Supernovae and Extinction Events

Our Solar System lives in a giant bubble probably carved out by multiple supernovae sometime in our history. We know this because we can detect the outer reaches of this bubble through measuring x-rays. Recent research by Melott, Marinho and Paulucci has suggested a possible link between the cataclysmic supernovae that created this bubble and a mass extinction event that occurred around 2.6 million years ago, when there was a significant loss in the number of large marine species. There has been speculation before on the role of supernovae in extinction events on Earth but the mechanism linking the two has not been pinned down. One of the problems being, that for the volume of high energy particles to cause an extinction event on Earth, the supernova would have to be within a few light years of us. The mechanism suggested is that the large amount of muons created during the supernova would bathe large animals on the Earth with a higher than normal radiation dose causing mutations in DNA and ultimately the animals die from this damage through cancerous tumours.

Previous studies had looked how the increased amount of ultra violet light hitting the upper atmosphere would remove ozone, therefore increasing the damage done by UV light to animals and plants on the surface. We wrote about some of these effects here and mentioned that research had looked at the possibility of extinction events being linked to supernovae. Melott, Marinho and Paulucci found that previous studies had not taken into account the importance of the supernova bubble known as the Local Bubble. Bubbles in the stellar medium are important for the recycling of gas in the galaxy and ultimately result in the inclusion of supernovae distributed heavy elements into new stars and planets – and maybe even life. In the life evolution of bubbles, like the one surrounding our Solar System, they grow bigger and bigger from supernovae until the super bubble pops out of the galactic disk. This causes the cooling gas of the expelled bubble to slowly be attracted back to the galactic disk where it is cool and dense and able to kick off new star formation processes.

Maybe it was a hot blue star like this one that blew up 2.6 million years ago (Credit: Me)

Scientists found that around 2.6 million years ago and back to about 8 million years ago there were quite a few supernovae kicking off in our neighbourhood of the galaxy. It seems that the early stars that blew up during this period cleared away much of the gas and formed this giant bubble we find ourselves in. At about 2.6 million years ago a supernova event occurred about 160 light years away that would have sent its cascade of high energy particles into the clear area of the bubble. If our Solar System was in this clear bubble, much like it is today, then the energy from this supernova would have had a much greater effect on the Earth than a similar supernova in the normal stellar medium. The researchers found that if they took the effect of our Local Bubble into account then the radiation being absorbed by larger animals on the Earth would be significantly greater than normal. For larger animals in the ocean such as whales, large sharks etc then the effect would have been more due to the larger surface area of these animals.

Melott, Marinho and Paulucci based their work on calculating the muon flux hitting the Earth. They found that at about the same time of the candidate supernova, 2.6 million years ago, the Earth was going through a magnetic pole flip which would have reduced the protective ability of the magnetic field – letting in a heap more muons. Now muons aren’t really a problem normally and millions of them pass through us all of the time and though they normally just pass through, some interact with the particles in our bodies to make a contribution to the overall background radiation we are exposed to. The researches calculated that this normal background muon exposure could have been up to 150 times higher around the time of the supernova 2.6 million years ago. This would have created a dose of about 1 Sieverts per 30 years. This is enough to dramatically increase the amount of DNA damage and the incidents of cancer within a few decades. They believe that this may have contributed to the 36% loss of marine megafauna.

The local bubbles and gas clouds around us (Credit: Linda Huff (American Scientist), Priscilla Frisch (U. Chicago))

Trying to piece together the puzzle regarding the effects of supernovae is quite tricky and requires collaboration between a number of different disciplines. Whereas we have thought that a supernova would have to be very close to us to have an effect on extinction it may be that supernovae occurring in our Local Bubble can have a much more significant impact than previously thought. It would be interesting to compare this to the early hominid development as well. This is a fascinating field and one worth watching as scientists continue to uncover interesting relationships between space events and life on Earth.