$100,000 quadrillion asteroid Psyche may have volcanoes of molten metal to thank for its shiny appearance
The asteroid Psyche may once have had vents that spewed molten metal – but only if it is chemically similar to rare, metal-rich meteorites, a new study suggests. The study, published online July 31 in the Geophysical Research Journal: Planetscould explain why the space rock has an unusual metallic cloak.
The potato-shaped asteroid Psyche, a member of the main asteroid belt between Mars and Jupiter, is unique in that it is extremely bright. Radar measurements indicate that on average its surface reflects nearly a third of the sunlight that shines on it, making it at least twice as reflective as most asteroids.
However, this shine may be mostly superficial. In 2020, calculations involving updated estimates of Psyche’s mass and volume suggested that the asteroid’s density was between 231 pounds per cubic foot (3,700 kilograms per cubic meter) and 256 pounds/cubic foot (4,100 kg/cubic meter). Although denser than most asteroids, its density is only about half that expected if Psyche were made entirely of iron and nickel. This, along with surface heat emission data, indicates that although Psyche’s mass is largely nonmetallic, the asteroid likely has a metal-rich cloak.
Scientists still don’t know exactly how Psyche acquired this coat. Although many phenomena could be responsible, the most favored at present is ferrovolcanism. “Ferrovolcanism is like normal volcanism except instead of molten rock, the ‘lava’ is molten metal,” said Courville, who was not involved in the new study.
First suggested in 2019, the idea is that during Psyche’s childhood, her metallic core solidified from the outside in, with the molten inner core gradually becoming richer in lighter elements. The difference in density between the solid exterior of the core and the molten interior would have created enough pressure for the latter to pierce the overlying mantle and surface of the asteroid, creating vents spewing molten metal.
But ferrovolcanism can only work with specific chemical compositions, because it depends greatly on the pressure that builds up in the inner core. Data from meteorites – space rocks that fall to Earth’s surface – help narrow the range of these chemical combinations.
To determine the chemistry that might have given rise to Psyche’s iron-spewing vents, the authors of the new study — Jaap Jorritsmaa doctorate. student at the Delft University of Technology and Win van Westrenen, professor of Earth sciences at Vrije University in Amsterdam, created computer models of the asteroid.
They considered that the asteroid had a chemical composition similar to that of three types of meteorites: EH chondrites (rare stony meteorites lacking iron), H chondrites (common stony meteorites with moderate amounts of iron) and mesosiderites (rare iron-rich meteorites). The researchers then performed simulations to determine which chemical composition was most favorable for the occurrence of ferrovolcanism.
The researchers found that most of Psyche must be rich in metals like iron, because low iron content would lead to tiny cores in which the internal pressure generated is insufficient to expel the magma. This meant that Psyche was more likely to have had ferrovolcanism if it contained large amounts of mesosiderite. H chondrites could also lead to releases of molten metal, but only if they had a relatively high density. In contrast, EH chondrites produced only tiny cores lacking iron-rich exteriors, making them unlikely to be part of Psyche.
Researchers expect NASA’s ongoing Psyche mission will provide evidence to support their conclusions. Scheduled to arrive at Psyche by July 2029, the spacecraft will spend two years photographing the asteroid and collecting spectroscopic data. Courville said these snapshots will reveal whether surface metal is found in large outcrops or flows, which would indicate that ferrovolcanism occurred in the space rock’s past.
