Deep-sea analysis deepens the mystery of rare ancient cosmic explosion

A long time ago, in the galaxy, quite close to us, there was a rare, very violent explosion.

We may never know what it was: it could have been an especially large supernova (star explosion) or a collision between two neutron stars, and it was at least 100 million years ago.

Whatever it was, it was a hot enough furnace to create some of the rarest elements in the universe. These elements were strewn across the sky by the violence of the explosion, eventually reaching earth, where they settled at the bottom of the ocean some ten million years ago, to be dug up and analysed in the 21^st century, said Dr Dominik Koll.

“It’s mesmerising that we can today measure signatures, in a sample that is tens of millions of years old, of a process from more than 100 million years ago,” said Dr Koll, who holds a joint appointment between the Research School of Physics and the Institute of Ion Beam Physics and Materials Research at Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in Germany.

The team’s findings are published in Nature, and detail how the explosion was more powerful than a standard supernova, as evidenced by the production of the heavy radioactive element plutonium-244.

We have only witnessed one standard supernova in our galaxy (in 1987) since the invention of the telescope, although we have seen them in other galaxies. The explosions that create plutonium-244 are thought to be 1000 to 10,000 time less frequent than standard supernovae.

Establishing the timeframe of the explosion required careful detective work, based on dating techniques similar to carbon dating, but using other radioactive elements.

Plutonium-244 itself is radioactive, with a half-life of 80 million years and was detected using the VEGA AMS facility at ANSTO. It is only produced by rare violent reactions in which neutrons are captured rapidly (inspiring the name ‘R-process’), although details of the process remain mysterious, Dr Koll said.

“The R-process has been replicated on Earth in thermonuclear explosions, but where it occurs in nature is still not clear.

“We know the process works, and how; but it is hard to model the cosmic conditions that can create these elements,” he said.

Establishing the timeframe of the R-process event required careful detective work, based on dating techniques similar to carbon dating, that use other radioactive elements.

Using a sample of ferromanganese crust extracted from the Pacific Ocean floor Dr Koll and the team had previously isolated another tracer of supernovae, iron-60 using the ANU Heavy Ion Accelerator Facility. Detailed modelling of the age of the sample layers was done at the DREAMS facility at HZDR using radioactive beryllium-10, which is created in the upper atmosphere.

Combining the data they found peaks of iron-60 at two specific depths in the crust, pointing to two nearby supernova in the last ten million years.

So the first hypothesis was that the plutonium-244 had been created in the same explosions. But the data did not match at all – instead, the plutonium-244 was evenly spread throughout the sample, suggesting an older event, which had spread its debris more evenly across the space around earth (the interstellar medium).

The next test was to look for curium-247, which is also radioactive and, just like plutonium, is created only in giant explosions. But the ANSTO analysis showed none at all.

Curium-247’s half-life is only 15.6 million years, less than a fifth of plutonium-244’s: its absence suggests that the event happened long enough ago for it all to decay, pushing back the date of the event to 100 million years ago, or more.

Dr Koll said that the assembly of the puzzle pieces to land on such an ancient creation date for the plutonium-244 had sent theorists back to the drawing board.

“We have ruled out four models with this finding, such as a recently proposed collision of the Solar System with a dense interstellar cloud.

“To be able to compute it, the models need to simplify the interstellar medium, but our data shows that nature is more complicated than that.

“We need to invest more in models and experiments to get to the truth!” he said.

This was first published by ANU Research School of Physics.