Hunt on for iron volcanoes on metallic asteroids


Two teams conclude some asteroids once spewed molten metal, as NASA prepares to investigate. Andrew Masterson reports.


An artist's impression of the metal asteroid, Psyche.

SSL/ASU/P Rubin/NASA/JPL-Caltech

Some asteroids in the solar system may have featured volcanoes that spewed out liquid iron, two research teams have independently concluded.

The findings of both sets of researchers are at this stage hypothetical, but a planned NASA mission set to launch in 2022 will provide a perfect opportunity to confirm, or rule out, the idea.

Metallic asteroids are common in the solar system’s asteroid belt. Astrophysicists agree that they began life early in the formation of the system, most likely as planetesimals – infant, small planets stripped of their external layers through violent collision, leaving only their molten inner cores.

In other words, at the moment of their violent formation they were floating blobs of liquid metal.

So far, so uncontentious, but planetary scientists Francis Nimmo and Jacob Abrahams at the University of California Santa Cruz wondered what happened next.

Clearly, the white-hot neo-asteroids would quickly start to cool down, so the researchers started modelling what that process would look like.

There were, in general, two scenarios: the metallic asteroids would crystalise and cool from the centre out, or from the outside in. The second model led to some interesting activity.

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

“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 researchers then set about modelling what asteroid volcanoes might look like. They realised that it all depended on the composition of the ejecta.

“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,” exclaims 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 pair published their results in the journal Geophysical Research Letters, and also presented at a recent conference.

While doing so, they discovered that another team, headed by Michael Sori from the University of Arizona’s Lunar and Planetary Laboratory, had reached very similar conclusions.

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

And, in astrophysical terms at least, the opportunity for unambiguous investigation is likely to present itself very soon.

In 2022, NASA will launch Psyche, an uncrewed mission to reach an asteroid of the same name – one of the 10 most massive bodies in the asteroid belt.

Psyche, the asteroid, has a diameter of about 210 kilometres and scientists think it is made entirely of iron and nickel.

The NASA mission is due to arrive at its – possibly volcanic – target in 2026.

  1. https://people.ucsc.edu/~janabrah/ferrovolcanism
  2. https://www.hou.usra.edu/meetings/lpsc2019/pdf/1625.pdf
  3. https://solarsystem.nasa.gov/asteroids-comets-and-meteors/asteroids/16-psyche/in-depth/
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