New Scans Give us a Better View of the Metal Asteroid Psyche

In 2022, NASA will launch a spacecraft to asteroid Psyche (16 Psyche), one of the largest in the asteroid belt, and the only known asteroid to be composed almost entirely of metals like iron and nickel.

Now, scientists have taken a new look at Psyche using the Hubble Space Telescope, conducting the first ultraviolet observations of this asteroid since the 1980s. Hubble has provided new insights into Psyche’s surface and composition, as well as possible activity taking place on Psyche’s surface.

“We were able to identify for the first time on any asteroid what we think are iron oxide ultraviolet absorption bands,” said planetary scientist Dr. Tracy Becker from the Southwest Research Institute, lead author of a new paper describing the observations. “This is an indication that oxidation is happening on the asteroid, which could be a result of the solar wind hitting the surface.”

Scientists have long speculated that this metallic asteroid might be the leftover surviving core of a protoplanet, where perhaps a violent collision with a planetesimal stripped off Psyche’s outer, rocky layers, leaving behind only the dense, metallic interior. This theory has been supported by estimates of Psyche’s bulk density, spectra, and radar surface properties, all of which show it to be an object unlike any others in the asteroid belt. Additionally, this composition of 16 Psyche is strikingly similar to that of Earth’s metal core.

The asteroid Psyche is one of the larger asteroids. Credit: Lindy T. Elkins-Tanton

“We’ve seen meteorites that are mostly metal, but Psyche could be unique in that it might be an asteroid that is totally made of iron and nickel,” Becker said in a press release. “Earth has a metal core, a mantle and crust. It’s possible that as a Psyche protoplanet was forming, it was struck by another object in our solar system and lost its mantle and crust.”

Becker and her team used Hubble to make high-resolution UV observations of Psyche taken in 2017 with the observatory’s Space Telescope Imaging Spectrograph (STIS). They observed the asteroid at two specific points in its rotation to view both sides of Psyche completely at ultraviolet (UV) wavelengths.

Figure 1. Orientation of Psyche at the time of the two HST observations. Image courtesy Becker et al.

They observed some small spectral differences between the sides, perhaps suggesting an unusual feature in the northern hemisphere, but overall, the spectra they observed showed iron to be the primary surface feature.  

“We did not find significant spectral variations with rotation, though the equatorial region of the asteroid may have a higher overall reflectance than the northern hemisphere,” the team wrote in their paper. Psyche also appeared increasingly reflective at deeper UV wavelengths.

“This is something that we need to study further,” Becker said. “This could be indicative of it being exposed in space for so long. This type of UV brightening is often attributed to space weathering.”

The team also compared the spectrum of Psyche with meteorite samples from specific database, but did not find any strong matches.

The observations were made in preparation for the Psyche mission, which will travel to the asteroid as part of an effort to understand the origin of planetary cores. The mission is set to launch in 2022 and arrive at Psyche in 2026. Since asteroid Psyche is so far from Earth — approximately 280,0000,000 km, we don’t have a clear understanding or clear images of this object. Metal asteroids are relatively rare in the solar system, and scientists believe Psyche could offer a unique opportunity to see inside a planet.

“What makes Psyche and the other asteroids so interesting is that they’re considered to be the building blocks of the solar system,” Becker said. “To understand what really makes up a planet and to potentially see the inside of a planet is fascinating. Once we get to Psyche, we’re really going to understand if that’s the case, even if it doesn’t turn out as we expect. Any time there’s a surprise, it’s always exciting.”

The study was published in The Planetary Science Journal and was presented at the virtual meeting of the American Astronomical Society’s Division for Planetary Sciences last week

Lead image caption: Artist impression of the massive asteroid 16 Psyche. Credit: Maxar/ASU/P. Rubin/NASA/JPL-Caltech.