Radioactive isotopes found in ocean sediments show that Earth has been traveling through a cloud of fallout from an ancient supernova for the past 33,000 years. A team of researchers led by Prof. Anton Wallner, a nuclear physicist at the Australian National University, found traces of iron-60 on the sea bed that could only have been of extraterrestrial origin.
Iron is so common an element that we tend to take it for granted, but it's actually quite a remarkable metal, if for no other reason than how it came to be.
As anyone who's taken school science can tell you, the two forms of nuclear energy work in what seems like the opposite direction. Fission releases energy by breaking down heavy atoms into lighter ones, and fusion releases energy by combining two light atoms into one heavier one.
The question is, where do these two processes meet and what happens when they do? The answer is they meet in the iron atom, where splitting it or fusing becomes much more difficult. In fact, iron atoms are so stubborn from an energy point of view that the only way to produce some isotopes in nature (in any quantity) is inside the heart of an exploding star – a supernova.
This is where all the iron on Earth came from, but there's a bit of a puzzle there. One radioactive isotope of iron, iron-60, has a half-life of 2.6 million years. This means that after Earth formed 4.6 billion years ago, all the iron-60 should have vanished in about 15 million years. The annoying thing is that scientists keep finding iron-60 on Earth. Where did it come from?
According to Wallner, the answer is that supernovae leave behind gigantic clouds of fallout, like the Crab Nebula, after they explode and Earth regularly drifts through these from time to time. By collecting sediment samples at two ocean locations, Wallner and his team used a mass spectrograph at the ANU Heavy Ion Accelerator Facility (HIAF) which shows that sediment cores dating back as far as 33,000 years ago show traces of iron-60.
Previous studies by Wallner had already found iron-60 traces going back to 2.6 million years, and possibly another at 6 million years, but there are indications that Earth has been traveling through a dense cloud of gas and dust called the Local Interstellar Cloud, (LIC). It may be from a recent supernova, and Wallner suspected that it contained iron-60.
In fact, the sediment indicates that iron-60 traces are spread throughout the past 33,000 years. This suggests that the isotope is evenly distributed in the LIC. It also raises the question of where the cloud came from. Was it from a relatively young supernova or was some other mechanism at work?
"There are recent papers that suggest iron-60 trapped in dust particles might bounce around in the interstellar medium," Prof. Wallner said. "So the iron-60 could originate from even older supernovae explosions, and what we measure is some kind of echo. More data is required to resolve these details."
The research was published in the journal PNAS.
Source: Australian National University