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Comment by zycon | Hacker News Reader

There's a hypothesis that Ordovician-Silurian extinction cca 450 milion years ago, one of the big five mass extinctions, was caused by a GRB.

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Is there any data to back this up? (not attacking, just genuinely curious, i love learning about mass extinctions)

Only extremely circumstantial evidence. Some guesses about which species died off vs being protected, some climate change signatures that are consistent with a high UV burst.

About the best you can say for it is "you can't prove it didn't happen". Which, given that it was half a billion years ago, may be the best you can get. There isn't a real smoking gun.

The leading hypothesis is more that a long-term climate change caused secondary effects in a vicious cycle. We know the climate change was happening; there's very strong evidence. But it's not clear exactly what caused it or exactly how it led to mass extinction.

No, and the idea is widely derided by paleontologists because it's pretty much untestable.

Would the Earth shield one side of itself? If yes by how much?

Seems like that would only help if the duration of the blast was less than 24 hours

Rotation around the axis doesn't expose the northern hemisphere to the southern sky, nor vice versa.

That depends on the orientation with regard to the poles. If the source were directly over a pole, only fifty percent of the planet would be directly exposed. If it were over the equator, however, the whole planet would be, if it were a magic GRB that lasted way longer than usual.

Or perhaps more like 12 hours, as half of the Earth would be exposed instantly.

But even then it depends on whether it's the direct radiation that's the issue or something more indirect (atmospheric changes, etc.)

The longest gamma ray bursts only last a few minutes.

Just how many nines reduction from peak intensity after a few minutes are we talking? Because I'd imagine 0.01% of a nearby gamma ray burst would still be bad news.

doesn't this depend on the blast's origin relative to the Earth's axis?

It will affect atmospheric chemistry. It will produce a ton of NO2 and make atmosphere unbreathable without equipment.

I imagine that killing half the planet is enough to send the "lucky" half into chaos and collapse.

So the upside is that anything that can get through 10k km of rock is very unlikely to interact with anything on the other side (eg neutrinos)

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