On the role of GRBs on life extinction in the Universe(arxiv.org)
arxiv.org
On the role of GRBs on life extinction in the Universe
http://arxiv.org/abs/1409.2506
52 comments
A possible answer to Fermi that I don't hear often: that there's plenty of life in the universe, but it's long-lived, and very, very slow.
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.
[1] http://en.wikipedia.org/wiki/Wow!_signal
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.
[1] http://en.wikipedia.org/wiki/Wow!_signal
Or maybe all signals are encrypted and compressed, and look like noise. Like most of our signals currently do.
Yes. I'd use pulsars as a carrier. ..Simply surround a few with a bunch of micro satellites make a sort of shutter...
Of course, for a long time we were simply unaware of radio signals. Maybe we're currently unaware of foo signals.
Of course, for a long time we were simply unaware of radio signals. Maybe we're currently unaware of foo signals.
Manifold: Space by Stephen Baxter covers this conjecture, for anyone interested in a fictional treatment.
The whole Manifold series is interesting. Each Manifold book stands in its own universe, but uses the same cast of characters with a different set of answers to Fermi.
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.
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.
One of his best novels, in my opinion. Fantastically huge ideas, and yet has pretty well developed characters for an SF novel. The ending is incredibly poignant.
Diaspora by Greg Egan also has a first-hand depiction of a gamma-ray burst hitting the Earth.
Diaspora by Greg Egan also has a first-hand depiction of a gamma-ray burst hitting the Earth.
If you're interested in Physics and Cosmology, Baxter is a very, very fascinating author.
I just find his style a bit dry. But it's worth it for the mind-bending ideas.
I just find his style a bit dry. But it's worth it for the mind-bending ideas.
And Jack McDevitt's The Devil's Eye also has a fictional treatment of gamma ray bursts, if I recall properly: http://www.amazon.com/The-Devils-Alex-Benedict-Novel/dp/0441...
"If you look at our current technology level, something strange has to happen to civilisations, and I mean strange in a bad way." - Elon Musk
that is generally known as "the great filter" theory, methinks, in the case of "the great filter is still in front of us".
Is it? Even accounting for this, you'd still expect the galaxy to be teeming with life, even with pessimistic values for the various other parameters.
This paper makes a lot of assumptions about the nature of life. Just off the top of my head, it takes as given that:
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.
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.
From the paper: "While life can take numerous other forms and could be much more resilient to radiation than on earth, we make here the conservative assumption that life is rather similar to the one on Earth. This common assumption is the basis for searches of Earth like exoplanets as places that harbour life. "
There are creatures that are resilient to radiation already on the earth.
http://en.wikipedia.org/wiki/Tardigrade
http://en.wikipedia.org/wiki/Tardigrade
Right, but these are the exceptions for a reason. Something life is exposed to only every hundred(s) millions of years or so is so infrequent that evolution selects against so long as it introduces inefficiencies (and nothing is free). Evolution is an extremely dumb optimization process, and will simply drift away from rad-protecting features as soon as the event is over.
True, in places where the only sources of radiation are rare events like GRBs. But I imagine there must be hospitable exoplanets somewhere in the universe that are, for one reason or another, subjected to more intense radiation constantly, in which case rad-protection would have a strong impact on evolution
[deleted]
Marine plankton created the ozone layer by turning CO2 into O2 which could then be transformed into O3 (Ozone). Which suggests plankton don't really need an ozone layer.
It's hypothesized that the Ordovician–Silurian extiction (end of trilobites) may have been caused by a GRB. This was ~450-440 million years ago. Here's to a 1 in 500 million year event! http://en.wikipedia.org/wiki/Ordovician%E2%80%93Silurian_ext...
I had a paper published on a model for the GRB that would have caused that event!
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.
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.
Maybe this is a dumb question, but what about life on the other side of the planet from where the GRB hits?
Not a dumb question! It's answered in the GRB wikipedia article:
"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...
"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...
That was my first question after I learned about GRB. According to wikipedia you linked, atmosphere might be sufficient to protect from gamma rays, unfortunately at a very high cost to stratosphere.
"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."
"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."
[deleted]
> When considering the Universe as a whole, the safest environments for life (similar to the one on Earth) are the lowest density regions in the outskirts of large galaxies and life can exist in only ~ 10% of galaxies. Remarkably, a cosmological constant is essential for such systems to exist.
Wow. That's an interesting side-effect.
Wow. That's an interesting side-effect.
If you are referring to the cosmological constant, not really: You still need rest of physical laws & constants for life to exist. Changes in the others could compensate for changes in the cosmological constant (for instance, strength of gravity).
I just thought it seemed a bit arbitrary to link the existence of life on that seemingly-unrelated constant.
I'm aware of all the rest of the "fine-tuning" (please mind the quotes, yadda-yadda).
I'm aware of all the rest of the "fine-tuning" (please mind the quotes, yadda-yadda).
When we talk about mass extinctions, we're mostly talking about marine life. Mass extinctions are dominated by a reduction in marine biodiversity, simply because that's what dominates the fossil record. (Beyond that, there's no land life for the first of the 5 major extinction events, and little data for land life for the next two.)
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, 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.
Half of the earth's surface would be fried instantly by a GRB. The other half would be protected, forget a few meters of water, by the bulk of the planet. The kicker, though is the sudden conversion of a large part of the atmosphere into nitric acid. (O2 + gamma -> NO, NO + H2O + O2 -> HNO3 - I'm not gonna balance that). That will cause serious imbalances in ocean chemistry and also be uncomfortable for terrestrial life on the other half of the planet as it diffuses over.
Many deep-water marine ecosystems are dependent on their neighbors in near-surface environments.
Sure! They're tied to the shallow marine ecosystem. Most marine productivity is within the photoic zone, as well. Marine ecosystems are typically not tightly tied to land life, though. (Sediment input, etc, is a different story, but while it's biologically mediated, it's not controlled by biologic factors.)
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.
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.
>However, marine organisms would be sheltered from gamma radiation.
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.
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.
It would be sheltered from gamma radiation. Even a few tens of centimeters of water is enough. (Phytoplankton is dispersed throughout the upper 50 meters or so, though it's concentrated closer to the surface.)
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.
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.
Is there really no way to find whether Earth got hit by gamma-rays? Nothing geologists can find, we have to guess?
One possibility is to look for evidence of neutron irradiation in rock layers that were exposed at the time. The signal would be weak:
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.
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.
I know little about this, but having said that could a very high burst produce affect elements that are mostly stable against the normal background radiation levels, but then decay into other elements when exposed to particular levels of gamma rays? So while general "rocks" might not have clear data, detecting the signatures of particular ratios of elements outside the normal baseline in a certain type of geological formation might provide evidence. Locating similar formations around the world could even triangulate to which side of the planet was hit.
dnautics commented on this thread that a gamma ray burst would suddenly convert free O2 in the atmosphere into NO, which would further react into nitric acid. Maybe this is detectable, either by the direct effects of HNO3, or by a sudden spike in nitrogen levels and the organisms that consume it.
> Amongst the different kinds of GRBs, long ones are most dangerous.
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 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?
"For [UV-B] radiation with a wavelength of 290 nm, the intensity at the top of the atmosphere is 350 million times stronger than at the Earth's surface." That's due to ozone (https://en.wikipedia.org/wiki/Ozone_layer).
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.
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.
It would (eventually, once the bacteria recovered) smell bad, but is this a human extinction event? The natural processes that form ozone wouldn't cease, and human action could supplement that. The specific types of plankton in a recovered ocean might be different, but surely we don't think all plankton would die out entirely over the entire earth?
I believe the conceptual problem most people are stuck on is the couple second at most gamma burst by itself is quite harmless to life on the surface. The guys on the ISS are likely totally screwed. Anyway massive radiation spike in the upper atmosphere has a rather well understood and very unfortunate chemical effect on the ozone layer, where its basically eliminated for about a calendar season.
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.
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.
Don't dead plants and animals fail to rot in sterile environments?
> but is this a human extinction event?
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.
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.
[deleted]
Whoa. In an odd way, it could turn out that destroying our natural ecosystems is the fastest way to prepare for a post-GRB environment. So maybe if our responsibility is to preserve intelligent life, the best route is to destroy the planet ASAP in a controlled manner, and learn to become completely self-sufficient without eco-system services... :/
While idea is remotely plausible, I think chance of that is really small. Destroying ecosystem will likely destroy human civilization, as we are still major part of that ecosystem.
The main reason species go extinct when the ecosystem collapses is the fact that most animals can't adapt to the new normal. Humans are among the most adaptable life forms imaginable, our civilization has a much better chance at surviving ecological collapse. Poor people might all be eating jellyfish stew, and certain economic sectors might disappear, but we should be just fine.
TLDR - this universe is seemingly hospitable to life, but in reality it's quite hostile.
It's intrinsically difficult to argue about the probability of life on a sample of one, and so I can't adjudicated between the Rare Earth hypothesis and the Common Life hypothesis any more than the next guy. But I do think I can say that the sneering dismissal of the Rare Earth hypothesis I've sometimes seen is unsound. The Copernican Principle that "we are not special" may have carried us a ways, but it is ultimately only a heuristic, not a scientific law, and it may have limits.
The Rare Earth hypothesis as explained in the book only mentions conditions required for past events leading up to the ascent of humans to occur, and of course this paper adds to the rarity required, but the hypothesis could also extend to conditions required for humans to colonize the galaxy. There's enough rocks out there with gravity large enough for humans to build sealed-city civilizations on, scattered at greater and greater distances away from earth so as to provide incrementally increasing challenges to get to them: the Moon, Mercury, Mars, the 4 Galilean moons of Jupiter, Saturn's Titan, and Neptune's Triton. Then there's the closest, Venus, which is uninhabitable even for sealed cities but provides the challenge of terraforming. When humans can successfully terraform Venus, they're ready for a one-way trip to the stars. And the paper suggests it will be outwards down the Orion spur.
Just to be clear, I speak more broadly of the idea in general that conditions are perhaps not right for life everywhere, rather than the specific claims of a particular book.
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.)
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.)
It's not an answer I like. But then, that's probably true for most potential answers to that particular question.