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.

Travelling to asteroid Psyche in 2029

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 speci...