Iron Magma Could Explain Psyche’s Density Puzzle

The metallic asteroid Psyche has mystified scientists because it is less dense than it should be. Now, a new theory by researchers including scientists at the University of Arizona could explain Psyche’s low density and metallic surface.
Psyche, the largest known metallic asteroid in the solar system, is located in the asteroid belt between Mars and Jupiter. Psyche appears to be composed largely of iron and nickel, rather than rocky rubble, like most asteroids, yet its density is estimated to be only about half that of an iron meteorite.

Metal-rich asteroids are thought to have 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,'” said Brandon Johnson, an associate professor of Earth and atmospheric sciences at Purdue University.

“This is a very new idea as of 2019. It’s a kind of volcanism where the magma is liquid metal instead of liquid rock,” said Michael Sori, an associate staff scientist at the UA Lunar and Planetary Laboratory.

The theory is detailed in a paper that was recently published in Nature Astronomy. The paper was co-authored by Johnson, Sori and Alexander Evans, an assistant professor of Earth, environmental and planetary sciences at Brown University.

“The first half of the paper is really theoretical. We show that the process is viable,” Sori said. “Then the second half, we give two examples of things we think it can explain. One is meteorites and the other is Psyche.”

Meteorites called pallasites are thought to be a mixture of core and mantle material, possibly blended together by ferrovolcanism. The pockets of liquid metal mixed with sulfur are less dense than surrounding solid material, producing an “excess pressure” and 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 said.

An upcoming NASA space mission to Psyche, planned for 2022, 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. The researchers theorize that the asteroid might consist of two layers, where a metal core is surrounded by a lower density mantle of rocky material.

“Ferrovolcanism may have transported core material to the surface, causing the radar detections of metal,” Johnson said.

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 the concept of ferrovolcanism is based on mathematical models. Sori’s role was applying these models to the asteroid Psyche.

Reference: “Ferrovolcanism on Metal Worlds and the Origin of Pallasites,” Brandon C. Johnson, Michael M. Sori & Alexander J. Evans, 2019 Sep. 16, Nature Astronomy [https://www.nature.com/articles/s41550-019-0885-x, preprint: https://arxiv.org/abs/1909.07451].