Is our system sailing through a radioactive dust cloud?

The Earth and entire Solar system might have been travelling through an interstellar dust cloud of radioactive material for the last 33,000 years – according to deep-sea sediment samples obtained and analysed from two different locations.

The new study (published in the journal, PNAS) was led by scientists from the Australian National University (ANU) Department of Nuclear Physics and included Australia Nuclear Science and Technology Organisation (ANSTO), and international collaborators.

The samples, which were reviewed at the ANU Heavy Ion Accelerator Facility (HIAF), showed the presence of the radioactive isotope Iron-60 (⁶⁰Fe), a product produced in the nucleosynthesis of supernovae explosions before being dispersed across the Universe.

“These clouds could be remnants of previous supernova explosions, a powerful and super bright explosion of a star,” said Professor Anton Wallner, a nuclear physicist at ANU.

Radionuclides found in Earth’s geological record, like ⁶⁰Fe, are not produced in significant quantities both on Earth or within the Solar system – but instead originate from previous stellar explosions in nearby regions of space (or regions the Sun and planets travel into along its orbit around the Galaxy).

The isotope of ⁶⁰Fe also decays in approximately 15 million years, so any traces found today are not residual values of the formation of the Planets from the original Solar nebula, and would have formed much later than Earth (aged at approximately 4.6-billion years old).

The Sun and our Solar system have, for the past few tens of thousands of years, entered a localised region of the interstellar medium which is slightly higher in hydrogen density. This region is known as the Local Interstellar Cloud (LIC) and the Sun and our system are expected to remain within it for another 10,000 – 20,000 years.

The cloud itself is not very dense, only about 0.3 atoms per cubic centimetre, but it has roughly the same temperature as the surface of the Sun (with a much lower specific heat capacity due to its low density).

Researchers have indicated that the LIC, which is approximately 30 light-years across in size, is flowing outwards from the Scorpius-Centaurus Association, located about 470 light years away which features a host of massive, younger stars like the stars of the Southern Cross (Crux).

Could the LIC be the remains of once massive stars that have exploded in supernovae, exhibiting heavy elements forged during stellar nucleosynthesis and now being spread across the Galaxy, and our localised region? In 2019, another study also found interstellar ⁶⁰Fe in deposits across Antarctica as well.  

Prior to this new study, Prof. Wallner and his team had also found traces of ⁶⁰Fe in 2016 from samples taken from Earth’s crust taken at locations across the Pacific, Atlantic and Indian Oceans which dated back to 2.6 million years and 6 million years ago.

In lieu of this, the research team decided to go back for more samples in more recent sedimentary layers and were able to find low levels of ⁶⁰Fe spread throughout the a geological epoch extending 33,000 years in the past from today’s levels.

The findings raise new questions about the origin of the radioactive isotope – if there was a recent supernova explosion, then where is the evidence of this event in the galaxy? If it wasn’t produced by a supernova explosion, then what could have created these isotopes? And what is the relation with the Scorpius-Centaurus Association?

“There are recent papers that suggest iron-60 trapped in dust particles might bounce around in the interstellar medium,” Professor Wallner said. 

“So the iron-60 could originate from even older supernovae explosions, and what we measure is some kind of echo. 

“More data is required to resolve these details.”