Cosmic rays pour down on Earth like a constant rain. We don't much notice these high-energy particles, but they may have played a role in the evolution of life on our planet. Some of the mass extinctions identified in the fossil record can be linked to an asteroid impact or increased volcanism, but many of the causes of those ancient die-offs are still open for debate.

"There may have been nearby astronomical goings-on that drastically increased the radiation on Earth," says Brian Fields from the University of Illinois.

A supernova going off 30 light years away could cause such a jump in radiation on our planet that could directly, or indirectly, wipe out huge numbers of species. Currently researchers are looking for possible evidence for this sort of cosmic foul play.

"Just finding dead beasties is not proof of a nearby supernova," Fields says.

A hard rain is going to fall
Cosmic rays are mostly high-energy protons, with some electrons, positrons and heavy nuclei mixed in. Their energies range over 14 orders of magnitude, with the most energetic cosmic rays flaunting a billion times more energy than is possible in man-made particle accelerators on Earth.

The current understanding is that most cosmic rays originate in the shock waves that emanate from supernova explosions. We can't precisely trace where a cosmic ray came from because its trajectory is bent by magnetic fields. In fact, a typical cosmic ray will bounce inside the galaxy's magnetic field for millions of years before eventually colliding with something... like Earth.

"Every square centimeter on the top of the Earth's atmosphere is hit by several cosmic rays per second," Fields says. "This is forever going on."

None of these "primary" cosmic rays ever reach us on the ground. Instead, they collide with atoms in the upper atmosphere, creating a shower of lower energy "secondary" particles.

Secondary effects
At sea level, the majority of cosmic ray secondaries are highly penetrating muons. About 10,000 muons pass through our bodies every minute. Some of these muons will ionize molecules as they go through our flesh, occasionally leading to genetic mutations that may be harmful.

At present, the average human receives the equivalent of about 10 chest X-rays per year from cosmic rays. We shouldn't be alarmed by this, since it is just part of the natural background radiation under which humans and our ancestors have been exposed to for eons. Indeed, cosmic-ray-induced mutations may sometimes be beneficial.

"It is clear that in some way cosmic rays shaped evolution of organisms on Earth," says Franco Ferrari from the University of Szczecin in Poland.

In a recent issue of the journal Astrobiology, Ferrari and Ewa Szuszkiewicz from the same university reviewed what we know about cosmic rays, and they argue that the current biological relevance of these particles is not necessarily representative of the past.

"It is very likely that organisms of early Earth possessed DNA that was unstable and could easily mutate under external agents, more so, perhaps, than the DNA of present-day bacteria," the authors write.

Cosmic ray storm
Not only might biology have been more susceptible to mutation long ago, but the cosmic rays might have been more intense in the past.

Evidence that the cosmic ray flux varies with time comes from ice-core measurements of carbon-14 and beryllium-10. These radioactive nuclei, which are created out of cosmic ray showers, show that the number of cosmic rays goes up and down by a factor of two in response to changes in solar activity. Specifically, when the Sun is less active, the solar wind is weaker and more cosmic rays are able to penetrate into the solar system.

Some scientists have tried to tie these modest cosmic ray fluctuations to global climate change. They claim that cosmic rays seed clouds, so when the cosmic ray flux is high, more clouds form and the planet cools off. The idea is controversial, however.

Other scientists are looking for evidence of more dramatic cosmic ray increases millions of years ago, which would have occurred too far in the past to have been recorded in ice cores.

Higher levels of cosmic rays would not only directly increase mutations in living species, they would also indirectly increase the level of ultraviolet radiation coming from the Sun. This is because cosmic rays generate nitrogen oxides, which catalyze the destruction of the ultraviolet-blocking ozone layer. "The increase in ultraviolet light is extremely damaging, and might be especially lethal to single-celled organisms," says Adrian Melott of the University of Kansas.

Life at the bottom of the food chain could be significantly diminished by a loss of ozone, and this could ripple up through the web of life causing widespread extinctions.

Ozone depletion could also arise from a nearby gamma ray burst. However, the radiation flash would last only a second, and the ozone would recover after a few years. In contrast, cosmic rays from a nearby supernova would likely bombard Earth for at least 1,000 years, according to Fields.

"An organism might be able wait out a gamma ray burst, but cosmic rays are going to affect many generations," he says.

Near miss
One way to tell whether an extinction event was due to cosmic rays is to look for radioactive isotopes that would have formed in a nearby supernova and then were blown onto our planet by the associated blast wave.

In 1999, a group from the Technical University of Munich in Germany detected iron-60 in rock samples from the deep ocean. This extremely rare iron isotope is forged in the fires of supernovae. It is also radioactively unstable, with a half-life of 1.5 million years, so it must have come from a fairly recent supernova.

From the iron-60's location and concentration, the German group later calculated that the putative supernova went off 2.8 million years ago at a distance of about 100 light years away. Fields believes this was probably too far away to have caused an extinction-level event.

"I'd call it a near miss," he says.

The cosmic rays from this supernova may have had an effect on the climate, but to cause serious biological damage, a supernova would need to explode within about 30 light years of Earth.

Cosmic ray roulette
Although 30 light years is small on a galactic scale, Fields thinks it likely that Earth has been caught in a supernova "kill radius" as many as a dozen times over our 4.5-billion-year history.

However, a nearby supernova is not the only way to increase the cosmic ray intensity. As our Sun orbits around the galactic center, it regularly passes through one of the galaxy's spiral arms where the cosmic ray radiation is higher than average, says Ferrari. Some researchers speculate that each passage through a spiral arm spawns an Ice Age on Earth through cosmic-ray-induced cloud formation.

In a similar vein, Melott and his colleagues found a possible link between the bobbing of our Sun up and down in the galactic plane and a 63-million-year cycle in fossil biodiversity. The hypothesis is that our solar system is exposed to more cosmic rays every time the solar system peaks out of one side of the galaxy.

However, Melott now thinks this bobbing may only play a small part, seeing as recent evidence points to a correlation between continental uplift and the observed biodiversity cycle.

More work is definitely needed to tie cosmic rays to extinction events. Melott says that the search continues for other radioactive isotope evidence of nearby supernovae, and his group is developing simulations of cosmic ray bombardment to see if there might be any recognizable pattern to the biological destruction.

"No one has calculated the full effects on the ground," he says.