Even 200 years after asteroid 16 Psyche was discovered, astronomers continue to puzzle over its formation.
Psyche is the 10th-most massive asteroid in the main belt between Mars and Jupiter, and the largest known metallic asteroid, at 140 miles in diameter. NASA's Psyche mission will arrive in 2029 to determine its origin. Psyche may be a leftover building block of an early planet, shredded by violent collisions, or a planetary fragment that once separated into layers before losing its rocky outer mantle.
Other hypotheses suggest Psyche is an ancient remnant that either started metal-rich or became a blend of rock and metal after repeatedly smashing into other asteroids. Each scenario has different implications for the origin of planets in the early Solar System.
To investigate these possibilities, researchers at the University of Arizona's Lunar and Planetary Laboratory ran simulations to predict how a large crater near Psyche's north pole could have formed under these competing ideas. In a study published in JGR Planets, the team outlines predictions designed to help scientists interpret the data that NASA's Psyche mission will collect when it arrives in 2029. Coupled with spacecraft observations, the predictions may help settle the mystery of Psyche's makeup once and for all.
"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."
Fewer than 10% of asteroids in the main belt are metal-rich, and of those, Psyche is the largest. However, to learn more about how that metal is distributed inside the asteroid, scientists will have to wait until the Psyche spacecraft gets there.
"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."
When an asteroid is porous, it is crushable and the impact energy is efficiently absorbed, leading to deeper, steeper craters, with less ejected material scattered across the surface. By comparing these simulated craters with what the spacecraft will observe, scientists will be able to investigate whether Psyche's interior is separated into layers of rock and metal, or instead a mixed-up jumble of materials.
Psyche's origins
The researchers liken their approach to walking into an abandoned pizza parlor, as Psyche and the other Main Belt asteroids are thought to be leftovers of 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."
In other words, if Psyche turns out to be an exposed planetary core where most of the rocky crust was stripped away, it will offer a window into a violent stage of planet formation that scientists cannot observe otherwise.
"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."
The researchers used the best shape model of Psyche, derived from telescope observations, to create a 3D target. They reproduced the formation of a specific concavity in the model, about 30 miles across by three miles deep, as a simulated impact in which Psyche was hit at speeds typical of asteroid belt collisions – about three miles per second. The team varied the size of the virtual impactors and tested the two models (metallic core and mixed rock-and-metal) to see which could reproduce the crater's known dimensions. Each scenario produced slightly different crater shapes and ejecta patterns.
"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 rock. Instead, they can contain large amounts of empty space or fractured material left over from past collisions. The team incorporated this porosity into their models and discovered it strongly affects not only the depth and shape of craters, but also the distribution of ejected material after 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.
The Psyche spacecraft carries instruments designed to study the asteroid's surface, gravitational field, magnetic field, and composition. In addition to crater shapes, which depend on internal structure and porosity, the simulations predict other observable patterns, including variations in density caused when impacts compress the asteroid's interior and the distribution of metal-rich debris blasted onto 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 Psyche mission is led by Arizona State University. Lindy Elkins-Tanton of the University of California, Berkeley, is the principal investigator. A division of Caltech in Pasadena, NASA's Jet Propulsion Laboratory is responsible for the mission's overall management, system engineering, integration and test, and mission operations. Maxar Technologies (now Intuitive Machines) in Palo Alto, California, provided the high-power solar electric propulsion spacecraft chassis.
Psyche is the 14th mission selected as part of NASA's Discovery Program, managed by the agency's Marshall Space Flight Center in Huntsville, Alabama. NASA's Launch Services Program, based at Kennedy, managed the launch service.
