The radioactive dust located in the depths of the ocean waves suggests that the Earth is moving through a massive cloud left by an exploded star.
Continuously, for the past 33,000 years, space has been seeding the Earth with a rare isotope of wrought iron in supernovae.
It is not the first time that the isotope, known as iron-60, has dusted our planet. But it contributes to growing evidence that this dust continues: we are still moving through an interstellar cloud of dust that could have originated from a supernova millions of years ago.
Iron-60 has been the focus of several studies over the years. It has a half-life of 2.6 million years, which means that it decays completely after 15 million years, so samples found here on Earth must have been deposited elsewhere. , as there is no way that iron-60 could have survived the formation of the planet 4.6 billion years ago.
And deposits have been found. Nuclear physicist Anton Wallner of the National University of Australia previously dated the deposits of the seabed to be 2.6 million and 6 million years ago, suggesting that the remains of supernovae had rained on our planet at this time.
But there is more recent evidence of this star dust, much more recent.
It has been found in Antarctic snow; according to the evidence, it had to fall in the last 20 years.
And, a few years ago, scientists announced that iron-60 had been detected in space around the Earth, measured over a 17-year period by NASA’s Advanced Space-Based Composition Explorer .
In 2020, Wallner found more, in five samples of deep-sea sediments from two sites dating back 33,000 years. And the amounts of iron-60 in the samples are quite consistent throughout the time period. But this finding raises more questions than it answers.
As you can see, the Earth is currently moving through a region called the local interstellar cloud, made up of gas, dust, and plasma.
If this cloud was created by exploding stars, it is reasonable to expect it to be dusting the Earth with a very light iron-60 rain. This is what Antarctic detection suggested; and this is what Wallner and his team were trying to validate by examining ocean sediments.
But if the local interstellar cloud is the source of iron-60, there should have been a sharp increase when the solar system entered the cloud, which, according to team data, has probably produced in the last 33,000 years. At the very least, the oldest sample should have had significantly lower iron-60 levels, but no.
It is possible, the researchers point out in their article, that the local interstellar cloud and the remains of supernovae coincide, rather than a structure, with the rest of the remains in the interstellar medium from the supernovae that go take place millions of years ago. This would suggest that the local interstellar cloud is not a weak supernova remnant.
“There are recent documents suggesting that iron 60 trapped in dust particles may bounce off the interstellar medium,” Wallner said last year.
“Thus, iron-60 could originate from even older supernova explosions, and what we measure is a kind of echo.”
Researchers point out that the best way to find out is to look for more iron-60, covering the gap between 40,000 years ago and about a million years ago.
If the abundance of iron-60 grows further in time, this would suggest ancient supernovae.
However, a more recent abundance would suggest that the local interstellar cloud is the source of iron-60.
The research has been published in Proceedings of the National Academy of Sciences.
A version of this article was originally published in August 2020.