Evidence of an astronomical “smoking gun” has been discovered that supports the idea that cosmic rays from a nearby supernova triggered climate change on Earth. The evidence comes from an unusual form of iron that was blasted through space by a supernova before eventually settling into the rocky crust beneath the Pacific Ocean
Gunther Korschinek, a physicist from the Technical University of Munich in Germany, leads a team who in 1999 found the first deposits of supernova matter on Earth. But it was impossible to date the supernova accurately from those samples, because the material was distributed through several different layers of rock.
The team has now analysed a different piece of ocean crust, where the supernova detritus is concentrated into a clear band of rock that can be accurately dated. The researchers found small but significant amounts of an isotope called iron-60 in the rock, which could only have come from a supernova.
When the iron-60 arrived from space, it was evenly distributed all over the Earth. But the signatures are only detectable in crust that has lain undisturbed for millions of years, such as certain parts of the Pacific Ocean floor. This particular crust was taken from an area a few hundred kilometres southeast of the Hawaiian Islands in 1980.
Korschinek estimates that the supernova was between about 100 and 200 light years away and happened 2.8 million years ago. The explosion can’t have been too close to Earth, or it would have delivered enough radiation to cause global climate change. Conversely, if the supernova was any further away, more of the iron-60 would have been filtered out by the thin wisps of matter drifting between the stars.
This means the supernova would have been at the right distance to spray out a stream of cosmic rays that could have increased the cloud cover on Earth. Korschinek calculates that there may have been 15% more cosmic rays arriving on Earth than normal for at least 100,000 years. This is not enough to actually kill anything, but was perhaps sufficient to change the Earth’s climate.
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