For years, scientists have struggled to explain their measurements and observations of asteroid Psyche. And one of the explanations thus obtained is now being brushed aside in a new study.

A steel wool asteroid: it may sound like science fiction. But some researchers really think that the asteroid Psyche has a steel wool-like interior. It could well explain the at first sight difficult to reconcile observations and measurements we have of the asteroid. But still it is, researchers now write in the magazine Geophysical Research Lettersvery unlikely.

Iron

The asteroid Psyche has been stirring things up for quite some time. The object – which is located in the asteroid belt between Mars and Jupiter – is giving off rather mixed signals. For example, researchers deduce from the light reflected by the asteroid that the surface consists almost entirely of metal. And that led scientists to suspect that we are dealing here with the remnants of a planetesimal that acquired a rocky crust and mantle during the formation of our solar system, but which was lost again by colliding with other planetary building blocks. In that scenario, Psyche would actually be an exposed iron core. And that is very interesting, because Psyche would then enable us to study such a core – which is also hidden in the heart of our earth, for example – once expanded and unimpeded by an overlying mantle and crust.

Porous

There is one problem, however: measurements of Psyche’s mass and density contradict the idea that we are dealing here with a massive iron ball. For example, researchers can deduce from the gravity that Psyche exerts on neighboring objects that it has a much lower density than you would expect from a 225 kilometer wide iron ball. It means that if Psyche is – as the surface seems to suggest – such a large iron ball, it must be very porous. A bit like steel wool: part metal, part empty space.

Unlikely

Although it could be based on the observations and measurements, scientists are now sweeping the steel wool theory off the table. Conditions in the young solar system weren’t conducive to forging steel wool-like asteroids, they say. “What we wanted to do in this study was see if it was possible for an iron object the size of Psyche to maintain that nearly 50 percent porosity,” said study researcher Fiona Nichols-Fleming. “And we found that was very unlikely.”

Cooling down

For the study, the researchers created a computer model that described how a large and porous iron object would evolve over time under different conditions. The simulations showed that an iron object the size of Psyche could only maintain a steel wool-like porosity if it cooled rapidly shortly after its creation. Without this cooling, the iron remained malleable in such a way that Psyche could pull it towards herself with her gravity, eliminating the empty spaces between them and thus giving the object a much higher density and lower porosity. But based on what we currently know about the conditions in the young solar system, it is highly unlikely that an object the size of Psyche could cool sufficiently in a short period of time.

Impact?

The study also dismisses the idea that Psyche became more porous some time after its creation – for example due to a large impact. Because such a drastic event would also have heated up the asteroid considerably. And in such a scenario too, the temperatures were so high that the iron became malleable and any extra porosity obtained by the impact was soon undone by Psyche’s own gravity.

sci fi?
If Psyche doesn’t have a steel wool-like structure, does that mean that steel wool asteroids are really science fiction? Certainly not, the researchers write in their research article. Smaller iron asteroids may be able to cool quickly enough to maintain high porosity.

iron volcanoes

But if Psyche doesn’t have a steel wool-like interior, how are we supposed to explain the observations and measurements we have of the asteroid? Based on their models, the researchers suspect that the asteroid has a hidden rocky component. That could also explain the relatively small density. However, it does raise the question of why the surface appears so metal-rich when viewed from Earth. The researchers do have ideas about that. For example, it could be that Psyche in principle had an iron core with a thin rocky mantle on top, which is then covered with a metal layer again by iron-spitting volcanoes.

More research is needed to uncover the true nature of Psyche. Fortunately, there is good news in that regard: NASA is sending a probe to the strange asteroid later this year. Also called Psyche for convenience, this probe should arrive at Psyche in early 2026 and eventually reveal how the asteroid works.