Iron-60 is a radioactive isotope of iron — it has 34 neutrons in its nucleus, whereas iron-56, the most common form of iron, has only 30. Iron-56 is stable, so once created it sticks around essentially forever. Iron-60, however, radioactively decays over time, emitting an electron from its nucleus to convert a neutron into a proton, leaving behind an atom of cobalt-60. It has a half-life of about 2.6 million years; if you have a pile of iron-60 then in 2.6 million years half of it will have decayed to cobalt.

That’s pretty fast, cosmically speaking, and there are two key things you need to know about iron-60^*. One is that it’s found in seabed sediments and ice cores, and the other is it is really only made one way: in supernovae. Exploding stars.

So the fact that we find it on Earth means there were once stars that exploded close enough that we have accumulated a detectable amount of iron-60, and that this happened recently enough that it didn’t all decay away into cobalt. I’ve written about this a few times; once in a Scientific American article and again back in my SYFY days. The basic idea is that massive stars in a nearby young cluster explode, and the debris sweeps over our planet, leaving behind trace amounts of iron-60.

The ratio of iron-60 found versus cobalt-60 can give an idea of how long ago this happened, but what didn’t occur to me is that this material could build up continuously over time. It did occur to some scientists, so they looked for it is seabed sediments and Antarctic ice cores.

They found the level of iron-60 varied in the samples on a scale of thousands to tens of thousands of years; for example, 40,000 and 80,000 years ago the amount of iron-60 found is less than there is today (that’s too short a time for radioactive decay to make a big dent) [link to journal paper].

This material should be pretty evenly distributed through space, so the variations seen must have some other source. The scientists posit it’s coming from the Local Bubble. There is a thin gas between the stars called the interstellar medium, and the sun happens to sit in the middle of a literal bubble in that gas carved out by the expanding debris from supernovae that exploded a dozen million years ago or so. However, the bubble isn’t completely evacuated; there are small density variations — cloudlets — inside it. The scientists think the iron-60 is stored up in these cloudlets as they’re hit by the supernova debris, and this stuff accumulates on Earth as we plow through them.

That’s pretty cool, if true. It would explain the variation — sometimes we move through a cloudlet, and sometimes we move through clearer space — and the timescales involved. The problem I see is that the number of iron-60 atoms is so small that it’s hard to get a really detailed analysis of exactly how much we hit, so it’s hard to get a good representation of the density of any given cloudlet, how many are out there, and so on. We know the clouds are out there, but it’s not clear what formed them (maybe the supernovae material plowing through interstellar material and sweeping them up…?), so it may be possible to learn more about them by studying the iron-60 accumulation on Earth.

It’s interesting that there’s a still a lot we don’t know about the nearby volume of space around the sun. We’re in the middle of it so it’s hard to study, but, happily, it does leave some clues we can use to figure things out.

^* Well, three things, I suppose, if the third is there’s nowhere near enough of this stuff to be dangerous to us, so don’t fret. They were counting individual atoms in the samples, so it’s not a huge amount.

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