Analog SFF, March 2012 (8 page)

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

9 “Deep-Ocean Crusts as Telescopes: Using Live Radioisotopes to Probe Supernova Nucleosynthesis” Fields B.D., Hochmuth K.A., and Ellis J. 2005 Astrophysical Journal 621, 902

10 “Modeling high-energy cosmic ray induced terrestrial muon flux: A lookup table” Atri D. and Melott A.L. 2011 Radiation Physics and Chemistry, 80, 701-703 dx.doi.org/ 10.1016/j.radphyschem.2011.02.020

11 “Severe Space Weather Events—Understanding Societal and Economic Impact” National Research Council, Space Studies Board 2008 National Academy of Sciences, Washington, D.C.

12 “Penetration of nearby supernova dust in the inner solar system” Athanassiadou, T., and Fields, B. D. 2011 New Astronomy, 16, 229-241

About the Author

Adrian Melott is currently Professor of Physics and Astronomy at the University of Kansas. He received his Ph.D. at the University of Texas in 1981. He was one of the pioneers in simulation of the formation of structure in a dark-matter dominated Universe. In 1996 he was named a Fellow of the American Physical Society “for groundbreaking studies of the origin and evolution of cosmic structure,” and in 2002 received the APS Joseph A. Burton Forum Award “to recognize outstanding contributions to public understanding or resolution of issues involving the interface of physics and society.” He was organizer and founder of Kansas Citizens for Science, which played a major role in restoring evolutionary biology to public science standards. Recently he has done research in “astrobiophysics,” publishing on the possible role of gamma-ray bursts and other astrophysical radiation sources in terrestrial mass extinctions, as well as investigating long-term biodiversity fluctuations. In 2007 he was named Fellow of the American Association for the Advancement of Science “for distinguished contributions to cosmological large-scale structure, for organizing public support for teaching evolution, and for interdisciplinary research on astrophysical impacts on the biosphere.” Melott would like to acknowledge and thank NASA for funding the research described in this article. He would like to thank his many collaborators on this research for their insights, and Jim Gunn for helpful suggestions on the manuscript.

Copyright © 2011 by Adrian Melott

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Where does diligence end and become something else?

I will never forget my mother's tattoos.

That was in the days before the nanospam, when your skin was your own and anything it said was yours. She had this multicolored starburst on her shoulder that she only showed in summer, when long sleeves gave way to short and then sleeveless. And there was a delicate gold watch on her wrist reading 5:45.

"Happy hour,” she said, when I was old enough to ask. “Because I always wanted to be at least forty-five minutes happy."

Then she'd hug me and add, in words I knew were good but whose subtext eluded me at the time, “It wasn't until you came around that I realized there was a better way."

I liked the watch best when I was young, before the dial started to move, before it turned digital. Even then, I'd been a retro-geek, though it would be a long time before I understood all the reasons simpler was better. And I liked the colors on her shoulder. Those were happy too. The right kind of happy. Though I didn't understand that for a long time either.

Perhaps that's why I took the job with Homeland Services. Their ‘wear isn't exactly retro, but you can style the emblem however you want and nobody dares spam you. Even the adwear you supposedly consent to each time you start your car or sip a mocha knows H.S. wearers, and keeps away. It's the job's greatest perk. Hell, it's the only perk. But at least it's a good one. I don't mind if my skin tells people I drive a nice car or wear designer jeans, but if I put on a pair of old sweats, I don't want it showing everyone the way to the nearest discount store. And the cheaper the product, the more garish its ‘wear.

But the perk came at a price. The alarm never seemed to go off except on Mondays when I was driving to work earlier than I wanted, having celebrated Saturday night on Sunday, if you know what I mean. As I said, this was before I figured out what my mother meant about the ways of being happy. What can I say? I was twenty-three and still on her old course, thankfully without a co-hab like my sire ("father” is too good a word) who'd vanished the moment his woman said “no” to an abortion. And yes, she told me that story. And yeah, it made me feel wanted. Or at least half-wanted. More than I'd have wanted me, which is another of those things you don't forget.