A tiny grain from asteroid Ryugu, brought back to Earth by Japan’s Hayabusa2 probe, has revealed a fiery mineral that no one expected. The mineral, djerfisherite, is normally found in environments with extremely high temperatures and low oxygen levels. However, the discovery of this mineral on an icy asteroid challenges long-held assumptions about asteroid formation and offers new insight into the early history of the Solar System.
A Surprising Discovery from the Ryugu Sample
The microscopic grain from Ryugu, which was returned to Earth by the Hayabusa2 mission in December 2020, has scientists rethinking their understanding of asteroids. Ryugu belongs to the C-type family of asteroids, which are rich in carbon and thought to have formed in the cold outer reaches of the Solar System. These grains, preserved in pristine condition, were expected to reveal the primitive conditions of the Solar System.
But inside one of these grains, scientists from Hiroshima University discovered djerfisherite, a potassium-bearing iron-nickel sulfide mineral. This mineral, which typically forms in very reduced environments at high temperatures, is not expected to be found in a body like Ryugu, which has never been thought to experience such conditions. The presence of djerfisherite is like finding a tropical seed buried in Arctic ice, a phenomenon that could indicate either an unexpected localized heat source on Ryugu or that the mineral came from elsewhere.
Challenging the Uniformity of Ryugu’s Composition
The discovery of djerfisherite raises new questions about the uniformity of Ryugu’s composition. Ryugu was long thought to have a relatively uniform make-up, given its cold, ice-rich environment. However, the presence of a high-temperature mineral like djerfisherite suggests that the asteroid may have experienced localized heating or that materials from different regions of the early Solar System mixed together.
Masaaki Miyahara, the first and corresponding author of the study, explained, “The discovery of djerfisherite in a Ryugu grain suggests that materials with very different formation histories may have mixed early in the solar system’s evolution, or that Ryugu experienced localized, chemically heterogeneous conditions not previously recognized.”
High-Temperature Origins: A Matter of Debate
The discovery of djerfisherite on Ryugu opens up two possible scenarios. The first is that the mineral was transported to Ryugu from another source, possibly from the inner regions of the Solar System where high temperatures were more common. The second possibility is that Ryugu’s parent body, which formed in the cold outer Solar System, was subjected to heat at some point, raising temperatures above 350°C and allowing the mineral to form in situ.
Research into other types of asteroids suggests that djerfisherite can form under extreme conditions, such as in the parent bodies of enstatite chondrites, which form in the warmer inner regions of the Solar System. Thermodynamic calculations indicate that in these cases, djerfisherite forms directly from high-temperature gas. However, Ryugu’s parent body was thought to have remained much cooler, and its formation was likely dominated by the presence of ice and carbon dioxide.
As Miyahara notes, “Ultimately, our goal is to reconstruct the early mixing processes and thermal histories that shaped small bodies like Ryugu, thereby improving our understanding of planetary formation and material transport in the early solar system.”
What’s Next for Ryugu Research?
The next steps in understanding the origins of this strange mineral are isotopic studies, which will allow scientists to trace the history of the Ryugu grains and better understand the heat conditions they were exposed to.
By investigating isotopic “fingerprints,” researchers hope to uncover the true story behind the mixing processes and thermal histories that shaped Ryugu and other primitive asteroids. This surprising discovery opens a new chapter in the study of asteroid formation, providing insights into the complexities of early Solar System bodies.
