Space Death Rays

Mass extinctions have often been linked to asteroid impact or increased volcanism but another cause has recently emerged- cosmic rays. These high energy particles constantly bombard the earth and are believed to have played a part in the evolution of life on our planet. A large number of species can be wiped out either indirectly or directly by increased radiations from a supernova going off as far as 30 light years away although extensive research on the subject is still going on.

High energy protons which constitute cosmic rays take their origin from supernova shock waves. As their trajectory is bent by magnetic fields their exact site of origin can’t be traced. In fact, they can bounce inside the galaxy’s magnetic field for millions of years before they collide with any other structure, for instance the earth. Every square centimeter of the Earth’s atmosphere is believed to be hit by several cosmic rays per second but these ‘primary rays’ do not reach the Earth surface. Instead they are converted into low energy ‘secondary’ particles after collision with atoms in the upper atmosphere.

An artist's rendering of particle showers forming after a primary cosmic ray smacks into the atmosphere. Credit: Simon Swordy (U. Chicago, NASA)

The secondary rays consist of highly penetrating muons (elementary particles similar to electrons) at sea level. A person is exposed to rays equal to about 10 chest X-rays per year and as they are highly ionizing, they can sometimes lead to genetic mutations that are harmful. But this happens very rarely and is not a cause for alarm as these rays are just part of the natural background radiation and have been so for decades. In fact, these rays have been found to induce beneficial mutations. It is thought that organisms of early Earth had DNA that was highly unstable and more susceptible to mutation by the cosmic rays thus strengthening the theory that these rays played an important part in shaping life on our planet

Cosmic rays are believed to have been more intense in the past and there are controversial theories about how they affected early life on Earth. Cosmic rays can increase cloud production according to one theory, which can cause more sunlight to bounce off the clouds into the space, thus cooling the planet. A contradicting theory is that it causes the opposite effect by depleting the ozone layer and allowing more solar UV radiation to enter the Earth’s surface creating hostile environment for life.

The Crab Nebula is a remnant of an exploded star, called a supernova. Cosmic rays are being accelerated at the outer edge of the expanding blast wave. Credit: NASA/ESA

Radioactive isotopes offer hints to whether an extinction event was due to cosmic rays. These would have originated from a nearby supernova and been carried to the earth by the resulting shock wave. Iron-60 which is an extremely rare iron isotope formed from the fires of a supernova has been found in rock samples from the deep ocean. From the isotopes concentration and location it was calculated that the supernova went off 2.8 million years ago at a distance of about 100 light years away. As this was more than 30 light years away it is unlikely that it caused a near extinction level event. But it was a near miss and cosmic rays from this supernova may have lead to a change in climate.

A supernova is not the only way to increase cosmic ray intensity. As the Sun orbits it frequently passes through one of the galaxies spiral arms. Here, the cosmic ray radiation is higher than average and according to one theory it is believed that passage through each spiral arm spawns an Ice Age through cosmic ray induced cloud formation.

A Spiral Galaxy. The sun keeps out of the way of the galaxies spiral arms which contain disruptive gravitational forces and radiation. Over 95 percent of stars in the galaxy wouldn't be able to support habitable planets simply because their rotation is not synchronized with the rotation of the galaxy's spiral arms.

In light of everything, it is clear that extensive research is still needed to say for sure that cosmic rays can indeed lead to extinction events and search is going on to find more Iron-60 deposits on Earth.

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1. The Supernova
2. Hubble’s 3D View of Deep Space