Vesta is simpler inside than previously thought

Vesta’s classification has vexed astronomers for decades, but a recent study is forcing planetary scientists to reevaluate its internal structure.

Located between Mars and Jupiter lies Vesta, the second-largest body within the main asteroid belt. For generations, astronomers thought Vesta wasn’t an ordinary asteroid: It also contained properties of a planet, with layers forming a crust, mantle, and core. However, a recent analysis using data from NASA’s Dawn spacecraft mission may force astronomers to throw that idea out the window.

After reassessing Dawn’s data, a team of researchers published a study last month in Nature Astronomy indicating that Vesta’s structure is more uniform than previously believed. Two possible reasons as to why Vesta might not have a multi-layered interior are currently being thrown around, but further evidence is needed to decide which one the observational data favors.

“The lack of a core was very surprising,” said co-author of the study Seth Jacobson, at Michigan State University in a press release. “It’s a really different way of thinking about Vesta.”

Refined data means refined results

Astronomers had thought that 329-mile-long (530-kilometer) Vesta was a body that never became a well-defined planet, its formation coming to a halt as a protoplanet. This claim was backed by early studies from NASA’s Dawn mission, which visited Vesta for over a year from 2011 to 2012. While there, Dawn mapped the gravity field of Vesta, acting as a probe whose motion scientists tracked using the spacecraft’s onboard cameras and Earth-based radio dishes. Dawn also measured the asteroid’s subtle wobbles in its spin due to its irregular shape.

But in their original analysis, scientists ran into a calibration issue with the data, struggling to combine the information from Dawn’s cameras (which yields its position relative to Vesta) and the radio tracking data (which indicates its motion relative to Earth via the Doppler effect). The new study fixes this issue, producing a more detailed gravity field that takes smaller-scale structures into account and more precise measurements of Vesta’s wobble. This is critical for inferring an important piece of information about Vesta — its moment of inertia.

A body’s moment of inertia quantifies an object’s resistance to changes in its rotational motion. For example, a figure skater spinning on ice changes their moment of inertia when their arms are pulled in or pushed away from their body, making their rotational speed increase and decrease, respectively. As most spinning space bodies are denser at their centers, their moment of inertia resembles that of a figure skater with their arms pulled in. But Vesta behaves more like a skater with their arms out. 

The new results revealed that Vesta does have a distinct boundary between its outer crust and its mantle beneath. But the mantle’s density is higher than previous models predicted and lacks any distinct stratification against the core. 

Two paths to the same destination

Astronomers are considering two scenarios to explain Vesta’s puzzling interior. One idea is that Vesta underwent incomplete differentiation, meaning Vesta’s outer layers may have been solid and strong enough to prevent dense accreted debris from sinking deeper into the interior.

The second scenario is that Vesta could be a second-generation object. In other words, two progenitor bodies could have collided, ejecting debris into space which eventually reformed into Vesta itself. The study proposes that as Vesta accreted material, the components of the mantle and core began mixing. “This idea went from a somewhat silly suggestion to a hypothesis that we’re now taking seriously due to this re-analysis of NASA Dawn mission data,” said Jacobson.

Both of these scenarios still have a long road ahead of them to be validated, but this paper shines a spotlight on the immense amount of information scientists can gather from a small body’s rotational motion. Missions like Psyche, NASA’s OSIRIS-APEX, and the European Space Agency’s Hera are scheduled to make similar measurements of more asteroids to hopefully solve some of these protoplanetary mysteries.