Not all asteroids are piles of rocky rubble as seen in Hollywood space operas and sci-fi video games. Some are made of much sterner stuff: iron and nickel, the raw materials that concentrate at the dense, hidden centers of worlds. These heavy-metal members of our solar system are thought to be fragments from the cores of long-vanished orbs broken up by giant impacts, emissaries from otherwise inaccessible depths of time and space that lie billions of years in the past—or thousands of kilometers beneath a world’s surface. Other than actually visiting such bizarre objects, there is no way to directly study these all-important building blocks of planets.
This is why NASA is now working to send a probe to Psyche, the largest metallic asteroid around the sun. Launching at the earliest in 2022, the eponymous spacecraft will orbit this 226-kilometer wide iron-nickel beast in the hopes of understanding the role it and other metallic asteroids played in assembling the solar system as we know it. By learning how Psyche formed, scientists may be able to explain the very hearts of other worlds, near and far, including our own familiar Earth. When the spacecraft arrives, though, it might be greeted with something decidedly alien: a landscape sculpted by “ferrovolcanism,” eruptions of liquid iron that occurred as the exposed core cooled.
A pair of recent studies now suggest that once upon a time, this heretofore unknown process occurred on Psyche and its cousins. Although its products have yet to be directly observed, it is already clear that this flavor of volcanism is like nothing scientists have ever encountered. As Brandon Johnson, an assistant professor of planetary sciences at Brown University, puts it, this style of heavy-metal eruption is like “turning the volcanism up to eleven.”
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Learning whether or not such eruptions actually took place on Psyche would do more than merely satiate scientists’ curiosity. Finding direct evidence of ferrovolcanism could rewrite the origin stories of planetary cores while potentially providing new answers to long-standing enigmas in the geophysical sciences. And so the hunt is on. Lindy Elkins-Tanton, the principal investigator on the NASA Psyche mission, now says she and her colleagues will search for signs of ancient ferrovolcanism on the metallic asteroid’s surface, in hopes of making a “mind-blowingly beautiful” discovery.
Chill Out, Psyche
Psyche has likely had an extremely eventful 4.5 billion years. Having formed near the birth of the solar system, it probably once had a silicate rocky shell surrounding a metallic core. Yet its journey to become a full-fledged planet was not meant to be.
“The early solar system was a bit like bumper cars,” says Matthew Genge, a senior lecturer and meteorite expert at Imperial College London. Plenty of other embryonic worlds would have smashed into proto-Psyche for millions of years, stripping away all of the material except the molten core. This core then continued to cool and ultimately freeze, leaving behind iron-nickel Psyche.
The extremity of that destructive history got some scientists thinking. Jacob Abrahams, a graduate student of planetary sciences at the University of California, Santa Cruz, was thinking about NASA’s upcoming visit to Psyche, and wondered what the cooling history of a metallic asteroid would have been like.
Taking physical properties of a simplified metal asteroid, Abrahams and his supervisor, Francis Nimmo, plugged them into a series of mathematical models and watched what happened if it cooled from the outside in. Upon becoming a stripped-down metal core, this asteroid progressively froze and contracted, creating cracks in its outer shell. The pockets of molten iron within, trapped by this solid outer layer, would do what all naturally buoyant fluids do and try to escape through cracks towards the surface. These virtual asteroids were Psyche-sized, leading Abrahams and Nimmo to suspect the same scenario might have unfolded in reality as well.
Liquid iron lava may sound strange, but “it would be incredibly weird if iron was the only thing that couldn’t be erupted out of something,” says Abrahams. And not only mathematics supports this idea—Mercury’s ancient past does, too.
The way a planet or asteroid cools makes all the difference to its evolution. More than four and a half billion years after Earth’s formation, its core is still cooling down, because our planet’s great bulk serves to reduce the loss of internal heat to a figurative trickle. Diminutive Mercury, however, is different. Its tiny size has let its internal heat escape, causing its relatively oversized iron-nickel core to freeze and shrink. This shrinkage pulled inward on the overlying mantle and crust, causing the entire planet’s surface to contract, snapping shut fissures that had been conduits for upward flows of magma. Surface volcanism should have ceased—yet Mercury still bears postcontraction signs (albeit ancient ones) of eruptive volcanic activity.
It seems then that magma can escape through pathways to the surface, even if plenty of fissures are squashed closed. If this applies to Mercury, then it likely applies to Psyche too. “Mercury is our main example of why this volcanism works,” says Abrahams. His study is in press with Geophysical Research Letters.
While these calculations were being carried out, Johnson, along with his colleagues, were also pondering the oddities of Psyche. The best estimate for Psyche’s density is only about half that of an iron meteorite, so either the asteroid is almost inexplicably porous, or something else profoundly altered its inner structure— but what? Falling down a “rabbit hole of scientific thought,” Johnson began to reconstruct the mechanics of how exactly Psyche had cooled down.
Johnson presented his independent research at the 50th Lunar and Planetary Science Conference earlier this year in The Woodlands, Texas. His team uses similar modelling to come up with an almost-identical hypothesis, finding that molten, sulfur-enriched iron and nickel could propagate to the surface via volcanic veins. They also suggest that Psyche’s strangely low density can be explained if it still has a rocky mantle layer, and ferrovolcanism has shunted metals up from the deep interior onto the surface.
Furthermore, back in the early days of the solar system, the radioactive isotope aluminum-26 kept the cores of many large rocky or metallic bodies molten. That would also make it “a driver for ferrovolcanism,” says Johnson.
Heavy Metal Volcanism
Elkins-Tanton explains that she and her colleagues had previously looked at the behavior of sulfur-rich magmas, but these first-order calculations on iron-rich fluids are novel and “really exciting.” They are “really articulate tattletales on the thermal history of the body in the way that other things aren’t,” she says—and the implications could be far-reaching.
Ferrovolcanism could change a planet’s composition. Genge points out that asteroids have fairly weak gravitational fields, meaning that plenty of their material can be ejected into space. Ferrovolcanic ejection of a core’s material into space would make this removal of material even more efficient. If an exposed core is allowed to clump together with other rocky material and forms a planet, that planet’s composition would be drastically different from, say, one whose core was never exposed by the violence of the early solar system.
Genge suggests this applies to trace elements in the core, like oxygen and sulfur, some of which are vital for biological processes. If ferrovolcanism drains them away, then this could place restrictions on the appearance and evolution of life on the resulting planet.
Ferrovolcanism could also explain strange features seen in meteorite samples on Earth. Pallasites are a class of meteorites thought to be a mix of core and mantle material, but how that mixture actually arises is far from clear. Johnson suggests that ferrovolcanism would be one way that core and mantle material could blend together. At the same time, chondrites—common stony meteorites—are also peppered with rounded, shiny inclusions known as chondrules, some of which are iron-nickel rich. Although suspected to have formed when metal-rich asteroids slammed into each other, causing buckshot-like sprays of liquid metal that then flash-froze in the icy chill of deep space, Genge suggests that such chondrules could be the result of ferrovolcanic processes instead.
When it comes to meteorites, plenty of mysteries remain. After all, meteorites are just the shrapnel of larger bodies, says Elkins-Tanton. “More ideas about what the bigger jigsaw puzzle may be will help us with the pieces we’ve already got,” she says.
Ancient Iron Rivers
The proof lies in the pudding. Modern-day ferrovolcanism on Psyche would be “a big surprise,” says Patrick Whelley, a volcanologist with NASA, adding that it is not an impossibility if there are enough radioactive isotopes still decaying deep inside. Still, it is more likely that the trapped heat within Psyche seeped out eons ago.
Its remains, however, might linger even today—even though no one expects to see a full-blown extinct volcano. Liquid iron’s viscosity is so low that it would be more likely to pool near fissures rather than build up anything akin to a familiar silicate-based cinder cone. Any patches of iron could be volcanic spatter still clinging onto Psyche, and it would be “very informative if we see them,” says David Rothery, a professor of planetary geosciences at The Open University. Yet such formations would more likely be obliterated by millions of years of bombardment from impacts. A conservative guess, he says, is that “we’re not going to see signs of obvious things looking like dead volcanoes when we look at Psyche…. But I’d love to be wrong.”
Measurements of Psyche’s magnetic field may offer a more promising avenue for finding past liquid-iron eruptions. The outside-in cooling regime could allow certain crystalline volcanic minerals nearer to the surface to preserve the tell-tale magnetic stamp of a partially-frozen liquid core. Those same minerals would also bear a unique hue, texture and geochemical fingerprint to identify them as ferrovolcanic products. These types of data should hopefully be within reach of NASA’s spacecraft when it slips into orbit around Psyche.
Although Abrahams reckons the huge suite of collected meteorites may be a more effective place to look for ferrovolcanic evidence, he still holds out hope for evidence gathered from that ancient metallic world. Metallic meteorites are in general the outcome of eons of gradual mixing, reducing and solidifying in a world-sized slow-cooker pot. Ferrovolcanic lavas, in comparison, are the quickly sautéed versions of the initial ingredients—in that they should be closer to a core’s original, pristine chemical composition. That means samples from Psyche may chronicle how cores evolved over time, revealing a lost chapter from the long, murky history of the birth of planets.
This is all somewhat speculative, and ferrovolcanism’s possible existence is currently just based on a few mathematical models. As we are yet to visit a metallic asteroid, it is difficult for anyone to say with confidence what we may ultimately discover. “We’re probably too naive to know what we’re going to learn yet,” says Rothery. “The things we find out might not be the things we imagined.”