Psyche, the iron giant of asteroids, may contain less iron than researchers thought

Asteroid 16 Psyche, which NASA plans to visit with a spacecraft in 2026, may be less heavy metal and more hard rock than scientists assumed, according to a new study by university researchers Brown and Purdue.

Psyche, which orbits the sun in the asteroid belt between Mars and Jupiter, is the largest of the M-type asteroids, composed mostly of iron and nickel, as opposed to the silicate rocks that make up most other asteroids. But seen from Earth, Psyche sends mixed signals about its composition.

The light it reflects tells scientists that the surface is indeed mostly metallic. This led to conjecture that Psyche could be the exposed iron core of a primordial planetary body – whose rocky crust and mantle were blown away by an ancient collision. However, measurements of Psyche’s mass and density tell a different story. The way its gravity pulls on nearby bodies suggests that Psyche is much less dense than a giant piece of iron should be. So if Psyche is indeed all metal, it should be very porous – kind of like a giant ball of steel wool with nearly equal parts empty space and solid metal.

“What we wanted to do with this study was see if it was possible for a body of iron the size of Psyche to maintain this nearly 50% porosity,” said Fiona Nichols-Fleming, holder of a Ph.D. student at Brown and senior author of the study. “We found that to be very unlikely.”

For the study published in Geophysical Research Letters, Nichols-Fleming worked with Alex Evans, an assistant professor at Brown, and Purdue professors Brandon Johnson and Michael Sori. The team created a computer model, based on the known thermal properties of metallic iron, to estimate how the porosity of a large body of iron would change over time.

The model shows that to remain highly porous, Psyche’s internal temperature would have to drop below 800 Kelvin very soon after its formation. At temperatures above this, iron would have been so malleable that Psyche’s own gravity would have collapsed most of the porous space into its mass. Based on what is known of conditions in the early solar system, the researchers say, it’s extremely unlikely that a body the size of Psyche – about 140 miles in diameter – could have cooled so quickly.

Additionally, any event that might have added porosity to Psyche after it formed—a massive impact, for example—would likely also have warmed Psyche above 800 K. Thus, any newly introduced porosity would be unlikely to last.

Taken together, the results suggest that Psyche is probably not an all-iron porous body, the researchers conclude. More likely, it harbors a hidden rocky component that lowers its density. But if Psyche has a rocky component, why does its surface look so metallic when viewed from Earth? There are few possible explanations, say the researchers.

One such possibility is ferrovolcanism – iron-spewing volcanoes. It’s possible, the researchers say, 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 to the surface, putting an iron coating on its rocky mantle. Previous research by Johnson and Evans has shown that ferrovolcanism is possible on a body like Psyche.

Either way, scientists will soon have a much clearer picture of this mysterious asteroid. Later this year, NASA plans to launch a spacecraft that will meet Psyche after a four-year journey through the asteroid belt.

“The mission is exciting because Psyche is such a bizarre and mysterious thing,” Nichols-Fleming said. “So all the mission will find will be really important new data points for the solar system.”

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