Earth may have been hit by a cosmic blast |

Earth may have been hit by a cosmic blast

Artist drawing of a gamma-ray burst, a cosmic explosion announcing the birth of a black hole. The Earth may have been hit by a gamma-ray burst about 1,200 years ago, according to researchers. Illustrates GAMMARAY (category l), by Phil Plait (c) 2013, Slate. Moved Wednesday, Jan. 23, 2013. (MUST CREDIT: NASA/Dana Berry/Skyworks Digital.)

Well now, here’s something I never thought I’d hear: The Earth may have been hit by a gamma-ray burst, a violent cosmic blast of energy, about 1,200 years ago, according to a report in Oxford Journals.

First off, this is nothing to panic about. If it happened at all, it was a long time ago, and unlikely to happen again for hundreds of thousands of years. And if it did happen, it didn’t cause any mass extinctions or anything like that — if it had, we’d have known about it earlier!

But it’s very interesting, and while I remain skeptical, the astronomers involved, Valeri Hambaryan and Ralph Neuhauser, make a compelling case. Here’s how it goes.

An analysis of Japanese trees indicates a sharp increase in their carbon-14 to carbon-12 ratio in the past. Using the tree rings as a guide, this occurred in the years 774-775 AD. What does this mean?

Most of the carbon around us is the carbon-12 flavor: Each atom has six protons and six neutrons in its nucleus. There’s a different kind of carbon, called carbon-14, which has, instead, eight neutrons and six protons. Carbon-14 is radioactive and decays into nitrogen over time.

The fact that there’s any carbon-14 on Earth means it must be made continuously to resupply the atoms that go away, and this is done by cosmic rays hitting nitrogen in our atmosphere. This just balances the amount that decays — think of it like a sink where you run the tap at the right amount to balance the water that drains away. The water level in the sink remains steady.

So we expect a certain amount of carbon-14 all around us. The Japanese trees have a sharp spike in them, about 10 times as much as usual. This also corresponds in time with a rise of carbon-14 seen in American and European trees, though the exact date is harder to pin down. It’s as if that sink you have suddenly has 10 times as much water in it. That water had to come from somewhere.

Not only that, but at the same time, something increased the amount of beryllium-10 (another radioactive element) in Antarctic ice by about 10 percent. That’s enough to be significant as well. What can make these big changes in elements all over the globe at the same time?

The best way to affect the whole Earth at the same time is to have something occur in space. It would take an extremely energetic (that is, violent) cosmic catastrophe to do it, something that could dump a lot of energy into Earth’s atmosphere. There are a few different things that could do this: a giant solar flare, a nearby star exploding or a gamma-ray burst.

A solar flare is unlikely; the energy needed to create the carbon-14 detected would be 20 times larger than any solar flare ever seen.

That’s possible, so I wouldn’t necessarily rule it out, but the odds are pretty slim. A nearby exploding star, or supernova, is almost certainly not the culprit.

That leaves a gamma-ray burst which happens when a black hole is born. There are several ways this can occur; the most common is for an extremely massive star to explode at the end of its life. The core collapses to form a black hole, and the forces involved send out two colossal beams of energy, like deadly lighthouse beams, into space.

If they’re pointed our way we see a flash of high-energy gamma rays. Hence the name.

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