Beyond the Sun Influence of Our Galactic Environment on Life on Earth

In this section we shall discuss two possible sources of SpW factors that may have influenced the evolution of life on Earth and such factors may be reflected in the fossil record: gamma ray bursts (GRBs) and cosmic rays.

Firstly, GRBs are powerful explosions that produce a flux of radiation detectable across the observable Universe. These events possibly originate in distant galaxies, and a large percentage likely arises from explosions of stars over 15 times more massive than our Sun. A burst creates two oppositely directed beams of gamma rays that race off into space. If a GRB were to take place within the Milky Way we would have to consider the possibility of mass extinctions comparable to the other known sources, such as the meteoritic collision with the Earth (cf., the next section), or a singular abundance of sulphur in the atmosphere due to the causes that are reviewed below. Mass extinctions have eliminated a significant fraction of life on Earth. For example, the most severe extinction of the past 500 million years occurred in the Late Permian (Erwin, 1994). The large masses of the first stars suggest that they may have produced supernovas at the end of their relatively short lifetimes. Such events may in principle be detectable as GRBs at very large red shifts, which may be detectable with the SWIFT satellite (Hartmann, 2005; Markwardt et al., 2005).

A number of other astrophysical objects also produce GRBs, such as quasars and neutron stars. Quasars were also forming at around z ~6, so part of the challenge is to identify the proper GRB source (Xu et al., 2005). GRB, together with meteoritic collisions, or an atmosphere that has gone through a transition unfavourable to the Earth biota are three likely causes that need to be discussed together, as we have attempted to do in the present review.

The Ordovician is the second oldest period of the Palaeozoic Era, thought to have covered the span of time between 505 and 440 million years before the present Ma BP. The late Ordovician mass extinction took place at approximately 440 Ma BP may be at least partly the result of a GRB. Due to expected depletion of the ozone layer arising from the incoming energetic flux, the solar ultraviolet radiation that is normally shielded would give rise to a severely modified ecosystem.

It is known that all marine animals suffered mass mortalities during the Late Ordovician Mass mortalities at the close of the Cambrian and late in the Ordovician resulted in the unique aspects of the Ordovician fauna.

The Swift mission, launched in November 2004, contributes to determine recent burst rates. During evolution of life certain events triggered large-scale extinctions. We consider one of the most remarkable possible candidates. The Late Ordovician extinction created new opportunities for benthic and planktonic marine fauna. Biological radiation during post-Ordovician glaciation led to many new taxa typical of the Silurian. GRBs within our Galaxy have been repeatedly suggested to be a possible threat to life on Earth (Thorsett, 1995; Scalo and Wheeler, 2002; Melott et al., 2004).

Some effects similar to those due to a nearby supernova should be expected. GRBs are less frequent than supernovae, but their greater energy output results in a larger region of influence, and hence they may pose a greater threat. It is likely (Melott et al., 2004) that in the last billion years (Ga), a GRB has occurred close enough to have dramatic effects on the stratospheric ozone, leading to detrimental effects on life through increases in solar ultraviolet (UV) radiation, which is strongly absorbed by ozone. A major question has been the timescale for atmospheric chemistry: most of the GRB influence comes in seconds or minutes as compared to months for the case of supernovae.

There is no direct evidence that such a burst activated the ancient extinction. The conjecture is based on atmospheric modelling (Thomas et al., 2005). The main conclusion to be derived from these calculations is that gamma-ray radiation from a relatively nearby star explosion, hitting the Earth for only ten seconds, could deplete up to half of the atmosphere's protective ozone layer. Recovery could take at least five years.

With the ozone layer damaged, ultraviolet radiation from the Sun could kill much life on land and near the surface of oceans and lakes, and disrupt the food chain. Nevertheless it is important to recall, as we shall do in the next two sections based on the fossil record that there are two other competing theories for mass extinctions during earlier geologic periods, such as the suggested Ordovician mass extinction.

A related issue of SpW besides GRBs is whether cosmic rays may have left their imprint in the fossil record. To answer this question we may recall some recent research related to the rationalization of observed cycles in the fossil diversity (Kirchner and Weil, 2005).

As the Earth's solar system travels around the centre of the Milky Way galaxy, it also wobbles up and down from the galaxy's disc. U.S. scientists found that these swings take about 62 million years to complete - thus, may expose the Earth to higher doses of dangerous cosmic ray that may also cause mass extinctions. One complete orbit around our galaxy takes the solar system about 225 million years to complete. So, we go through about four of these cycles above and below the galactic plane during one orbit around the galaxy. (The galactic plane as the plane that is contained within the equator of the Milky Way galaxy, with the centre of the galaxy being the origin of this galactic coordinate system.) The modulation of the cosmogenic nuclide production expected from the galactoverti-cal motion of the solar system was evaluated earlier (Vanzani et al., 1987). The time distribution of 10Be concentration predicted by the model appears to be consistent with the data of deep-sea sediments.

The translation between the northern and southern sides of the galactic plane happens due to mass and gravity. When the solar system is on the northern side of galactic plane, the galactic mass located in the southern part uses its gravity to pull the solar system back down. Similarly, the northern galaxy mass, through the gravitational force, displaces the solar system from the southern side. A large amount of fossil data that covered an era of over 500 million years has been published (Sepkoski, 2002). Further studies suggest that living things on the Earth have been at their greatest risk of extinction every 62 million years or so for the past 542 million years (Rohde and Muller 2005).

This suggests that living in the south side of the galactic plane of the Milky Way may be safer for humans and all living things here on the Earth. Cosmic rays strike the Earth on their travels from a large cluster of galaxies in the direction of the Virgo constellation (Medvedev and Melott, 2007). Our own galaxy is moving toward the Virgo constellation in the northerly direction. So, when the solar system is on the north side of the Milky Way's plane, we are being bombarded by more cosmic rays from the Virgo constellation. The more cosmic rays that hit the Earth, the more that these energetic particles could possibly cause various problems such as changes in weather and climate, damage to DNA within humans and other animals, and mass extinctions. This work suggests that mass extinctions may very likely correspond to peaks in cosmic rays when the Earth is at its maximum northerly distance from the galactic plane.

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