Each supernova sprays a cocktail of exotic elements across the cosmos.© NASA

Cosmic fallout from an exploding star dusted the Earth about 2.8 million years ago, and may have triggered a change in climate that affected the course of human evolution. 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 Earth¹. 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.

"We've looked at all the possibilities and we can't find anything else that could produce such quantities," Korschinek says. The researchers report their results in the latest issue of Physical Review Letters².

"It represents an experimental triumph and a milestone in this field," says Brian Fields, an astrophysicist from University of Illinois at Urbana-Champaign. He argues that the result marks the birth of a completely new area of research, which he calls "supernova archaeology"³.

Close shave

Comets and meteorites also deliver matter to Earth, but they always come from within our Solar System. Supernovae are the only known source of interstellar debris. "The very fact that a supernova can dump material on the Earth is, to my mind, rather spectacular. It demonstrates that the Earth is not independent of its cosmic environment," says Fields.

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. It was collected by oceanographers who were investigating the rocks as a potential source of rare mineral ores.

Korschinek estimates that the supernova was between about 100 and 200 light years away and happened 2.8 million years ago, give or take 300,000 years. The explosion can't have been too close to Earth, or it would have delivered enough radiation to cause mass extinctions. 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.

Cooling rays

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.

The increase in cloudiness would have cooled the surface, tying up water as ice at the poles and leading to a dryer climate in Africa. Climate records in rock cores match the dates of the supernova event.

"Some people believe this climate change in Africa wasa driving force in our own evolution," adds Korschinek. The argument isthat a drier climate in the continent would have forced humans toadapt⁴, and to spread out to other, wetter areas.

The team is now looking for other unusual isotopes in the crust sample, which may reveal more about the type of star that caused the supernova. But there are probably 10,000 times fewer of these atoms than of the iron-60, says Korschinek, so they will be extremely difficult to measure. "We're sweating, and I don't know if we will succeed," he says. 

University of Illinois, Urbana

• References

  1. Knie K., et al. Phys. Rev. Lett., 83. 18 (1999). | Article | ISI | ChemPort |
  2. Knie K., et al. Phys. Rev. Lett., 93. 171103 (2004). | Article | PubMed | ChemPort |
  3. Fields B. D., Hochmuth K. A., Ellis J., Preprint, http://arXiv.org/astro-ph/0410525 (2004).
  4. DeMenocal P. B., Science, 270. 53 - 59 (1995).