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For many years, researchers regarded Vesta, a prominent body within the solar system’s asteroid belt, as more than just a simple asteroid. It was believed to possess structured layers akin to a planet, consisting of a crust, mantle, and core.
Astrophysicists have long investigated Vesta to glean insights into the formation of early planets and to understand what Earth may have resembled in its infancy.
Recent findings from Michigan State University challenge this established perspective.
A research team from NASA’s Jet Propulsion Laboratory (JPL) has published a study in Nature Astronomy revealing that Vesta’s core structure is unexpectedly uniform. This revelation has taken the scientific community by surprise, as many had labeled Vesta a protoplanet that never fully developed into a true planet.
“The absence of a core is quite astonishing,” stated Seth Jacobson, an assistant professor in Earth and Environmental Sciences at MSU and a co-author of the paper. “This prompts a significant shift in how we interpret Vesta.”
So, what exactly is Vesta? The researchers propose two potential scenarios that warrant further investigation.
The first theory suggests that Vesta underwent incomplete differentiation; it began the melting process necessary for developing distinct structural layers but was unable to complete it. The second hypothesis, put forth by Jacobson at an astronomy conference a few years prior, posits that Vesta may actually be a fragment from a planet that was in the process of forming in the early solar system.
During the conference, Jacobson proposed that certain meteorites might be remnants from collisions that occurred while planets were coalescing. Although he mentioned Vesta as part of this speculation, he had not seriously considered it at the time.
“What once seemed like an offhand suggestion has evolved into a credible hypothesis, bolstered by this new analysis of data from NASA’s Dawn mission,” Jacobson remarked.
More than an asteroid
In general, most asteroids are composed of archaic chondritic material, giving them a gravel-like appearance in the cosmos. In contrast, Vesta is largely covered in volcanic basaltic rocks, suggesting that it underwent a process known as planetary differentiation, where metal settles to the center to form a core.
NASA launched the Dawn spacecraft in 2007 to examine Vesta and Ceres, the two largest entities in the asteroid belt, intending to deepen our understanding of planet formation.
Dawn orbited Vesta from 2011 to 2012, measuring its gravitational field and capturing high-resolution images to produce a detailed surface map. After similarly studying Ceres, the mission concluded in 2018, leading to various published findings.
According to Jacobson, as researchers continued to analyze this data, they became better equipped to process it effectively. They discovered methods to improve the calibration of their measurements, enhancing their understanding of Vesta’s composition. This motivated Ryan Park, a senior research scientist and principal engineer at JPL, and his team to reprocess the measurements of Vesta.
“For years, we grappled with inconsistencies in gravity data derived from Dawn’s observations of Vesta,” noted Park. “After nearly a decade refining our calibration and processing techniques, we achieved significant alignment between the data from Dawn’s Deep Space Network and its onboard imaging. We were excited to confirm the robustness of the data in revealing Vesta’s internal structure. Our findings illustrate that Vesta’s history is much more intricate than previously assumed, influenced by distinctive processes like interrupted planetary differentiation and subsequent collisions.”
Planetary scientists estimate a celestial body’s core size by measuring its moment of inertia, a physics concept indicating how hard it is to alter an object’s rotational speed. Jacobson exemplified this with a figure skater manipulating their speed by adjusting their arm positions.
Analogously, a celestial object with a denser core operates differently than one without a core at all. With this framework, the research team assessed Vesta’s rotation and gravitational field and found that it behaved in a manner inconsistent with having a core, challenging prior assumptions regarding its formation.
Two hypotheses
While neither hypothesis has been fully explored or debunked, both present challenges that necessitate additional research. Although incomplete differentiation could be a factor, it doesn’t align with the characteristics of meteorites collected over time.
“We are highly confident these meteorites originated from Vesta,” Jacobson pointed out, “and they do not exhibit clear signs of incomplete differentiation.”
An alternative explanation hinges on the notion that massive collisions occurred during the formation of terrestrial planets, generating debris that included rocks from melting and, like Vesta, lacking a core.
Jacobson’s lab is investigating the implications of these colossal impacts, working alongside his graduate student, Emily Elizondo, to examine the idea that some asteroids in the asteroid belt may originate from the remnants of forming planets.
This theory is still speculative. Further modeling and analysis are required to substantiate the notion that Vesta is an ancient fragment of a nascent planet. Researchers can refine their study of Vesta meteorites to better test these hypotheses, Jacobson added, alongside enhanced studies utilizing new methodologies on existing Dawn mission data.
This paper marks the beginning of a transformative line of inquiry, Jacobson argued, with the potential to significantly alter the scientific community’s perspective on differentiated celestial bodies.
“The collection of Vesta meteorites is no longer just a sample from an asteroid that failed to become a planet,” Jacobson declared. “These may represent fragments of a primordial planet that never reached full development. The identity of this planet remains uncertain.”
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