Radioactive dust deep beneath the ocean waves suggests Earth is moving through a massive cloud left behind by an exploded star.
Constantly, for the last 33,000 years, space has littered the Earth with a rare isotope of iron forged in supernovae.
It is not the first time that the isotope, known as iron-60, has dusted our planet. But it does add to a growing body of evidence that such dust formation is underway – 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 it decays completely after 15 million years – so any samples found here on Earth must have been deposited from elsewhere as there is no way to survive iron-60 after the formation of the planet 4.6 billion years ago.
And deposits have been found. Nuclear physicist Anton Wallner of the Australian National University previously dated the seafloor deposits to 2.6 million and 6 million years ago, suggesting that debris from supernovae had descended on our planet at that time.
But there is more recent evidence of this stardust – much more recent.
It has been found in the Antarctic snow; according to the evidence, it must have fallen in the past 20 years.
And a few years ago, scientists announced that iron-60 had been detected in space around Earth, measured over a 17-year period by NASA’s space-based Advanced Composition Explorer.
In 2020, Wallner found more of the stuff, in five deep-sea sediment samples from two sites dating back 33,000 years. And the amounts of iron-60 in the samples are fairly consistent over the entire time period. But this finding actually raises more questions than it answers.
Earth, you see, is currently moving through an area called the Local Interstellar Cloud, which is made up of gas, dust and plasma.
If this cloud was created by exploding stars, it is reasonably expected to sprinkle the Earth with a very faint iron-60 rain. This is what the Antarctic detection suggested; and this is what Wallner and his team tried to validate by examining the ocean sediments.
But if the Local Interstellar Cloud is the source of the iron-60, then there must have been a surge as the solar system entered the cloud – which, according to the team’s data, likely occurred in the past 33,000 years. The oldest sample should have had at least significantly lower levels of iron-60, but it wasn’t.
It is possible, the researchers note in their paper, that the Local Interstellar Cloud and the supernova debris coincide, instead of one structure, leaving the debris behind in the interstellar medium of supernovae that took place millions of years ago. That would suggest that the Local Interstellar Cloud is not a faint supernova remnant.
“There are recent articles suggesting that iron-60 trapped in dust particles could bounce around in the interstellar medium,” Wallner said last year.
“So the Iron-60 could come from even older supernova explosions, and what we’re measuring is some sort of echo.”
The best way to find out, the researchers note, is to look for more iron-60, which bridges the gap between 40,000 years ago and about a million years ago.
If the abundance of iron-60 increases further back in time, it would indicate ancient supernovae.
However, greater abundance more recently would suggest that the Local Interstellar Cloud is the source of the iron-60.
The research is published in the Proceedings of the National Academy of Sciences.
A version of this article was originally published in August 2020.