On the role of GRBs on life extinction in the Universe
arxiv.org
arxiv.org
It's not an answer I like. But then, that's probably true for most potential answers to that particular question.
Diaspora by Greg Egan also has a first-hand depiction of a gamma-ray burst hitting the Earth.
I just find his style a bit dry. But it's worth it for the mind-bending ideas.
I got on a Baxter kick after powering through his Long Earth books (ostensibly coauthored with Terry Pratchett, though Pratchett's fingerprints are rather non-existent after first book). As long as you're ok with Baxter's strong dislike of organized religion, he seems to be one of the better modern science fiction writers.
If you assume that intelligence inevitably separates from its physical substrate into some form of persistent data, this is the logical way to get around the light-speed limit: you go dormant and simply wait the thousand years it takes to get a reply, or the hundred-thousand years it takes to arrive at a new solar system.
Such intelligences would only interested in speaking to other "Space Ents" (anything else must seem like a fruit fly in comparison), and it's prohibitively expensive to saturate the galaxy with signal, so maybe you send targeted pings at promising worlds every few thousand years. Maybe that's the Wow! signal [1]. Or maybe we just haven't been around long enough to get our first hello.
Of course, for a long time we were simply unaware of radio signals. Maybe we're currently unaware of foo signals.
a) organisms are sensitive to UV radiation
b) atmospheric ozone is the mechanism that protects the host planet against excessive UV radiation
c) the planet's atmosphere contains lots of nitrogen
d) the planet has large oceans of water (since the paper notes the main significance of high UV flux would be to kill off marine plankton)
These all hold true for Earth, but not for places like Europa or Titan, let alone more exotic environments.
http://arxiv.org/abs/astro-ph/0504158
When the GLAST (now James Webb Telescope) data came in, it pointed to a small (~1%) anisotropy of UHE GRBs toward high metalicity galaxies. Our model was completely isotropic. Unfortunately, we didn't have time to rework the anisotropy into our model's assumptions before our funding fell through.
... and that is the story of my brief experience as a scientist.
"Life surviving an initial onslaught, including those located on the side of the earth facing away from the burst, would have to contend with the potentially lethal after-effect of the depletion of the atmosphere's protective ozone layer by the burst." https://en.wikipedia.org/wiki/Gamma-ray_burst#Rate_of_occurr...
"The greatest danger is believed to come from Wolf–Rayet stars, regarded by astronomers as likely GRB candidates. When such stars transition to supernovae, they may emit intense beams of gamma rays, and if Earth were to lie in the beam zone, devastating effects may occur. Gamma rays would not penetrate Earth's atmosphere to impact the surface directly, but they would chemically damage the stratosphere."
Wow. That's an interesting side-effect.
I'm aware of all the rest of the "fine-tuning" (please mind the quotes, yadda-yadda).
However, marine organisms would be sheltered from gamma radiation.
From a geological viewpoint, this seems very unconvincing as a significant cause of any the major mass extinction events. It's interesting for extraterrestrial life and future society in general, though.
However, even a fraction of a meter of water will form a very effective barrier to gamma radiation (One meter reduces it to ~2e-17 times it's original intensity).
Even if it were to kill off everything in the upper few tens of centimeters of water, it wouldn't decimate marine productivity. Most of the productivity in the oceans is tied to the ecosystem in the photoic zone, but not to the upper few centimeters of the water column.
Edit: I just realized they're mostly referring to the effect of increased UVB radiation _after_ the gamma ray burst removes the ozone layer, rather than the effect of the gamma ray burst itself. Because UV radiation is much longer wavelength, it would have an effect on the upper few meters of the water column, rather than the upper few centimeters, which could be significant. I'm still unconvinced that it's a "good" explanation for mass extinctions, given that we have several others (e.g. long-term sea level fluctuations), but it makes more sense than I thought at first.
Phytoplankton may not since it's on the surface (for sunlight access) and at 50% of atmospheric oxygen comes from ocean algae, at least as much as plants.
UVB from sunlight after the gamma ray burst strips away the issuing layer is what people are referring to. Because it's longer wavelength, it could still have an effect.
1) The gamma ray spectrum from GRBs looks like it has a long high-energy tail: http://www.mpa-garching.mpg.de/lectures/ADSEM/WS0304_Kienlin...
2) For gamma energies about about 8 MeV, you start producing neutrons from gamma-nucleus interactions (this is why cancer patients who are treated with high-energy beams are very slightly radioactive for a while after each treatment: high energy photons blow neutrons out of a few of their nuclei, leaving short-lived daughter nuclei in their place.)
3) For an extinction-level gamma-ray event the GRB fluxes are estimated as ~10 kW/m^2 as the top of the atmosphere and 160 W/m^2 at the surface of the exposed side of the Earth. That's 160/1.6E-13 = 1E15 MeV/s.m^2. If 10E-6 gammas generate a neutron (totally ballpark guess, but the spectrum is only down by about 1E-3 from peak at around 10 MeV, and neutron production cross-sections at around 10 MeV are on the order of a milli-barn for nitrogen and some heavier elements, so a 1E-3 fraction going into neutron production is not unreasonable) that's 1E9 neutrons/m.s. Given GRBs last for a couple of seconds, therefore, one would expect to see a layer of neutron-activated rock that was exposed to a fair neutron flux.
4) But... all rock is exposed to a background flux of a few neutrons per second, so over a few hundred years there would be a background signal that swamped this burst signal.
Ergo, it is unlikely that this mechanism could be used to detect a signature of GRB extinction events on Earth. The additional neutron activation signal would be completely swamped by the natural background signal in almost any reasonable circumstances.
So what effect would a long, direct GRB have on earth? Are we talking scorched, molten earth and boiled seas, or put extra sunscreen on for a week?
So we would need some really solid sunscreen. Also the oceans would turn into a big vat of plankton chowder, which would make for an exciting last few years for all of us.
Seems like there's no prognosis as to what consequences a GRB event may have on humans specifically in the paper quoted on Wikipedia:
“The greatest danger is believed to come from Wolf–Rayet stars, regarded by astronomers as likely GRB candidates. … if WR 104, at a distance of 8,000 light-years were to hit Earth with a burst of 10 seconds duration, its gamma rays could deplete about 25 percent of the world's ozone layer. This would result in mass extinction, food chain depletion, and starvation.”
I suppose this topic is hard to speculate on, as it requires substantial knowledge on a variety of disciplines.
A demonstration can be provided COTS from marine reef tanks, you can buy UV sterilizers which continuously zap flowing water with UV, quite effectively killing all floating algae and pretty much anything else floating in the water.
The problem in the ocean is for some months they'll literally be nothing for plankton to eat at all, and the sheer level of rot and rot byproducts would be pretty spectacular making life really difficult. And the recolonization process once UV levels drop enough to make it possible will result in some really weird blooms, and algae blooms lead to busts so the oscillations will likely be pretty exciting after the initial crash. A few million years later they'll be a lovely layer of petrochemicals to mine.
On land, the couple month UV hit would be very much like hitting the entire surface of the planet with defoliants. You might survive the lack of food, but no clean water for awhile and no harvest this season and everything green pretty much just died and started washing away in the rain might be a bigger mess. In a way the best place to hang out might be a desert. All the cacti will die, but there won't be many mudslides. A forest might be a good place to chill out too, all the leaves will die and the trees themselves might die but the roots won't rot for a long time, keeping dirt in place. Grassland areas will be pretty much screwed.
In particular because it both easy and tempting for some people to overspecify the conditions for life and for others to underspecify. On the one hand it is trivially easy to get odds as long as you want by overspecifying exact details of human ascent. On the other hand, looking up into the sky it's hard not to notice that if life were really so easy and abundant it really shouldn't look like that up there. (Or Great Filter handwave handwave. But anyhow, something is up, and the general idea of Rare Earth is still in the running.)