More than two centuries after asteroid 16 Psyche was first identified, scientists are still trying to determine how it formed.
Located in the main asteroid belt between Mars and Jupiter, Psyche ranks as the 10th-most massive asteroid and the largest known object made primarily of metal, measuring about 140 miles across. NASA's Psyche spacecraft is scheduled to arrive in 2029 with the goal of uncovering its origin. Researchers suspect it could be a remnant of an early planet that was torn apart by massive collisions, or possibly a fragment of a once-layered body that lost its outer rocky shell.
Other ideas suggest Psyche may have formed as a metal-rich object from the start, or that it became a mix of rock and metal after repeated impacts with other asteroids. Each possibility points to a different story about how planets formed in the early Solar System.
Simulating Craters to Reveal Psyche's Interior
To explore these possibilities, scientists at the University of Arizona's Lunar and Planetary Laboratory created simulations to understand how a large crater near Psyche's north pole may have formed. Their findings, published in JGR Planets, provide predictions that will help researchers interpret data collected by NASA's Psyche mission when it arrives. By combining these simulations with real observations, scientists hope to finally determine what Psyche is made of.
"Large impact basins or craters excavate deep into the asteroid, which gives clues about what its interior is made of," said Namya Baijal, a doctoral candidate at the LPL and first author of the paper. "By simulating the formation of one of its largest craters, we were able to make testable predictions for Psyche's overall composition when the spacecraft arrives."
Although metal-rich asteroids make up less than 10% of the main belt, Psyche is the largest among them. Still, researchers will need direct spacecraft measurements to understand how that metal is distributed throughout its interior.
Why Porosity Matters in Asteroid Impacts
"One of our main findings was that the porosity -- the amount of empty space inside the asteroid -- plays a significant role in how these craters form," said Baijal. "Porosity is often ignored because it's difficult to include in models, but our simulations show it can strongly affect the impact process and shape of craters left behind."
Asteroids with more internal voids tend to absorb impact energy more effectively, which leads to deeper and steeper craters and less debris scattered across the surface. By comparing simulated crater features with those observed by the spacecraft, scientists can test whether Psyche's interior is layered, with distinct metal and rock regions, or instead a more chaotic mixture of materials.
Clues to Planet Formation in the Early Solar System
The research team compares their approach to examining the remains of an abandoned pizza shop. Psyche and other main belt asteroids are thought to be leftover building blocks from planet formation. "The cooks have long left, but you can look at what's left behind -- the ovens, scraps of dough, the toppings -- and make inferences about how the pizzas were made," said Erik Asphaug, a professor in LPL and co-author of the study. "We can't get to the cores of Earth or Mars or Venus, but maybe we can get to the core of an early asteroid."
If Psyche turns out to be the exposed core of a former planet, stripped of its outer layers, it would provide rare insight into a violent phase of planetary evolution that is otherwise impossible to observe directly.
"We tested two main interior structures for Psyche," said Baijal. "One is a layered structure with a metallic core and a thin, rocky mantle, which likely formed if a violent collision stripped away the outer layers. The other is a uniform mixture of metal and silicate, created by a more catastrophic impact that mixed everything together, like some metal-rich meteorites found on Earth."
Modeling a Massive Impact on Psyche
Using detailed shape models based on telescope data, the researchers built a 3D representation of Psyche and recreated the formation of a large crater roughly 30 miles wide and three miles deep. In their simulations, the asteroid was struck at speeds typical for the asteroid belt, about three miles per second. They tested different impactor sizes and compared two internal structure models (metallic core and mixed rock-and-metal) to see which could best match the observed crater.
"We found that an impactor about three miles across would create a crater of the right dimensions," Baijal said. "The crater's formation is consistent with both scenarios of Psyche's makeup."
Unlike planets, many asteroids are not solid. Instead, they often contain fractured material and empty spaces from past collisions. By including porosity in their simulations, the researchers showed that it has a major influence on crater formation and how debris is distributed after an impact.
"By rigorously treating Psyche's shape, porosity and composition, this work represents a true watershed moment for our capacity to realistically simulate impacts into unique types of asteroids," said Adeene Denton, a postdoctoral researcher and another co-author of the study.
NASA's Psyche Mission and What Comes Next
The Psyche spacecraft is equipped to measure the asteroid's surface, gravity, magnetic field, and composition. In addition to crater shapes, the simulations predict other features scientists can look for, such as density variations caused by impacts compressing the interior and the spread of metal-rich debris across the surface.
"When the spacecraft arrives at Psyche in a few years, the geochemists, geologists and modelers on the team will all be looking at the same object and trying to interpret what we see," said Asphaug. "This work gives us a head start."
The mission is led by Arizona State University, with Lindy Elkins-Tanton of the University of California, Berkeley, serving as principal investigator. NASA's Jet Propulsion Laboratory, a division of Caltech in Pasadena, manages mission operations, system engineering, and testing. The spacecraft platform was built by Maxar Technologies (now Intuitive Machines) in Palo Alto, California.
Psyche is the 14th mission selected under NASA's Discovery Program, managed by the agency's Marshall Space Flight Center in Huntsville, Alabama. NASA's Launch Services Program at Kennedy handled the launch.
