Like cosmic steel foundries, metallic asteroids in the early days of the solar system may have had volcanoes spewing molten iron instead of lava. Based on models of how metallic asteroids form and cool, a new study by planetary scientists at UC Santa Cruz opens the window on such a spectacular possibility.

Asteroids tend not to have a reputation for being dynamic, but when the solar system was forming, things were very different. For a time, it was a very violent place as comets, asteroids, and planet-sized objects whizzed about, colliding with one another with a force that's hard to imagine.

One result of this is metallic asteroids. Made up largely of nickel and iron, these asteroids are believed to have been created when protoplanets formed into molten masses where metals sank to the core and lighter elements rose like slag to form a crust. These were then struck by other planet-sized objects, smashing them apart and leaving behind blobs of molten iron with little or no crusts.

In order to better understand this process in the run up to NASA's planned 2022 mission to the metallic asteroid Psyche, Francis Nimmo, professor of Earth and planetary sciences, asked graduate student Jacob Abrahams to develop some simple models of how such molten asteroids cool and solidify. However, these models presented some surprises.

"One day he turned to me and said, 'I think these things are going to erupt,'" says Nimmo. "I'd never thought about it before, but it makes sense because you have a buoyant liquid beneath a dense crust, so the liquid wants to come up to the top. 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."

However, these volcanoes would be a bit different from their terrestrial counterparts, not only because such ferrovolcanic asteroids are small and have a short active life, but their eruptions were different.

"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," says Abrahams. "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 researchers say that it's difficult to find traces of past volcanic activity because the asteroids have been subjected to billions of years of wear. Though the activity could have produced color variations and volcanic vents, these are probably not very prominent today. Because of this, the most likely place to find evidence of ferrovolcanism may be from iron meteorites found 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," says Nimmo. "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 research was published in Geophysical Research Letters.