Analog SFF, March 2012 (7 page)

Serious problems begin to show up in some plants and wildlife. However, a greater potential threat exists: phytoplankton, which power the food chain in the ocean, are dying rapidly. It appears that a major drop in marine life is in the offing, because nearly all of it depends on the energy captured by the phytoplankton. On land, there is serious mortality to food crops, but some of them seem resistant to the UVB. It will take months or years to know how it will work out.

Famine greatly reduces the world population. After five years or so, things begin to recover. However, many species have vanished permanently.

A Universe of Radiation

Geologists tend to like to think of the Earth in isolation. Therefore, they only grudgingly accepted the overwhelming evidence for a catastrophic impact about 65 million years ago, which was a key element in the vanishing of the dinosaurs (carried live on all the cable science channels!) along with many other species. This was one of many mass extinctions—up to 19 can be identified, depending on the criteria we use. Most people would identify the “Big Five” that stand out above the others in intensity. Often they are accompanied by a wholesale change in the ecology, even the kind of sedimentary rocks that are formed. In nearly all cases, the cause is controversial, with no consensus at all. There is a current idea that certain events may reach the “mass” extinction level when the biosphere is under an additional stress, and is hit by some sudden challenge.

I've become interested in this topic. I'm a physicist, and while the roots of my interest came from physics, it's led me into paleontology, and some very fruitful collaborations. One of the things we
expect
to happen to the Earth is it to be hit by some bursts of radiation from elsewhere. Our review of average rates of events has identified three major external radiation threats: supernovae, Solar proton events, and gamma-ray bursts.

For a long time, people have considered the threat from an “ordinary” supernova. As no doubt most readers know, there are many kinds of supernova. Typically, they constitute a large release of energy from a star collapsing at the end of its fuel-burning lifetime, or sometimes gradually adding mass from a companion until it exceeds the limit of stability. This was portrayed decades ago in a pair of science-fiction novels by Charles Sheffield,
Aftermath
and
Starfire
, with surprisingly accurate descriptions of the effects. Sheffield was a physicist who worked on application to space projects.

Even the Sun is a possible danger. The worst Solar flare we know of, in 1859, was called the Carrington Event. It's reasonably well documented. The problem here is that we don't know how much worse they get. A Solar proton event much worse than Carrington could be very bad for life on Earth, particularly humans.

However, my interest in and work on this topic began with gamma-ray bursts. In 2003, I changed fields and started this investigation. The story of how that happened will frame my description of the threat.

In the Beginning

It began calmly enough, when a prospective physics faculty member was a candidate at KU. One of the things we do is have them give a talk about their research. He explained his research on the mechanisms of gamma-ray bursts, which are the most powerful known explosions in the Universe. I asked him, “So what would happen if one of these goes off in our Galaxy?” Laconically, he said “It's not good,” which brought a few laughs. So, he joined our faculty, and the next year we decided to have a look at the effects. It turned out that some work had already been done on this possibility, but there was room for a lot more.

I should explain that up to this point nearly all my work had been in cosmology, investigating the formation of structure on the largest scales where clumping is seen. When I got involved in the radiation burst threat, it proved so interesting that it completely changed my research. I haven't looked back.

My opening scenario at the beginning of this article was based on the effects of a dangerously-near gamma-ray burst.

A First Look at Gamma-Ray Bursts

In the 1960's, most nuclear-capable nations signed a treaty not to test nuclear weapons in the atmosphere or in space. Naturally a way was needed to monitor compliance. The US launched the
Vela
series of satellites, which looked for bursts of X-rays and gamma-rays, which are associated with nuclear weapons. The surprise was in detecting such bursts within a few months of being operational; the consternation in the intelligence community was that there was no other evidence of explosions. No radiation in the atmosphere, no rumors, no seismic wave—nothing. Later, directional capability was added, and people were relieved to learn that the bursts were coming from the sky.[1]

The sources remained a mystery for a quarter-century. It was learned that one or so per day can be detected, and that they originate pretty much uniformly all over the sky. This means they are either local or cosmic. We see nearby, bright stars all around us. The distribution of faint, dim stars include the Milky Way, which is the disk of our Galaxy seen edge-on. But if we survey very distant galaxies, they are once again all around us. The gamma-ray burst distribution tells us they are not galaxy-wide, but could be either near us or at cosmological distances.

They are quite powerful. Their power enabled them to be mistaken for nuclear weapons on the ground! That's why most people assumed they were within a few light-years. In this case, the minority turned out to be right. In the late ‘90s a series of source identifications connected them with very distant galaxies and the large energies mentioned earlier. During the few seconds of the burst they radiate to us about 1045 Joules, more gamma-ray energy than the entire rest of the Universe.Later, it was deduced that they are beamed, with the energy going out to maybe .01 of the solid angle around them. So you can knock off two zeros from the energy, but there are one hundred going off for every one you see.

Radiation at the Earth

It's not too difficult to figure out that if there are a few per day (pointed at us) in the Universe, then there should be some number, maybe one every few hundred thousand years, in our galaxy; then after some millions of years, you'd expect there to be one pointed at us. “This is not good,” as was recognized not long after the cosmological option, with its implied high power for the sources, was accepted.[2-4]

The implied energy release is something like the mass of the Sun converted to energy in a few seconds. Nowadays, there are two generally accepted theoretical explanations for the origin of the two kinds of GRBs. “Long” bursts, longer than two seconds, probably come from the collapse of very large, rapidly spinning stars to black holes when they run out of fuel. Energy is released in the material before the black hole state is reached. “Short” bursts, less than two seconds, probably come from the merger of pairs of neutron stars or possibly involve dwarf stars, with the two relics in orbit, gradually spiraling toward one another until they merge in a spectacular explosion. But for the Earth, it's the results that matter.

So could a gamma-ray burst be implicated in one or more mass extinctions? Is there any evidence to support such an idea?

Mulling Over the Effects on Life

The University of Kansas happens to have a top-notch bunch of paleontologists, and it's an easy place to work across the artificial boundaries between academic disciplines. I invited a group of them, as well as Mikhail Medvedev, the plasma astrophysicist whose lecture had set this off, to get together and talk. Within a few meetings, we were able to get down to the physics of extinction. It turns out that the easiest way a gamma-ray burst can affect life on Earth is indirect: by modifying the atmosphere. Such energetic photons won't travel very far in the air, so life on the ground doesn't get hit. Instead, the energy is given up in tearing apart molecules of nitrogen, oxygen, etc. Unless the GRB is improbably close, on the ground you just get a burst of blue and UV light, possibly blinding, but only on one side of the Earth.

The effects of breaking up the nitrogen molecule (N2) are severe. Normally, life on Earth is surrounded by nitrogen in this form, but can't use it because the molecule is so tightly bound. So if you want to make plants grow, the first thing you probably do is add nitrogen fertilizer. The energy of gamma-rays and X-rays is easily enough to break up the nitrogen molecule; once you have done that, the nitrogen will react with everything it can. A large fraction of it will first encounter oxygen, and form oxides of nitrogen. There are many of these: Most familiar are the “nitrous” that some dentists use, and the brown component of smog that used to be produced in quantity by automobile engines. These normally exist in very small amounts in the atmosphere, but will be produced in great quantities under the conditions of a galactic GRB. My graduate student at the time, now Professor Brian Thomas, has done numerous computations of exactly what happens in the atmosphere.

These compounds are mostly produced high in the atmosphere. They might darken the sky just a little. As they are gradually removed, they will deposit a bit of nitrate on the ground. But the effect on the ozone layer can be devastating. High up in the stratosphere there is a layer of ozone. This ozone layer filters out about 90% of the Sun's ultraviolet-B, which has wavelengths about 380-420 nm. UVB can cause severe burning of skin. It is absorbed by protein and most importantly DNA. It breaks chemical bonds, and can lead to cancer and mutation. The main reason the world has been moving away from chlorofluorocarbons (such as early versions of Freon) is that it also depletes the ozone layer. Because of this problem, a lot of research has been done on the effects of UVB on phytoplankton, the creatures that live near the surface of the ocean and produce most of the basic food there. Even small increases, such as 10%, can produce measurable rates of mortality.

A Profile of Extinction

KU paleontologists Bruce Lieberman and Larry Martin zeroed in on identifying what the characteristics of a GRB-mediated extinction would look like. Since a few meters of water will block most UVB, we realized that such an event would hit hardest those species living in shallow water or near the surface in the open ocean. That led immediately to the suggestion of the end-Ordovician extinction as a candidate for such a radiation event.6

That extinction, one of the top few in severity, came about 440 million years ago. Life had just begun to tentatively colonize the land. The extinction took out the majority of species that existed at that time. Trilobites were hard-hit. The thing that grabbed our attention was that it seemed to selectively target those organisms that would be hardest-hit by UVB—organisms that lived in shallow water, or lived near the surface of the ocean, or whose larvae lived near the surface. The event was also accompanied by glaciation, which is often blamed for the event. But glaciation happens all the time without bringing on mass extinctions. It's possible that the “smog” effect might have tipped the Earth over into glaciation, but this needs to be checked with global climate models.

How Likely Is Such an Event?

Did it Really Happen?

It is possible to be more precise, using our computations of the effect of a GRB on the atmosphere. A long-burst GRB pointed at us from a distance of about 6,000 light years would produce about 30% global average ozone depletion. That's enough to easily double the amount of UVB at sea level, which would spell the end for many organisms. The atmosphere will recover in five to ten years, but that is many generations for phytoplankton. A food chain crash in the ocean could easily take place in such a time. Short-burst GRBs are less powerful, but more common, and so actually constitute a substantial threat as well. When we put all the numbers together,[7] we conclude that an average rate of something like one such intense extinction level event is expected every 200 million years from GRBs. In the 500 million years or so that we have a good fossil record, it is therefore unlikely that we have escaped having at least one such event.

How can we test this idea? There is bad news and good news. The bad news is that such events seem to be very “clean.” So far, no one has been able to figure out any marker. Any radioisotopes that might have been produced should have long since decayed away. We can calculate how much nitrate should be produced, and have even tested our work against deposits from Solar flares found in ice cores drilled in Greenland. Unfortunately, there's no ice on Earth older than about 800,000 years. We might get a handle on Solar flare rates, but it's unlikely that a gamma-ray burst would show up in that “short” a time interval. Since nitrates are so soluble in water, they'd be washed away from most of the land surface soon after the deposit started. So far then, no one has figured out the kind of residue that might constitute a “smoking gun” for such a radiation event.

The good news is that we can at least be a little more quantitative in what we expect in the pattern of extinction from astrophysically induced UVB. There are some regularities: the ozone-depleting compounds tend to stay in the hemisphere, northern or southern, where they are produced. The ozone depletion tends to be worse near the poles, and its pattern varies according to the latitude over which the burst went off. On the other hand, there's more sunlight near the equator. We can combine these to produce a pattern of UVB intensity on the post-GRB Earth as a function of latitude. So we can test the pattern against the Ordovician extinction event. It turns out that a study already existed in which extinction intensity was tabulated against latitude. (The latitude then—I'm sure all the readers of this piece know that the continents move around over hundreds of millions of years).

We decided to use the “falsification” approach. There are various fashions in the philosophy of science, mostly useless, but one fruitful method is to ask whether you can rule out an idea. In this approach, we can't prove anything is true, but we can disprove ideas, and the ones we can't disprove eventually become “scientific truth.” The ones that can't be tested at all are regarded as not scientific. Anyway, we can simulate the effects of GRBs at various latitudes. If none of them can reproduce the pattern found in the data, the idea is falsified.