Metallic asteroids are the cooled cores of disrupted planetesimals. They originated early in the history of our Solar System when planets were beginning to form. University of California Santa Cruz planetary researchers Jacob Abrahams and Professor Francis Nimmo think that as the metal cooled and solidified, volcanoes spewing liquid iron could have erupted through a solid iron crust onto the surface of the metallic asteroid.

“Three major types of crustal material are observed in the Solar System,” Abrahams and Professor Nimmo said.

“The most familiar, silicate crust, is found on the terrestrial planets and their moons, most asteroids, and on Jupiter’s moon Io.”

“The other common type of crust is formed from ices, primarily water ice, on the surfaces of most outer solar system moons, some asteroids, and most Kuiper belt objects.”

In addition to ice and silicates, a less common third type of crust is present in the Solar System: metallic core material left behind following collisional disruption of the mantle of a planetary body.

These metallic objects have been detected in the asteroid belt and contribute substantially to the meteorite record. However, our understanding of their surfaces is very limited — the first detailed images of a metallic asteroid will come from NASA’s Psyche spacecraft, which is scheduled to launch in 2022.

“A protoplanet or planetesimal involved in a catastrophic collision could be stripped of its rocky outer layers, exposing a molten, iron-rich core. In the cold of space, this blob of liquid metal would quickly begin to cool and solidify,” Professor Nimmo said.

“In some cases it would crystallize from the center out and wouldn’t have volcanism, but some would crystallize from the top down, so you’d get a solid sheet of metal on the surface with liquid metal underneath.”

“As for what the iron volcanoes would look like, it depends on the composition of the melt,” Abrahams said.

“If it’s mostly pure iron, then you would have eruptions of low-viscosity surface flows spreading out in thin sheets, so nothing like the thick, viscous lava flows you see on Hawaii.”

“At the other extreme, if there are light elements mixed in and gases that expand rapidly, you could have explosive volcanism that might leave pits in the surface.”

The best opportunity to find evidence of ferrovolcanism on metallic asteroids might actually come from studying iron meteorites already in collections on Earth.

“There are lots of these metallic meteorites, and now that we know what we’re looking for, we might find evidence of volcanism in them,” Professor Nimmo said.

“If material got erupted onto the surface, it would cool very fast, which would be reflected in the composition of the meteorite. And it might have holes in it left by escaping gas.”

The study was published in the journal Geophysical Research Letters.

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Jacob N.H. Abrahams & Francis Nimmo. Ferrovolcanism: Iron Volcanism on Metallic Asteroids. Geophysical Research Letters, published online April 8, 2019; doi: 10.1029/2019GL082542