
On the role of GRBs on life extinction in the Universe - ca98am79
http://arxiv.org/abs/1409.2506
======
daveloyall
Sounds like an answer to the Fermi Paradox if I ever heard one.

It's not an answer I like. But then, that's probably true for most potential
answers to that particular question.

~~~
SerpentJoe
Manifold: Space by Stephen Baxter covers this conjecture, for anyone
interested in a fictional treatment.

~~~
bashinator
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.

~~~
Florin_Andrei
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.

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idlewords
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.

~~~
guelo
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. "

~~~
happyscrappy
There are creatures that are resilient to radiation already on the earth.

[http://en.wikipedia.org/wiki/Tardigrade](http://en.wikipedia.org/wiki/Tardigrade)

~~~
maaku
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.

~~~
dandelany
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

------
e0m
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...](http://en.wikipedia.org/wiki/Ordovician%E2%80%93Silurian_extinction_event#Gamma_ray_burst_hypothesis)

~~~
nitid_name
I had a paper published on a model for the GRB that would have caused that
event!

[http://arxiv.org/abs/astro-ph/0504158](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.

------
Houshalter
Maybe this is a dumb question, but what about life on the other side of the
planet from where the GRB hits?

~~~
tomr_stargazer
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...](https://en.wikipedia.org/wiki/Gamma-
ray_burst#Rate_of_occurrence_and_potential_effects_on_life_on_Earth)

~~~
TrainedMonkey
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."

------
Florin_Andrei
> _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.

~~~
dkural
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).

~~~
Florin_Andrei
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).

------
jofer
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.

~~~
fiatmoney
Many deep-water marine ecosystems are dependent on their neighbors in near-
surface environments.

~~~
jofer
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.

------
hcarvalhoalves
Is there really no way to find whether Earth got hit by gamma-rays? Nothing
geologists can find, we have to guess?

~~~
tjradcliffe
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...](http://www.mpa-
garching.mpg.de/lectures/ADSEM/WS0304_Kienlin.pdf)

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.

~~~
geuis
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.

~~~
hcarvalhoalves
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.

------
infogulch
> 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?

~~~
idlewords
"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](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.

~~~
jessaustin
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?

~~~
VLM
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.

~~~
daveloyall
Don't dead plants and animals fail to rot in sterile environments?

------
patcon
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... :/

~~~
TrainedMonkey
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.

~~~
vinceguidry
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.

------
Florin_Andrei
TLDR - this universe is seemingly hospitable to life, but in reality it's
quite hostile.

~~~
jerf
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.

~~~
vorg
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.

~~~
jerf
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.)

