Observations of an asteroid group indicate that they may have metal-rich surfaces, but those asteroids with measured densities are about half as dense as expected for a metal body. These low densities require either that the asteroids either have such high porosities that half of their volume is empty space or that the asteroids are a mix of metal and other lower density materials, such as silicate rock. The asteroid 16 Psyche, the largest of this group, is of particular interest due to its recent selection for study by a dedicated NASA mission in 2026. To understand if indeed a high porosity metal body is possible, a team of planetary researchers from Brown University and Purdue University modeled the removal of porosity over time for a range of temperatures, initial porosities, and masses of the body. Their results indicate that all iron asteroids must cool to and remain at low temperatures prior to a porosity-adding event in order to retain porosity on timescales longer than a few million years. Accordingly, a pure iron Psyche would not have sufficiently cooled to retain high porosities when large porosity adding events are most likely to have occurred in the early Solar System. Hence, Psyche is not likely to be a pure iron core.
An artist’s concept of the asteroid Psyche, which lies in the main asteroid belt between Mars and Jupiter. Image credit: NASA / JPL-Caltech / ASU.
Psyche, a metal asteroid about 226 km (140 miles) in diameter, is one of the most intriguing targets in the main asteroid belt.
The asteroid orbits the Sun between the orbits of Mars and Jupiter at a distance ranging from 378 to 497 million km (235-309 million miles) from the Sun.
It takes about five Earth years to complete one orbit of the Sun, but only a bit over four hours to rotate once on its axis.
Psyche is the largest of the M-type asteroids, which are composed chiefly of iron and nickel as opposed to the silicate rocks that make up most other asteroids.
But when viewed from Earth, Psyche sends mixed signals about its composition.
The light it reflects tells planetary scientists that the surface is indeed mostly metal. That has led to conjecture that Psyche may be the exposed iron core of a primordial planetary body — one whose rocky crust and mantle were blasted away by an ancient collision.
However, measurements of Psyche’s mass and density tell a different story.
The way its gravity tugs on neighboring bodies suggests that Psyche is far less dense than a giant hunk of iron should be.
So if Psyche is indeed all metal, it would have to be highly porous — a bit like a giant ball of steel wool with nearly equal parts void space and solid metal.
“What we wanted to do with this study was see whether it was possible for an iron body the size of Psyche to maintain that near-50% porosity. We found that it’s very unlikely,” said lead author Fiona Nichols-Fleming, a Ph.D. student in the Department of Earth, Environmental and Planetary Sciences at Brown University.
As a metallic asteroid such as Psyche cooled and solidified, iron volcanoes may have erupted onto its surface. Image credit: Elena Hartley.
In the study, Nichols-Fleming and her colleagues created a computer model, based on known thermal properties of metallic iron, to estimate how the porosity of a large iron body would evolve over time.
The model shows that to remain highly porous, Psyche’s internal temperature would have to cool below 800 K very shortly after its formation.
At temperatures above that, iron would have been so malleable that Psyche’s own gravity would have collapsed most of the pore space within its bulk.
“Based on what is known about conditions in the early Solar System, it’s extremely unlikely that a body of Psyche’s size could have cooled so quickly,” the authors said.
“In addition, any event that may have added porosity to Psyche after its formation — a massive impact, for example — would likely have also heated Psyche back up above 800 K.”
“So any newly introduced porosity would have been unlikely to last.”
Taken together, the results suggest that Psyche probably isn’t a porous, all-iron body. More likely, it’s harboring a hidden rocky component that drives its density down.
But if Psyche does have a rocky component, why does its surface look so metallic when viewed from Earth?
“There are few possible explanations,” the scientists said.
“One of those possibilities is ferrovolcanism — iron-spewing volcanoes. It’s possible that Psyche is actually a differentiated body with a rocky mantle and an iron core.”
“But widespread ferrovolcanic activity may have brought large amounts of Psyche’s core up to the surface, putting an iron coating atop its rocky mantle.”
The results appear in the journal Geophysical Research Letters: Planets.
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Fiona Nichols-Fleming et al. Porosity Evolution in Metallic Asteroids: Implications for the Origin and Thermal History of Asteroid 16 Psyche. Geophysical Research Letters: Planets, published online February 1, 2022; doi: 10.1029/2021JE007063
