Iron volcanoes may have erupted on asteroids of metal

NASA’s mission to the asteroid Psyche will search for evidence of previous eruptions and signs of ‘ferrovolcanism’ on the iron-nickel meteorite.

As a metallic asteroid such as Psyche cooled and solidified, iron volcanoes may have erupted onto its surface (Elena Hartley)

Metallic asteroids are suspected to have begun their existence as blobs of molten iron floating in space, but scientists now think that as the metal cooled and solidified, volcanoes spewing liquid iron could have erupted through a solid iron crust onto the surfaces of these asteroids.

This new theory resulted from an analysis by planetary scientists at UC Santa Cruz whose investigation was prompted in part by NASA’s plans to launch a probe to Psyche — the largest metallic asteroid in the solar system.

Francis Nimmo, professor of Earth and planetary sciences, says he was interested in the composition of metallic asteroids indicated by analyses of iron meteorites. This prompted him to set graduate student Jacob Abrahams to work on the development of some simple models demonstrating how the asteroids cooled and solidified.

Nimmo says: “One day he turned to me and said, ‘I think these things are going to erupt.

“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.”

The findings are discussed in a paper due to published in Geophysical Research Letters.

Metallic asteroids originated early in the history of the solar system when planets were beginning to form. 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.

Nimmo explains: “In some cases, it would crystallize from the centre 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.”

Abrahams adds that the appearance of the volcanoes would depend on the composition of the melt at this stage. He explains: “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.”

NASA’s Psyche mission is scheduled to launch in 2022 and will reach the asteroid in 2026. Signs of past volcanism that researchers could look for include variations in the colour or composition of the material on the surface, and possibly features that look like volcanic vents. Abrahams believes that large volcanic cones are unlikely, however.

This is because metallic asteroids would have solidified fairly quickly after their formation, therefore leaving plenty of time (billions of years) for any surface features of volcanism to be degraded.

Psyche is about 300 kilometres across and is made of almost pure nickel-iron metal (NASA/JPL-Caltech/UCLA/MPS/DLR/ID)

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

Nimmo says: “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.

“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.”

When they presented their findings at a recent Lunar and Planetary Science Conference, Abrahams and Nimmo discovered that another research team had independently arrived at similar conclusions about the possibility of ferrovolcanism.

Abrahams concludes: “It’s not a shocking idea, but we’d just never thought about iron volcanism before, so it’s something new and interesting to investigate.”

This isn’t the first time Psyche — three times the distance from the Sun as the Earth is — has made the news.

In 2016, an observation of the metal asteroid — with a diameter of 210 kilometres — by the NASA Infrared Telescope Facility located in Hawaii seemed to show evidence of water or hydroxyl on its surface.

The source of these molecules is something of a mystery, but researchers have suggested several different theories as to how they may have come to reside there.

This includes the suggestion that water-rich minerals may have been delivered by carbonaceous asteroids that impacted Psyche at some point in its distant past.

The Psyche mission could uncover these mysteries and give us important insight into the formation of planets.

Original research: https://doi.org/10.1029/2019GL082542