Do Supernova Explosions Impact Earth Every Few Hundred Million Years?

Sn1987a_impact2 A University of Kansas research team is exploring the energy of cosmic rays and a possible link to massive prehistoric extinction events. Fossils and cosmic rays appear to have nothing in common. But Adrian Melott, a professor at the University of Kansas, is doing work with high energy cosmic rays to investigate the possibility that one may be linked to the other.

"There are a lot of things that can happen to the Earth that would cause it to get hit by more high-energy cosmic rays," says Melott. "A supernova fairly nearby (within about 30 light-years) is an obvious one. Another one would be a gamma ray burst in our galaxy that's pointed at us. And some people think that as we move up and down in the disc of the galaxy, when we get to the top we would get hit by more high-energy cosmic rays. So we don't know. We have a general idea of the effects on the atmosphere, but people haven't modeled it very much. Normally they don't matter, because most of the cosmic rays that hit us are medium and low energy."

Cosmic rays are particles, usually protons, that arrive individually, not in the form of a "beam" or "ray" as is often depicted in science fiction movies. Under certain conditions, many high-energy cosmic rays could strike the Earth's atmosphere. So the core of the project, explains Melott, uses the computer time to run a program that simulates atmospheric interactions to study what happens to the atmosphere from a flux of high-energy cosmic rays "so that we can later use this to better understand what happened in these kind of events."

The biggest effect of cosmic rays on the Earth's atmosphere, says Melott, is on the ozone layer. When radiation events hit the atmosphere—which is 80 percent nitrogen—the bonds between nitrogen atoms break. The nitrogen atoms will then react with anything they can, he explains, and a substantial number of them will react with oxygen molecules or atoms, making oxides of nitrogen, which sets up a chemical reaction that converts ozone back to oxygen. However, stratospheric ozone is a vital shield against ultraviolet light from the sun. So the ozone depletion causes more ultraviolet B light (UVB) than normal to travel through the atmosphere to the ground.

"A strong event like a gamma ray burst from a nearby supernova causes even more depletion, resulting in a doubling of the global average UVB," says Melott, "which all the people who experiment on plants and animals with UVB tell us would be a disaster."

Fortunately, the likelihood of our having a gamma ray burst of that intensity is, he says, "on average, approximately every few hundred million years."

Scientists have already established that things that live on or near the surface of an ocean are very sensitive to UVB. Melott says the common scientific opinion is that the obvious big danger to aquatic life is that a lot of single-celled phytoplankton will be killed off very quickly in high-energy cosmic ray events.

"And that is the basic food source in the ocean," he notes. "If those things are killed off, then the things that eat them, that eat them, that eat them, et cetera, are killed off, too. You could have a food chain crash."

Melott and team are using a two-dimensional latitude and altitude time-dependent atmospheric model developed by NASA to study atmospheric chemistry changes.

There are cosmic rays which occasionally hit the Earth which are about a million times more energetic than what they may get there in their simulations. "So if there is a gamma ray burst, for example, aimed at the Earth, there is good reason to believe we'd be hit by a mass of protons," Melott added. "The same is true if a supernova went off really nearby. The thing is, up until now, the effects of the protons have not been dealt with, except as very crude approximations. We hope to improve that understanding."

As they increase the energy, the simulation run time also increases. To propagate a single proton through the atmosphere currently takes several hours, "and we are running thousands of them because we have to average over large numbers to get reasonable and stable answers." Although the simulations take a long time to run, Melott says the output is very simple in the end.

One extinction event that happened 440 million years ago, at the end of the Ordivician Era, was one of the three worst mass extinctions, notes Melott. It had some characteristics that he and his colleagues thought made it a candidate for a radiation event. For instance, he says, one of the things that happened was that creatures that lived on the surface or that had larvae that lived on the surface were killed in this extinction.

"In 2009," continues Melott, "we tried to simulate this event. We had some data that others had published in Paleobiology that showed the extinction rate as a function of latitude. One of the things Dr. Thomas and I knew from our simulations was that the amount of UVB you get depends on latitude. We had this data they had compiled on the percentage of extinction as a function of latitude so we said, 'Can we duplicate that?' Their data basically says extinction was worst at mid-latitudes, and not as bad near the poles and not so bad near the equator.

"We did a bunch of simulations, gamma ray bursts going off at different latitudes, and we found that if one had gone off over the South Pole it would match their data perfectly. From this pattern we could then say the Northern Hemisphere wouldn't be affected very much. We now can predict that if people look at fossils in certain areas that were north of the equator they'll find the extinction event looks different; it won't have the sudden extinction pulse. That's the prediction for the fossil data. We don't have any answers yet, but there are people looking at it."

Biodiversity has been going up and down on the earth for 500 million years, and this is unexplained. One possible explanation is that the Sun and the Earth are bobbing up and down in the galaxy.

"That's known," says Melott, "and the time-scale for that motion is around 60 million years. It seems that these times of low biodiversity are when the Sun and the Earth are displaced to galactic north. We have developed a hypothesis that there are increased cosmic rays on the northern side of the galaxy that we are normally shielded from by the galaxy's magnetic field. And when we bob up we're exposed, so every 60 million years or so the Earth gets a dose of these cosmic rays. Part of the reason for doing these cosmic ray simulations is to assess this idea for reasonableness. That's just getting started. We have a big allocation and we're just starting to chew into it, but that's what it's for. Those simulations require a more complex computation that takes more time. But that's where our research is going."

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