Radioactive material deep beneath the ocean waves indicates that the earth is moving through a massive cloud left behind by an exploding star.
For the past 33,000 years, space has been constantly sowing the earth with a rare iron isotope forged in supernovae.
This is not the first time that the isotope known as iron-60 has dusted our planet. But it does contribute to a growing body of evidence that such dust continues – we are still moving through an interstellar cloud of dust that could have formed 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 expires completely after 15 million years – so any specimens found here on Earth must be deposited from elsewhere, as there was no way to make an iron-60 of the formation of the planet 4.6 billion years ago.
And deposits were found. Nuclear physicist Anton Wallner of the Australian National University dated the seabed precipitation earlier to 2.6 million and 6 million years ago, suggesting that debris from supernovae was raining down on our planet at this time.
But there is more recent evidence of this stellar substance – much more recently.
It was found in the Antarctic snow; according to the evidence, it must have fallen over the past 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 more of those things, in five samples of deep-sea sediments from two sites dating back to 33,000 years ago. And the amounts of iron-60 in the samples are fairly consistent throughout the period. But this finding actually raises more questions than it answers.
The earth is moving, you see, currently through a region called the local interstellar cloud, which consists of gas, dust and plasma.
If this cloud was created by exploding stars, it is reasonable to expect that it will dust the earth with a very faint rain of iron-60. This is what the Antarctic exploration suggested; and this is what Wallner and his team tried to validate by exploring the ocean sediments.
But if the local interstellar cloud is the source of the iron-60, there must have been a sharp increase when the solar system entered the cloud – which, according to the team’s data, probably took place within the last 33,000 years. At the very least, the oldest sample should have had significantly lower levels of iron-60, but it was not.
The researchers note that it is possible that the local interstellar cloud and the supernova debris coincide, rather than one structure, with the debris left behind in the interstellar medium of supernovae that occurred millions of years ago. This suggests that the local interstellar cloud is not a faint supernova remnant.
“There are recent newspapers that suggest that iron-60 trapped in dust particles can reflect in the interstellar medium,” Wallner said last year.
“The iron-60 can therefore arise from even older supernova explosions, and what we measure is a kind of echo.”
The best way to find out, the researchers note, is to look for more iron-60, which covers the gap between 40,000 years and about a million years ago.
If the iron-60 abundance grows longer in time, it would represent ancient supernovae.
However, more abundance recently indicated that the local interstellar cloud is the source of the iron-60.
The research was published in the Proceedings of the National Academy of Sciences.
A version of this article was originally published in August 2020.