For 33,000 years, Earth has passed through a cloud of weakly radioactive dust from a supernova explosion. Evidence for this phenomenon is found in the oceans of our planet, according to a new study.
Researchers at the Australian National University (ANU) found traces of iron-60 in sediment on the seabed at two different locations. The traces both date back 33,000 years.
ANU nuclear physicist Prof Anton Wallner explains: “These clouds could be the remnants of ancient supernova explosions, which take place when a star implodes in a very powerful and bright way.”
This discovery could help to understand what surrounds our Solar System, but it also poses essential questions.
What is 60 iron?
This is proof of the passage of a supernova, or even several supernovae, because this isotope of iron is only formed when stars die in a supernova. It has four more neutrons than the common form of the chemical element iron found on Earth.
Since iron 60 is not found on Earth, the traces found in the ocean must have been shipped to our planet from a place beyond our Solar System.
Iron-60 is a radioactive iron isotope with a half-life (the time it takes for a substance to lose half of its physiological activity) is 2.6 million years. It also breaks down after 15 million years. Since Earth is 4.6 billion years old, this means that traces of iron-60 found on our planet must have landed in supernovae before ending up on the seabed.
The researchers in question had already discovered traces of iron 60 that are 2.6 million years old, and possibly others dating back 6 million years.
This therefore suggests that the Earth passed through several clouds composed of debris from surrounding supernovae.
What is a supernova?
It is the complete explosion of a star. At the end of a supergiant star’s life cycle, the energy source is depleted, the pressure drops and collapses so quickly that the outer part of the star explodes, creating a powerful light that can be briefly longer. brilliant than entire galaxies.
Such a supernova explosion is the source of heavy elements in the Universe, and it leaves remains, such as the Crab Nebula or Cassiopeia A.
When famous astronomer Carl Sagan said, “We are made of stardust,” he was referring to the carbon, nitrogen and oxygen created in stars and the heavy elements found in star remains. dead. To be more precise, the American scientist could have said that: “We are all made of nuclear waste” (but it is admittedly less poetic).
According to the researchers, the culprit could be the Local Interstellar Cloud, also known as the Local Plush, in which the Solar System moves and whose composition and origin are still unknown.
What is the Local Interstellar Cloud?
It is a cloud of dust 30 light years wide that the Solar System is currently traversing. It should be out of it in 10,000 to 20,000 years. It is composed largely of hydrogen, the most abundant molecule in the Universe.
Earth likely picked up iron-60 particles from this radioactive stardust cloud as it passed through it. Last year, researchers in Antarctica discovered similar stardust containing iron-60.
However, new research suggests that there is evidence showing that some traces of iron-60 do not come only from the local Interstellar Cloud, but also from further afield.
This raises two questions:
•If the local Interstellar Cloud was not formed by a supernova, where does it come from?
•Is the iron 60 actually distributed evenly in space?
Anton Wallner specifies: “Recent studies suggest that iron 60 trapped in dust particles could rebound in the interstellar medium. So the 60 iron could come from even older supernova explosions, and what we’re measuring would be some kind of echo. “