Analog SFF, March 2012

by Dell Magazine Authors

  * * * *

  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.

  We found[8] that we had an exact match—within the error bars of the data—if a burst went off over the South Pole. Therefore the idea survived this test, which it could have easily failed. This doesn't prove that it happened, but the idea continues to be viable.

  * * * *

  A Universe of Radiation: Stars Matter

  Of course, Sheffield had to resort to some shenanigans in his novels to get a nearby star that has none of the characteristics we need for a viable supernova precursor to explode and irradiate the Earth. Over time, though, it's very likely that we were close enough to a supernova to sustain serious damage. If you run the numbers,7 again it's every few hundred million years that we're likely to be within the 30-odd light years needed for an extinction level event. Discovery of the deposits left as a result of the end-Cretaceous asteroid impact came as a side effect of the search for deposits from a supernova. Even more distant supernovae, which might have moderate effects on Earth, can leave deposits.[9]

  There's likely to be a lot of background radiation. For every extinction-level event from a GRB or supernova, we expect about ten times as many more which are intense enough to stress the biosphere. (Our standard for such “stress” is to produce about as much ozone depletion as humans recently did with their “gas attack” on the atmosphere, which is known to have done some damage to phytoplankton, and possibly to amphibians as well.)

  Supernovae are thought to be one of the primary sources of the “galactic” cosmic rays we observe—the highest energy group, that are mostly protons moving close to the speed of light. Their paths are bent and twisted by the galaxy's magnetic field. There are about three supernovae per century in our galaxy, and the cosmic rays’ travel time across it is in the order of 50,000 years at the speed of light. So we see a blended mix of cosmic rays from near and far, recent and past. If a supernova were very close, we would get an intense burst of cosmic rays, adding to the danger from the X-rays and gamma rays.

  Cosmic rays produce air showers, in which they spawn a growing zoo of elementary particles until it hits the ground. Most of the big detector experiments study this zoo, and indirectly infer what kind of primary started it. We have been looking at the kind of high-energy air showers that might result from a nearby supernova. The thing that has emerged as a substantial new threat is muons.[10] Muons which are rather like a kind of heavy electron, can penetrate up to about a kilometer of water. So they don't interact much, but there are so many of them in the cosmic ray air showers that they contribute about half the biologically effective radiation dose from cosmic rays. In proximity to a nearby supernova, they would be greatly increased, and would constitute a direct radiation threat to all life except possibly that deep underground or in the deepest part of the oceans.

  * * * *

  "Beware of Your Friends” Jeremiah 9:4

  And then there's the danger from our own Sun, described earlier as the Carrington Event. It's reasonably well documented. In 1859, the Sun was seen to visibly brighten. This was followed by a burst of protons emitted by the Sun. The aurora was seen in Jamaica! The protons collided with the Earth's magnetic field, causing large fluctuations, and an electromagnetic pulse on the ground. The new telegraph technology was affected: induced electric currents in wires set fires in a number of telegraph offices. An operator in England (in a kind of “hey, watch this” moment) showed that he could disconnect his battery and continue to send telegraph messages using “cosmic electricity.” The Carrington Event can be clearly seen as a nitrate spike in Greenland ice core data. We've done computations that show there would have been ozone depletion a bit worse than what we did recently with chlorofluorcarbons, but which would have healed in five years or so. There may have been a perturbation in the oceans, or an uptick in skin cancer rates, but the records apparently aren't good enough to detect these things.

  Despite its severity, the amount of atmospheric ionization from Carrington is quite small compared with that which would result from an extinction-level GRB or supernova. We don't know just how bad Solar proton events have been, or how often. There just isn't very much information, except for events of Carrington intensity and below. There are hints of big flare events on some stars similar to the Sun, but the situation is poorly understood and confusing. Our lack of information on the rate of such events locally just sets in at about the level at which they become seriously dangerous to life.7 Of course, we can set some limits by observing that mass extinctions on Earth don't happen frequently. Larry Niven captured something of what might be expected from a huge event in his story “Inconstant Moon,” which later became an Outer Limits episode.

  But ancient life didn't have electromagnetic technology. An event like this today would seriously damage our infrastructure, causing failures of electrical power, destruction of transformers, and crippling or destroying many satellites on which we rely for communications and weather information.[11] Unshielded computer disks could be erased. A severe event could cripple the world economy for months at least. So modern humans are highly vulnerable to Solar proton events that would have earlier caused only a modest ozone depletion, from which life would easily recover. The extinction level threshold for modern humans, from such events, is much lower than that for any life form that has existed here before. How many of us would survive if there were no electricity for months?

  * * * *

  Advance Warning or Preparations?

  We can easily spot large stars that are likely to go supernova, especially if they are close enough to do serious damage. None are. Gamma-ray bursts aren't completely understood by any means, but it's likely that a long burst precursor would be obvious to us, since it would have to be a large star with large angular momentum, in its last years before going supernova. It's likely that such an event close enough to harm us could be recognized, even if it is a few thousand light-years away. Unfortunately, our recent analysis7 suggests that the short burst type is a greater threat. This type is thought to originate in neutron star or dwarf star mergers. These could easily progress in the dark with next to no signals until near the end, if then.

  Solar flares (and Solar proton events) are not well enough understood to predict. There are some regularities, but it's safe to say we would have at the most a few hours to days, once the protons were on their way to the Earth. Maybe we could harden some of our computer equipment, vital records, or power down the electric grids.

  A very nearby supernova would definitely irradiate the Earth with muons on the ground, but that threat is not at all likely within the human time frame. The kind of event that could hit us in addition to crippling electromagnetic technology is ozone depletion and UVB damage—from either a Solar event or a short gamma ray burst. The rate of the former is unknown. The rate of the latter can be estimated, and it isn't likely to happen soon. But it is virtually certain to happen, and without warning.

  For all of the extinction-level events, there are likely to be about ten times as many that would qualify as a disaster, with a substantial number of deaths and economic dislocations. Our historical time frame is only a few hundreds or thousands of years at best, depending on how willing we are to interpret accounts verging on mythology. Ice cores can give us information on atmospheric ionization and irradiation levels back a few hundred thousand years, but the interpretation is full of ambiguities. Isotopic anomalies might tell us about nearby supernovae back a few million years[9],[12]. We really don't have much good information on what has happened, or—what is bound to concern people most—what is likely to happen.

  If there is some advance warning, preparations are possible. People would need to stay out of the sunlight and set up food stockpiles to last years. A government-level nuclear war shelter would do the job. However, a large greenhouse would make a fine shelter: Glass filters out nearly all the UVB, and given the right conditions, one could grow food inside.
The Svalbard Global Seed Vault means that losses of terrestrial plants could be made up. Humans are clever, and no doubt many would muddle through.

  Watch the skies! But don't forget your welder's goggles. . . .

  * * * *

  References

  1 “Observations of Gamma-Ray Bursts of Cosmic Origin” Klebesadel R.W., Strong I.B., and Olson R.A. 1973, Astrophysical Journal Letters 182, L85

  2 “Terrestrial Implications of Gamma-Ray Burst Models” Thorsett S.E. 1995 Astrophysical Journal Letters 444, L53

  3 “Life Extinctions by Cosmic Ray Jets:” Dar A., Laor A., and Shaviv N. 1998 Physical Review Letters 80, 5813

  4 “Astrophysical and Astrobiological Implications of Gamma-Ray Burst Properties” Scalo J.M., and Wheeler J. 2002 Astrophysical Journal 566, 723

  5 “Phanerozoic Biodiversity Mass Extinctions", Bambach R.K. 2006 Annual Review of Earth and Planetary Sciences 34, 127

  6 “Did a gamma-ray burst initiate the late Ordovician extinction?” Melott A.L. and 8 others, 2004 International Journal of Astrobiology 3, 51

  7 “Astrophysical Ionizing Radiation and the Earth: A Brief Review and Census of Intermittent Intense Sources” Melott A.L. and Thomas B.C., 2011 Astrobiology 11, 343 doi: 10.1089/ast.2010.0603.

  8 “Late Ordovician geographic patterns of extinction compared with simulations of astrophysical ionizing radiation damage” Melott A.L. and Thomas B.C, 2009 Paleobiology 35, 311