonian.
Paleontologists have been debating fiercely which one of the above mechanisms was responsible for the major mass extinctions. But, the geological records indicate that different combinations of such events, that is, impacts of large meteorites or comets, gigantic volcanic eruptions, drastic changes in global climate and huge sea regressions/sea rise seem to have taken place around the time of the major mass extinctions. Can there be a common cause for such events?
The orbits of comets indicate that they reside in an immense spherical cloud (‘the Oort cloud’), that surrounds the planetary with a typical radius of R ~ 100,000 AU. The statistics imply that it may contain as many as 1012 comets with a total mass perhaps larger than that of Jupiter. The large radius implies that the comets share very small binding energies and mean velocities of v < 100 ms−1. Relatively small gravitational perturbations due to neighbouring stars are believed to disturb their orbits, unbind some of them, and put others into orbits that cross the inner solar system. The passage of the solar system through the spiral arms of the Galaxy, where the density of stars is higher, could have caused such perturbations, and consequently, the bombardment of the Earth with a meteorite barrage of comets over an extended period longer than the free fall time. It has been claimed by some authors that the major extinctions were correlated with passage times of the solar system through the Galactic spiral arms. However, these claims were challenged. Other authors suggested that biodiversity and extinction events may be influenced by cyclic processes. Raup and Sepkoski (1986) claimed a 26–30 Myr cycle in extinctions. Although this period is not much different from the 31 Myr period of the solar system crossing the Galactic plane, there is no correlation between the crossing time and the expected times of extinction. More recently, Rohde and Muller (2005) have suggested that biodiversity has a 62 ±3 Myr cycle. But, the minimum in diversity is reached only once during a full cycle when the solar system is farthest away in the northern hemisphere from the Galactic plane.
Could Galactic GRBs generate the major mass extinction, and can they explain the correlation between mass extinctions, meteoritic impacts, volcano eruptions, climate changes and sea regressions, or can they only explain the volcanic-quiet and impact-free extinctions?
Passage of the GRB jet through the Oort cloud, sweeping up the interstellar matter on its way, could also have generated perturbations, sending some comets into a collision course with the Earth.
The impact of such comets and meteorites may have triggered the huge volcanic eruptions, perhaps by focusing shock waves from the impact at an opposite point near the surface on the other side of the Earth, and creating the observed basalt floods, timed within 1−2 Myr around the K/T and P/T boundaries. Global climatic changes, drastic cooling, glaciation and sea regression could have followed from the drastic increase in the cosmic ray flux incident on the atmosphere and from the injection of large quantities of light-blocking materials into the atmosphere from the cometary impacts and the volcanic eruptions. The estimated rate of GRBs from observations is approximately 103 year−1. The sky density of galaxies brighter than magnitude 25 (the observed mean magnitude of the host galaxies of the GRBs with known red-shifts) in the Hubble telescope deep field is approximately 2 × 10−5 per square degree. Thus, the rate of observed GRBs, per galaxy with luminosity similar to that of the Milky Way, is RGRB ~ 1.2 × 10−7 year−1. To translate this result into the number of GRBs born in our own galaxy, pointing towards us, and occurring in recent cosmic times, one must take into account that the GRB rate is proportional to the star formation rate, which increases with red-shift z like (1 + z)4 for z < 1and remains constant up to z ~ 6. The mean red-shift of GRBs with known red-shift, which were detected by SWIFT, is ~2.8, that is, most of the GRBs were produced at a rate approximately 16 times larger than that in the present universe. The probability of a GRB pointing towards us within a certain angle is independent of distance. Therefore, the mean rate of GRBs pointing towards us in our galaxy is roughly RGRB /(1 + z)4 ~ 0.75x 10−8 year−1, or once every 130 Myr. If most of these GRBs take place not much farther away than the distance to the galactic centre, their effect is lethal, and their rate is consistent with the rate of the major mass extinctions on our planet in the past 500 Myr.
The observation of planets orbiting nearby stars has become almost a routine. Although current observations/techniques cannot detect yet planets with masses comparable to the Earth near other stars, they do suggest their existence. Future space-based observatories to detect Earth-like planets are being planned. Terrestrial planets orbiting in the habitable neighbourhood of stars, where planetary surface conditions are compatible with the presence of liquid water, might have global environments similar to ours, and harbour life. But, our solar system is billions of years younger than most of the stars in the Milky Way and life on extra solar planets could have preceded life on the Earth by billions of years, allowing for civilizations much more advanced than ours. Thus Fermi’s famous question, ‘where are they?’, that is, why did they not visit us or send signals to us? One of the possible answers is provided by cosmic mass extinction: even if advanced civilizations are not self-destructive, they are subject to a similar violent cosmic environment that may have generated the big mass extinctions on this planet. Consequently, there may be no nearby aliens who have evolved long enough to be capable of communicating with us, or pay us a visit.
• Solar flares do not comprise a major threat to life on the Earth. The Earth’s atmosphere and the magnetosphere provide adequate protection to life on its surface, under water and underground.
• Global warming is a fact. It has drastic effects on agricultural yields, cause glacier retreat, species extinctions and increases in the ranges of disease vectors. Independent of whether or not global warming is of anthropogenic origin, human kind must conserve energy, burn less fossil fuel, and use and develop alternative non-polluting energy sources.
• The current global warming may be driven by enhanced solar activity. On the basis of the length of past large enhancements in solar activity, the probability that the enhanced activity will continue until the end of the twenty-first century is quite low (1%). (However, if the global warming is mainly driven by enhanced solar activity, it is hard to predict the time when global warming will turn into global cooling.
• Within 1–2 billion years, the energy output of the sun will increase to a point where the Earth will probably become too hot to support life.
• Passage of the sun through the Galactic spiral arms once in 140 Myr approximately will continue to produce major, approximately 30 Myr long, ice ages.
• Our knowledge of the origin of major mass extinctions is still very limited. Their mean frequency is extremely small, once every 100 Myr. Any initiative/decision beyond expanding research on their origin is premature.
• Impacts between near-Earth Objects and the Earth are very infrequent but their magnitude can be far greater than any other natural disaster. Such impacts that are capable of causing major mass extinctions are extremely infrequent as evident from the frequency of past major mass extinctions. At present, modern astronomy cannot predict or detect early enough such an imminent disaster and society does not have either the capability or the knowledge to deflect such objects from a collision course with the Earth.
• A SN would have to be within few tens of light years from the Earth for its radiation to endanger creatures living at the bottom of the Earth’s atmosphere. There is no nearby massive star that will undergo a SN explosion close enough to endanger the Earth in the next few million years. The probability of such an event is negligible, less than once in 109years.
• The probability of a cosmic ray beam or a gamma-ray beam from Galactic sources (SN explosions, mergers of neutron stars, phase transitions in neutron stars or quark stars, and micro-quasarejections) pointing in our direction and causing a major mass extinction is rather small and it is strongly constrained by the frequency of past mass extinctions – once every 100Myr.
• No source in our Galaxy is known to threaten life on the Earth in the foreseeable future.
Alvarez, L.W., Alvarez, W., Asaro, F., and Michel, H.V. (1980). Extraterrestrial cause for the Cretaceous tertiary extinction, Science, 208, 1095–1101.
Benton, M.J. (1995). Diversification and extinction in the history of life, Science, 268, 52–58.
Carslaw, K.S., Harrison, R.G., and Kirkby, J. (2002). Cosmic rays, clouds, and climate. Science, 298, 1732–1737.
Courtillot, V. (1990). A Volcanic Eruption, Scientific American, 263, October 1990, pp. 85–92.
Courtillot, V., Feraud, G., Maluski, H., Vandamme, D., Moreau, M.G., and Besse, J. (1998). Deccan Flood Basalts and the Cretaceous/Tertiary Boundary. Nature, 333, 843–860.
Dado, S., Dar, A., and De Rújula, A. (2002). On the optical and X-ray afterglows of gamma ray bursts. Astron. Astrophys., 388, 1079–1105.
Dado, S., Dar, A., and De Rújula, A. (2003). The supernova associated with GRB 030329. Astrophys. J., 594, L89.
Dar, A. (2004). The GRB/XRF-SN Association, arXiv astro-ph/0405386.
Dar, A. and De Rújula, A. (2002). The threat to life from Eta Carinae and gamma ray bursts. In Morselli, A. and Picozza, P. (eds.) Astrophysics and Gamma Ray Physics in Space (Frascati Physics Series Vol. XXIV, pp. 513–523 (astro-ph/0110162).
Dar, A. and De Rújula, A. (2004). Towards a complete theory of gamma-ray bursts. Phys. Rep., 405, 203–278.
Dar, A., Laor, A., and Shaviv, N. (1998). Life extinctions by cosmic ray jets, Phys. Rev. Lett., 80, 5813–5816.
Drees, M., Halzen, F., and Hikasa, K. (1989). Muons in gamma showers, Phys. Rev., D39, 1310–1317.
Du and De Rujula, A. (2004). Magnetic field in galaxies, galaxy dusters, and intergalactic space in: Physical Review D 72, 123002–123006.
Ellis, J., Fields, B.D., and Schramm, D.N. (1996). Geological isotope anomalies as signatures of nearby supernovae. Astrophys. J., 470, 1227–1236.
Ellis, J. and Schramm, D.N. (1995). Could a nearby supernova explosion have caused a mass extinction?, Proc. Nat. Acad. Sci., 92, 235–238.
Erwin, D.H. (1996). The mother of mass extinctions, Scientific American, 275, July 1996, p. 56–62.
Erwin, D.H. (1997). The Permo-Triassic extinction. Nature, 367, 231–236.
Fields, B.D. and Ellis, J. (1999). On deep-ocean60Fe as a fossilofa near-earth supernova. New Astron., 4, 419–430.
Galante, D. and Horvath, J.E. (2005). Biological effects of gamma ray bursts: distances for severe damage on the biota. Int. J. Astrobiology, 6, 19–26.
Gurevich, A.V. and Zybin, K.P. (2005). Runaway breakdown and the mysteries of lightning. Phys. Today, 58, 37–43.
Hildebrand, A.R. (1990). Mexican site for K/T Impact Crater?, Mexico, Eos, 71, 1425.
Kirkby, J., Mangini, A., and Muller, R.A. (2004). Variations of galactic cosmic rays and the earth’s climate. In Frisch, P.C. (ed.), Solar Journey: The Significance of Our Galactic Environment for the Heliosphere and Earth (Netherlands: Springer) pp. 349397 (arXivphysics/0407005).
Meegan, C.A. and Fishman, G.J. (1995). Gamma ray bursts. Ann. Rev. Astron. Astrophys., 33, 415–458.
Melott, A., Lieberman, B., Laird, C., Martin, L., Medvedev, M., Thomas, B., Cannizzo, J., Gehrels, N., and Jackman, C. (2004). Did a gamma-ray burst initiate the late Ordovician mass extinction? Int. J. Astrobiol., 3, 55–61.
Morgan, J., Warner, M., and Chicxulub Working Group. (1997). Size and morphology of the Chixulub Impact Crater. Nature, 390, 472–476.
Officer, C.B., Hallan, A., Drake, C.L., and Devine, J.D. (1987). Global fire at the Cretaceous-Tertiary boundary. Nature, 326, 143–149.
Officer, C.B. and Page, J. (1996). The Great Dinosaurs Controversy (Reading, MA: Addison-Wesley Pub. Com.).
Raup, D. and Sepkoski, J. (1986). Periodic extinction of families and genera. Science, 231, 833–836.
Rohde, R.A. and Muller, R.A. (2005). Cycles in fossil diversity. Nature, 434, 208–210. Ruderman, M.A. (1974). Possible consequences of nearby supernova explosions for atmospheric ozone and terrestrial life. Science, 184, 1079–1081.
Scalo, J. and Wheeler, J.C. (2002). Did a gamma-ray burst initiate the late Ordovician mass extinction? Astrophys. J., 566, 723–737.
Sepkoski, J.J. (1986). Is the periodicity of extinctions a taxonomic artefact? In Raup, D.M. and Jablonski, D. (eds.), Patterns and Processes in the History of Life. pp. 277–295 (Berlin: Springer-Verlag).
Sharpton, V.L. and Marin, L.E. (1997). The Cretaceous-Tertiary impact crater. Ann. NY Acad. Sci., 822, 353–380.
Shaviv, N. (2002). The spiral structure of the Milky Way, cosmic rays, and ice age epochs on earth. New Astron., 8, 39–77.
Shaviv, N. and Dar, A. (1995). Gamma ray bursts from Minijets. Astrophys. J., 447, 863–873.
Smith, D.S., Scalo, J., and Wheeler, J.C. (2004). Importance of biologically active Aurora-like ultraviolet emission: stochastic irradiation of earth and mars by flares and explosions. Origins Life Evol. Bios., 34, 513–532.
Solanki, S.K., Usoskin, I.G., Kromer, B., Sch,ússler, M., and Bear, J. (2004). Unusual activity of the sun during recent decades compared to the previous 11000 Years. Nature, 431, 1084–1087.
Svensmark, H. (1998). Influence of Cosmic rays on earths climate. Phys. Rev. Lett., 81, 5027–5030.
Thomas, B.C., Jackman, C.H., Melott, A.L., Laird, C.M., Stolarski, R.S., Gehrels, N., Cannizzo, J.K., and Hogan, D.P. (2005). Terrestrial ozone depletion due to a milky way gamma-ray burst, Astrophys. J., 622, L153-L156.
Thorsett, S.E. (1995). Terrestrial implications of cosmological gamma-ray burst models. Astrophys. J. Lett., 444, L53-L55.
van den Bergh, S. and Tammann, G.A. (1991). Galactic and extragalactic supernova rates. Ann. Rev. Astron. Astrophys., 29, 363–407.
