The radioactive dust deep beneath the ocean waves suggests that the Earth is moving through a massive cloud left behind by an exploding star.
Continuously, for the past 33,000 years, space has seeded 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 helps to increase the amount of evidence that such dust is in progress – we are still moving through an interstellar cloud of dust that could have come 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 it decomposes completely after 15 million years – so any evidence found here on Earth must have been deposited elsewhere, because there is no iron. -60 that could have survived the formation of the planet 4.6 billion years ago.
And deposits were found. Nuclear physicist Anton Wallner of the Australian National University previously dated the seabed to 2.6 million and 6 million years ago, suggesting that supernova debris was raining on our planet at this time.
But there is more recent evidence of this star dust – much more recent.
It was found in the snow of Antarctica; according to the evidence, it must have decreased in the last 20 years.
And a few years ago, scientists announced that iron-60 was detected in space around the Earth, measured over a period of 17 years by NASA’s Space-based Advanced Composition Explorer.
In 2020, Wallner found several things in five deep sediment samples from two locations dating back 33,000 years. And the amounts of iron-60 in the samples are quite consistent over the entire period of time. But this finding actually raises more questions than answers.
You 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 the explosion of stars, then it is reasonable to expect the Earth to dust with a very light rain of iron-60. Here is what the detection of Antarctica suggested; and that’s what Wallner and his team tried to validate by examining ocean sediments.
But if the Local Interstellar Cloud is the source of iron-60, there should have been a sharp rise when the Solar System entered the cloud – which, according to team data, has occurred over the past 33,000 years. At the very least, the oldest sample should have had significantly lower levels of iron-60, but it did not.
It is possible, the researchers note in their work, that the Local Interstellar Cloud and the remnants of supernovae are coincident, rather than a single structure, the remnants remaining in the interstellar environment of supernovae that occurred millions of years ago. This would suggest that the Local Interstellar Cloud is not a weak supernova remnant.
“There is recent work to suggest that iron-60 trapped in dust particles could bounce around the interstellar medium,” Wallner said last year.
“So iron-60 could come from even older supernova explosions, and what we’re measuring is a kind of echo.”
Researchers note that the best way to find out is to look for more iron-60s, covering the gap between 40,000 years ago and about a million years ago.
If the abundance of iron-60 increases further back in time, this would suggest ancient supernovae.
However, a greater abundance more recently would suggest that the Local Interstellar Cloud is the source of iron-60.
The research was published in The works of the National Academy of Sciences.
A version of this article was originally published in August 2020.