Volcanoes are badass, but are they truly metal? If one new theory is right, the answer was once yes: Some of the solar system’s primordial worlds may have had volcanoes that oozed molten iron and nickel, an eruptive process that might account for the formation of pallasites, some of the most beautiful and enigmatic meteorites yet known.
The model, published on Monday in Nature Astronomy, might also help explain some of the bizarre characteristics of 16 Psyche, a 278-kilometre-wide asteroid with a uniquely metallic surface. Within this decade, the study’s ideas will be put to the test. In 2022, NASA will launch Psyche, an orbiter that will rendezvous with its namesake asteroid in 2026 and map it in detail.
“I was really happy to see [this study] come out, because these are calculations that, if somebody else didn’t do them, we’d need to do them,” says Lindy Elkins-Tanton, a planetary scientist at Arizona State University who is the principal investigator of NASA’s Psyche mission.
“[The Psyche mission] is the whole reason we were getting excited, and thinking about Psyche not as just some point-of-light asteroid in the sky, but as an actual, physical world,” adds lead study author Brandon Johnson, a planetary scientist at Purdue University. “No matter what the Psyche mission finds, it’s bound to be exciting.”
Radar measurements of Psyche’s surface strongly suggest that the asteroid has a metallic sheen. This led scientists to suspect that it’s the exposed core of a planetesimal, one of the many mini-worlds that formed in the early solar system. One idea for Psyche’s formation holds that it started out with an iron-nickel core and a rocky mantle, and then it lost its mantle during an onslaught of collisions in the solar system’s rough-and-tumble infancy.
But since 2017, two studies of Psyche’s volume have found that the asteroid’s density is around 4.2 grams per cubic centimetre, which is about 45 per cent lower than you’d expect from a solid mass of iron and nickel. If Psyche is all-metal, it must be extremely porous. Another possibility is that, despite its metallic surface, Psyche is still holding on to at least some of its lighter, rocky mantle. But if that’s the case, why does its surface look so core-like?
“It’s still too dense to be just a rock, but not dense enough to be just metal,” Elkins-Tanton says. “The question is, what is it?”
According to the new model, an explanation might lie in how Psyche cooled solid billions of years ago, during the first 10 to 100 million years of its existence. When Psyche’s core was still hot enough to be liquid, this metallic ooze—a mix of iron, nickel, and sulfur—may have solidified from the outside in. As it did, the core’s iron and nickel would have crystallised, spreading toward Psyche’s centre like frost branching across a windowpane.
However, iron and nickel don’t crystallise in the presence of sulfur, so as these metals continued to solidify, the core’s sulfur would have become more concentrated in the remaining pockets of molten metal. As these melts accumulated more sulfur, they could have remained liquid at lower temperatures. Pure iron begins to solidify below 1,538 degrees Celcius, but the right iron-sulfur mix can remain liquid at temperatures as low as 988 degrees Celcius.
Johnson and his colleagues calculate that, depending on the size of these melt pockets, the molten metals within them could have felt immense pressure, on the order of hundreds of pounds per square inch. Much like squeezing a tube of toothpaste, this pressure could have powered the molten metal through the overlying mantle and even onto the object’s surface if the mantle was less than 48 kilometres or so thick—creating features akin to volcanic vents. If humans could have floated over such seeps, Johnson says, they’d probably have seen large fissures and oozing, glowing pools of iron and nickel.
This theoretical process, which the team has named ferrovolcanism, could help explain Psyche’s oddball appearance and density. Johnson and his colleagues Michael Sori and Alexander Evans find that Psyche’s observed density makes sense if the asteroid has an iron core topped by a 25-kilometre-thick rocky mantle. And if Psyche does have a rocky mantle, ferrovolcanic seeps could have veneered it with metal from the asteroid’s core.
“One of the most interesting parts, I think, is the implication that Psyche might not be a ball of iron and nickel—that it might not be a naked core,” Johnson says. “It might have a mantle tens of kilometres thick, if our ideas are correct.”
Elkins-Tanton applauds the calculation, but she cautions that the extent of such volcanism remains unknown, while radar data suggest that much of Psyche’s surface is metallic.
“Maybe it is the core of a planetesimal and a lot of the rock was stripped off, but it was stripped off unevenly, [or] maybe it’s a big jumble—there’s a whole lot of possibilities,” she says. “We’re not going to look at a ball bearing in space.”
Still, the ferrovolcanism theory is attractive in part because it might also help explain pallasites, an enigmatic category of meteorite. Pallasites consist of green-brown crystals—the mineral olivine—embedded in a matrix of iron and nickel. Sometimes, the trapped crystals are even peridot, the name given to gemstone-quality olivine. Since olivine forms in the mantle, scientists have long thought of pallasites as mixes of mantle and core that were sent hurtling across the solar system by ancient collisions, with some bits landing here on Earth. But pallasites’ precise origin story has remained a subject of debate.
“Essentially, this ferrovolcanism would be a natural way to get core material mixing with the mantle,” Johnson says.
A meteor from the annual Perseid meteor shower shoots by the Arcminute Microkelvin Imager Radio Telescope near Cambridge, England.
PHOTOGRAPH BY NIGEL BLAKE, ALAMY
However, getting molten iron and minerals to mix only gets you partway to a pallasite, Elkins-Tanton says. These meteorites also tend to lack other minerals, but in the timeline of planetesimal formation, there’s only a small window when crystals of just olivine would abound in an object’s deep mantle.
“If you can just get the iron-nickel magmatism to erupt just into that layer, then I think that mechanism works,” she says. “But in my opinion at least, it doesn’t solve all the mysteries of pallasites.”
Come 2026, the Psyche orbiter will start to put these mysteries to the test. By picking up on differences in the asteroid’s surface gravity, scientists will be able to tell whether the asteroid’s interior consists of layers or is porous metal through and through. It will also be able to map Psyche’s surface composition. If it spots olivine on the surface, it’d suggest that Psyche created some of the solar system’s pallasites, perhaps even some of the ones we’ve found on Earth.
Though data from the mission are still years away, Johnson is thrilled that we’ll finally be seeing this unusual asteroid up-close: “This is what planetary scientists live for, just seeing a world come into view.”
Lead Image: In 2026, NASA's Psyche mission will arrive at 16 Psyche, one of the solar system's most massive asteroids. The asteroid's sheen suggests that it could be a piece of an early planet's iron-nickel core, or it could have oozed molten iron across a thin mantle soon after forming.
ILLUSTRATION BY NASA