快猫短视频

Washington DC

COSMIC rays from a nearby supernova triggered a mass extinction on Earth 2
million years ago, say researchers who have found the 鈥渟moking gun鈥�.

In 1999, Klaus Knie and colleagues at the Technical University of Munich
discovered atoms of iron-60 in cores taken from the ocean floor. Iron-60 has a
half-life of just 1.5 million years, so the atoms must have formed recently in
geological terms.

Knie suggested that the iron was fallout from a nearby supernova about 2
million years ago鈥攁round the time many marine molluscs became extinct. But
astronomers did not know any star system that could have been responsible.

Now Narcisco Ben铆tez at Johns Hopkins University in Baltimore, and
Jes煤s Ma铆z Appell谩niz of the Space Telescope Science
Institute, also in Baltimore, have traced the motion of a cluster of young stars
formed from the debris of around 20 supernovae that exploded over the past 20
million years. The cluster, called the Scorpius-Centaurus OB association, is now
a safe 350 light years away, but the group say it passed within 130 light years
of Earth about 2 million years ago鈥攃lose enough for it to have been to
blame.

Donald Clayton, an astronomer at Clemson University in South Carolina, says
the theory makes sense. 鈥淭he amount of iron-60 found in deposits is about what
you might expect from a supernova going off about 100 light years away,鈥� he
says.

The supernova would also have bombarded Earth with high-energy cosmic rays,
damaging the ozone layer that usually protects life from ultraviolet radiation.
Ben铆tez estimates the damage could have lasted up to 1000 years, long
enough to wipe out susceptible species.