FT235

Is the destruction of mankind written in the stars?

A supernova is among the most dramatic astronomical events imaginable. For a few brief weeks or months, a single star can burn more brightly than an entire galaxy, consuming all its fuel in one brief, cataclysmic explosion. Astronomers have sometimes wondered whether such an event in our vicinity might have serious consequences for life on Earth. The general consensus has been that there is no danger… but now they are not so sure.

A supernova was one of the initial candidates for the cause of the great mass extinction 65 million years ago in which the dinosaurs were wiped out, the so-called K-T boundary event. However, by the 1980s geological evidence seemed overwhelmingly on the side of a giant asteroid impact.

Further reassurance was provided by a 2003 study looking at how close a supernova would have to be to harm the planet. [1] Scientists from NASA and Kansas University used data from recent supernovæ to calculate that one would have to be just 26 light years away before the cosmic rays and gamma radiation would pose a threat to the Earth’s ozone layer. Previous estimates assumed that supernovæ would be dangerous even at much greater distances and the Earth would be affected by these events relatively frequently. But 26 light years is right on our cosmic doorstep; Dr Neil Gehrels of NASA’s Goddard Space Flight Center says that such an event would only occur once every 600 million years.

However, a discovery by Narciso Benítez of Johns Hopkins University suggests that a supernova event did cause one major extinction two million years ago. [2] Geologists have noted that the Pliocene-Pleistocene boundary, which involved the extinction of plankton, molluscs, and other marine life, coincides with an unusual concentration of the isotope Iron-60.

Suspicion fell on a group of stars known as the Scorpius-Centaurus OB association. Supernovæ within this group have produced the ‘local bubble’, an immense cloud of gas. Although the stars are now 450 light years away, a couple of million years ago they were about 130 light years from Earth. That’s not enough for the atmosphere-blasting effect that Dr Gehrels was looking at, but it’s sufficiently close for a supernova to damage the ozone layer. The effect would not be global extinction, but vulnerable species – such as plankton – could be killed off.

The ozone layer is currently weakened due to the effects of CFCs and other man-made pollutants, and this has produced a measurable increase in the amount of ultraviolet light reaching the Earth’s surface with a corresponding rise in skin cancers. This indicates that any sort of additional damage might have unexpectedly significant consequences.

A supernova doesn’t just produce a wave of high-intensity radiation; it is literally the explos­ion of a star, and the shrapnel may also be dangerous. Dr Richard Firestone of the US Lawrence Berkeley National Laboratory has suggested that the extinction of the mammoth 13,000 years ago was caused by the aftermath of a supernova. [3]

The fireball in question was 250 light years away, and would have been no more than a star that flared briefly and then disapp­eared forever. Seven thousand years later, the initial shockwave hit: a shower of iron-rich grains that peppered the entire Solar System. Travelling at 10,000km (6,214 miles) a second, most would have simply burned up in the atmosphere – but some got through, leaving a pattern of miniature impact craters.

The most serious effects occurred another 21,000 years later. According to Firestone, debris from the event coalesced into a large number of comet-like objects. “Our research indicates that a 10km [six mile]-wide comet, which may have been composed from the remnants of a supernova explosion, could have hit North America 13,000 years ago,” says Firestone.

He has identified a layer of spherical metal particles from this date at several sites across North America. They are composed of a mix of carbon, titanium, vanadium and other elements; in particular, there is a high percentage of the unstable isotope Potass­ium-40, which is rare in the Solar System but is characteristic of material formed by supernovæ.

The impact of the massive comet somewhere in North America would have caused widespread damage and temporary climate change. Small animals that breed and mature rapidly would have been able to recover their populat­ion levels fairly quickly, but larger mammals like mammoths might have been tipped into extinction. Others have (of course) quest­ioned Firestone’s interpretation of the data. [4]

As if this weren’t enough, a subtler and stealthier risk from stellar explosions has been identified. During the collapse stage that precedes the supernova, an intense burst of neutrinos would be produced. These are very small, elusive particles that barely interact with other matter – billions of them pass through the Earth without stopping. But Dr Juan Collar of CERN has calculated that a nearby collapse would produce so many neutrinos that their impact would trigger 12 cancers per kilogram in any animal tissue, “severe enough to kill a vast percentage of large animals with a frequency comparable to that of most major extinctions.” [5]

Again, Dr Collar’s calculations have been debated, and our knowledge of neutrino interact­ions is still evolving. However, there is certainly plenty to supp­ort the old notion that a bright new star is a significant portent – and not necessarily a good one.

NOTES 

1 www.universetoday.com/2003/01/15/ supernova-wont-destroy-the-world/.
2 http://physicsworld.com/cws/article/ news/5046.
3 www.lbl.gov/Science-Articles/Archive/NSD-mammoth-extinction.html.
4 www.starburstfound.org/YDextinct/p1.html.
5 http://query.nytimes.com/gst/fullpage.html? res=9E06EFDF1F39F930A15752C0A960958260&sec=health&spon=