In a continuous manner, for the past 33,000 years, space has been sowing Earth with a rare isotope of iron acting as a supernova. It is not the first time that the isotope, called iron-60, has dusted Earth, but it does add to an increasing collection of evidence that such dusting is continuing, and that we are still traveling through an interstellar cloud of dust that could have come from a supernova that exploded millions of years ago.
Iron-60 Has a Half-Life
Iron-60 has been the topic of numerous research throughout the years. It has a half-life of 2.6 million years, meaning it entirely decays after 15 million years, so any traces found on our planet must have been deposited from elsewhere because there is no way any iron-60 could have kept living after Earth formed about 4.6 billion years ago.
Nuclear physicist Anton Wallner of the Australian National University previously dated seabed deposits to between 2.6 million and 6 million years ago, which implies that debris from supernovae ended up on our planet back then. However, there is a more recent proof of this stardust, found in the Antarctic snow, and according to the evidence, it had to have fallen in the last 20 years.
Now Wallner has discovered more of this stardust, in five samples of deep-sea sediments from two different places dating back to 33,000 years ago. The amounts of iron-60 in the findings are rather consistent over the whole time period. However, this discovery actually births more questions than it answers.
To begin with, Earth is now moving through an area known as the Local Interstellar Cloud, comprised of gas, dust, and plasma. If this cloud was generated by exploding stars, then it is logical to expect that it is raining Earth with a very slight rain of iron-60. This is what the Antarctic finding suggested, and this is what Wallner and his team were trying to confirm by analyzing the ocean sediments.
The Local Interstellar Cloud – a Remnant of Supernovae?
On the other hand, if the Local Interstellar Cloud is the origin of the iron-60, there should have been an acute increase in it when the Solar System entered the cloud, which as per the team’s data, has probably taken place in the last 33,000 years. To say the least, the oldest sample should have had a lot lower levels of iron-60, but it did not.
It is feasible, however, that the Local Interstellar Cloud and the supernova remnants are coincident; instead of one structure, with the debris remaining in the interstellar medium from a supernova that happened millions of years ago. That would imply that the Local Interstellar Cloud is not dim supernova debris.
“There are recent papers that suggest iron-60 trapped in dust particles might bounce around in the interstellar medium,” Wallner said. “So the iron-60 could originate from even older supernovae explosions, and what we measure is some kind of echo.”
The best method to use in order to find out is to look for more iron-60, bridging the gap between 40,000 years ago and around a million years ago
The research has been published in the Proceedings of the National Academy of Sciences.