Space is a hostile and chaotic place, and yet on our little rock, life – as Richard Attenborough famously said – finds a way. Unless a gentle shift in stellar fortune BRUTALLY DESTROYS IT.
We know about the threat of comets, aliens, the inevitable nova-ing of our star and the eventual heat death of the universe, but space also has some more subtle tricks…this post explores one of them.
Two million years ago a slightly underwhelming marine extinction event occurred: fossil evidence shows a mass death of molluscs and Plankton which is considered to be the marker between the Pliocene and Pleistocene epochs. What puzzled palaeontologists who studied this extinction was that it occurred in conjunction with a spike in levels of the iron isotope 60Fe in the sediments where these fossils were found. Were the extinction and the isotope related?
91% of naturally occurring iron on earth is the 56Fe isotope. The next most common isotopes of iron (which differ from one another by the number of neutrons in their nucleus) are 54Fe, 57Fe and 58Fe. 60Fe is has a far greater rate of decay than these, and it’s not found on earth, it’s produced during supernovas. It appears that two million years ago our solar system entered a region of space known as the local bubble. This is a peanut shaped ‘hole’ in space where particles are even more scarce than in the surrounding void. This cloud of hot, sparse gas was created by one or several supernovas. One of these may have been Geminga: a star in the Gemini constellation that supernova’d 3-4 million years ago, leaving behind a neutron star carcass and emitted a death-rattle in the form of substantial gamma radiation.
What does marine life care about ancient supernovas? When we entered the local area of this (or these) supernova, we would have experienced a marked increase in exposure to ionizing radiation in the forms of neutral electromagnetic radiation and charged cosmic rays. Both forms of radiation increase the level of Nitric Oxide (NO) in the atmosphere by increasing the number of N- and O+ ions available. NO is an unstable molecule that acts as a catalyst for the breakdown of ozone. As we know from our current problems with the ozone hole above Antarctica, this greatly increases the amount of UV radiation that enters our atmosphere. For the light-sensitive, photosynthesising Plankton that are hugely important to the entire biosphere, this proved deadly 2 million years ago. It’s surprising that the knock-on effects of increased CO2 and decreased oxygen resulting from the mass Plankton death weren’t greater.
Detecting past supernovas is challenging: the light they emit dwindles after around a million years and debris from the explosion can travel in unpredictable directions, making the point of origin hard to identify. The presence of 60Fe in different layers of the earth’s crust is only a crude indicator of our planet’s experience of them. The discovery of the local bubble has provided a new marker to look for (the actual makeup of our surrounding space), but this technique is still far from refined.
Astronomers predict that supernovas occur in our galaxy every 10-100 years. It is expected that for a nova to have a significant impact on life on earth it would need to be less than 10 parsecs (32.6 light years) from us*. Our current position in the bubble is a very good place to be: without the normal levels of interstellar gas we can see extraordinary numbers of stars – our skies would be practically blank in normal space. More practically, the Helosphere cast by the sun which protects us from many harmful forms of radiation can also extend further with this lowered resistance. Life on earth would be a lot harsher were we travelling through normal space. We probably won’t ever know how many other supernovas have happened in close proximity to our planet. How many times have distant explosions shaped and destroyed life in our fragile biosphere?
Next time you look up in wonder at the night sky, thank the local bubble for its benevolence, but mourn the Plankton which it heartlessly annihilated.
More space-based death will be explored in further posts. Thanks for reading!
* It has also been hypothesised that a supernova occurring closer than 25 light years away would completely destroy all life here!
“Could a nearby supernova explosion have caused a mass extinction?”
JOHN ELLIS AND DAVID N. SCHRAMM (Proc. Natl. Acad. Sci. Astronomy USA Vol. 92, pp. 235-238, January 1995)
“Our Local Galactic Neighborhood” NASA. Interstellar.jpl.nasa.gov. 2000-02-08. Retrieved 01/04/2014
Near Eath Supernovas, NASA http://science1.nasa.gov/science-news/science-at-nasa/2003/06jan_bubble/