Imagine a time in the Solar System’s past where the asteroids were not solid rock, but blobs of molten iron. It sounds strange, but that may have been the case. And in the right conditions, some of those asteroids would have sprouted volcanoes. One of those asteroids, Psyche, is the destination for a NASA mission.
That’s the conclusion that scientists at the University of Santa Cruz came to in a new study soon to be published in Geophysical Research Letters.
The scientists are Francis Nimmo, Professor of Earth and Planetary Sciences at the University of California, Santa Cruz (UCSC), and graduate student Jacob Abrahams. They embarked on this research because of NASA’s plans to send a spacecraft to the asteroid Psyche. Psyche is a unique asteroid; not only is it the largest asteroid in our Solar System, but it seems to be the exposed iron and nickel core of an early planet.
In the chaos of the early Solar System, chunks of rock collided and sometimes combined. Over time, they formed larger and larger chunks which eventually formed into the rocky planets. In fully-formed planets like Earth, the iron and nickel core is buried under a silicate crust. It’s the same on all the terrestrial planets and moons, and on most asteroids.
But some planetesimals would have suffered catastrophic collisions, and their outer layers would have been stripped away, leaving the exposed molten iron-rich core. That core would have cooled in the frigid temperatures in space, with the surface likely solidifying first. This is what they think happened to Psyche.
Professor Francis Nimmo, UCSC Earth and Planetary Sciences.
“One day he turned to me and said, ‘I think these things are going to erupt.'”
In advance of NASA’s mission to Psyche, Nimmo became interested in metallic asteroids. He was interested in their composition, and in what clues to that composition could be provided by meteorites here on Earth. He had graduate student Jacob Abrahams run some models of how the asteroids formed and cooled.
It turns out, at least some of these asteroids would have experienced volcanic eruptions of molten iron.
“One day he turned to me and said, ‘I think these things are going to erupt,'” Nimmo said in a press release. “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.”
These ferro-volcanoes wouldn’t have erupted on all of these exposed core asteroids. They would have to have been massive enough that the surface cooled and solidified quickly, while the core remained molten. But not all them would have behaved that way.
“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,” Nimmo said.
In these metallic asteroids, the solidified crust would have compressed the buoyant, molten core. That would have kept the molten iron locked into the core. But over time, there would have been changes in the crust.
The crust would cool, and the compression and cooling would have formed stress cracks and faults in the crust. The molten core was more buoyant, and would be under pressure. As cracks and faults formed, the molten iron would have escaped, creating ferro-volcanoes.
“…nothing like the thick, viscous lava flows you see on Hawaii.”Professor Francis Nimmo, UCSC Earth and Planetary Sciences.
We don’t know what exactly these ferro-volcanoes would’ve looked like. But according to the two researchers, there are at least a couple of possibilities. largely depending on the composition of the melted rock.
“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,” Abrahams said. “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.”
It’s unlikely that NASA’s Psyche mission will be able to see any clear evidence of this ferro-volcanism. It all happened billions of years ago, and collisions and wear and tear would have shaped the surface of asteroids like Psyche since then. “It’s not clear what they might look like now,” Abrahams said.
But there still may be some signs of these ferro-volcanoes. There could be variations in the color of material on the surface. There may even be some volcanic vents. But don’t expect any lava cones.
The Psyche mission isn’t strictly about finding evidence of volcanic activity; it has other science objectives. In fact, according to Nimmo, the best place to find proof of ferro-volcanic activity on asteroids like Psyche might be in metallic asteroids here 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,” 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 pair of scientists presented these results at a recent Lunar and Planetary Science Conference, and found that another research team had come to the same conclusion. An abstract of that team’s results is here.
These are interesting results, partly because of the picture they paint. A planetesimal, or proto-planet, travelling through the ancient Solar System, undergoes a catastrophic collision. The surface is stripped away, leaving only a molten core. The surface cools, trapping the molten core underneath, only to escape in ferro-volcanoes as stresses force cracks in the surface.
“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,” Abrahams said.
NASA’s Psyche mission is scheduled to launch in 2022. In 2026 it will arrive at Psyche, then spend 21 months orbiting the asteroid, mapping its surface features and studying its other properties. The spacecraft will include several instruments:
- Multispectral Imager
- Gamma Ray and Neutron Spectrometer
- X-Band Gravity Science Investigation
The Psyche mission will also test a new laser communication system called Deep Space Optical Communication (DSOC.)