Iron is one of the most abundant elements in the Universe, along with lighter elements like hydrogen, oxygen, and carbon. Out in interstellar space, there should be abundant quantities of iron in its gaseous form. So why, when astrophysicist look out into space, do they see so little of it?
First of all, there’s a reason that iron is so plentiful, and it’s related to a thing in astrophysics called the iron peak.
In our Universe, elements other than hydrogen and helium are created by nucleosynthesis in stars. (Hydrogen, helium, and some lithium and beryllium were created in Big Bang nucleosynthesis.) But the elements aren’t created in equal amounts. There’s an image that helps show this.
The reason for the iron peak has to do with the energy required for nuclear fusion and for nuclear fission.
For the elements lighter than iron, on its left, fusion releases energy and fission consumes it. For elements heavier than iron, on its right, the reverse is true: its fusion that consumes energy, and fission that releases it. It’s because of what’s called binding energy in atomic physics.
That makes sense if you think of stars and atomic energy. We use fission to generate energy in nuclear power plants with uranium, which is much heavier than iron. Stars create energy with fusion, using hydrogen, which is much lighter than iron.
In the ordinary life of a star, elements up to and including iron are created by nucleosynthesis. If you want elements heavier than iron, you have to wait for a supernova to happen, and for the resulting supernova nucleosynthesis. Since supernovae are rare, the heavier elements are rarer than the light elements.
It’s possible to spend an extraordinary amount of time going down the nuclear physics rabbit hole, and if you do, you’ll encounter an enormous amount of detail. But basically, for the reasons above, iron is relatively abundant in our Universe. It’s stable, and it requires an enormous amount of energy to fuse iron into anything heavier.
We know that iron in solid form exists in the cores and crusts of planets like our own. And we also know that it’s common in gaseous form in stars like the Sun. But the thing is, it should be common in interstellar environments in its gaseous form, but we just can’t see it.
Since we know it has to be there, the implication is that it’s wrapped up in some other process or solid form or molecular state. And even though scientists have been looking for decades, and even though it should be the fourth-most abundant element in the solar abundance pattern, they haven’t found it.
Now a team of cosmochemists from Arizona State University say they’ve solved the mystery of the missing iron. They say that the iron has been hiding in plain sight, in combination with carbon molecules in things called pseudocarbynes. And pseudocarbynes are tricky to see because the spectra are identical to other carbon molecules which are abundant in space.
The team of scientists includes lead author Pilarasetty Tarakeshwar, research associate professor in ASU’s School of Molecular Sciences. The other two members are Peter Buseck and Frank Timmes, both in ASU’s School of Earth and Space Exploration. Their paper is titled “On the Structure, Magnetic Properties, and Infrared Spectra of Iron Pseudocarbynes in the Interstellar Medium” and is published in the Astrophysical Journal.
“We are proposing a new class of molecules that are likely to be widespread in the interstellar medium,” said Tarakeshwar in a press release.
The team focused in on gaseous iron, and how only a few atoms of it might join with carbon atoms. The iron would combine with the carbon chains, and the resulting molecules would contain both elements.
They also looked at recent evidence of cluster of iron atoms in stardust and meteorites. Out in interstellar space, where it is extremely cold, these iron atoms act kind of like “condensation nuclei” for carbon. Varied lengths of carbon chains would stick to them, and that process would produce different molecules than those produced with gaseous iron.
We couldn’t see the iron in these molecules, because they masquerade as carbon molecules without iron.
In a press release, Tarakeshwar said, “We calculated what the spectra of these molecules would look like, and we found that they have spectroscopic signatures nearly identical to carbon-chain molecules without any iron.” He added that because of this, “Previous astrophysical observations could have overlooked these carbon-plus-iron molecules.”
Not only have they found the “missing” iron, they may have solved another long-lived mystery: the abundance of unstable carbon chain molecules in space.
Carbon chains that have more than nine carbon atoms are unstable. But when scientists look out into space, they find carbon chains with more than nine carbon atoms. It’s always been a mystery how nature was able to form these unstable chains.
As it turns out, it’s the iron that gives these carbon chains their stability. “Longer carbon chains are stabilized by the addition of iron clusters,” said Buseck.
Not only that, but this finding opens a new pathway for building more complex molecules in space, such as polyaromatic hydrocarbons, of which naphthalene is a familiar example, being the main ingredient in mothballs.
Said Timmes, “Our work provides new insights into bridging the yawning gap between molecules containing nine or fewer carbon atoms and complex molecules such as C60 buckminsterfullerene, better known as ‘buckyballs.'”
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