The metallic asteroid Psyche has mystified scientists because it is less dense than it should be, given its iron-nickel composition. A new theory offers an explanation.

Unlike most asteroids, Psyche appears composed largely of iron and nickel instead of rocky rubble. Researchers think metal-rich asteroids formed when primordial planetesimals collided, stripping away much of the outer material and leaving behind the inner metallic cores, which then cooled and solidified from the outside-in.

During this cooling process, an alloy of residual melted pockets of iron, nickel, and lighter elements like sulfur, might have flowed to the surface through fluid-filled cracks called dikes, coating a topmost, rocky layer.

“We refer to these processes collectively as ‘ferrovolcanism,'” says Brandon C. Johnson, an associate professor of Earth and atmospheric sciences at Purdue University.

Researchers believe meteorites called pallasites are a mixture of core and mantle material, which ferrovolcanism possibly blends together. The pockets of liquid metal, mixed with sulfur, are less dense than surrounding solid material, producing an “excess pressure,” possibly causing the propagation of dikes and allowing ferrovolcanism to occur.

The researchers determined how far these dikes would have to propagate to make volcanism possible.

“Our calculations suggest that ferrovolcanic eruptions may be possible for small, metal-rich bodies, especially for sulfur-rich melts and bodies with mantles thinner than about 35 kilometers or bodies where the mantle has been locally thinned by large impact craters,” Johnson says.

An upcoming NASA space mission to Psyche will help scientists test this theory. The ferrovolcanic eruptions might explain Psyche’s low density, which exists despite radar and other scientific evidence of a metallic surface composition.

Psyche, the largest known metallic asteroid in the solar system, is located in the asteroid belt between Mars and Jupiter. Its density is estimated at only about half that of an iron meteorite.

The researchers theorize that the asteroid might consist of two layers, with a lower density mantle of rocky material surrounding a metal core. “Ferrovolcanism may have transported core material to the surface, causing the radar detections of metal,” Johnson says.

The research is ongoing, with future work harnessing more sophisticated modeling to study how ferrovolcanism might occur, and possibly probing Psyche’s evolution.

No spacecraft has yet to visit a metallic asteroid, and researchers base the concept of ferrovolcanism on mathematical models. NASA plans to launch the space probe in 2022. The mission could allow scientists to confirm the theory and answer questions about the role of metallic asteroids in the evolution of the solar system.

The researchers detail the theory in Nature Astronomy. Additional coauthors are from the University of Arizona’s Lunar and Planetary Laboratory and Brown University.

Source: Purdue University

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