In a surprising turn of events, scientists from Hiroshima University discovered a rare mineral called djerfisherite—a potassium-bearing iron-nickel sulfide—in samples from the asteroid Ryugu. The samples were delivered to Earth by the Japanese Aerospace Exploration Agency's (JAXA) Hayabusa2 mission, marking a milestone in the study of asteroids and the early solar system.
The unexpected finding shook the scientific community's understanding of the formation of space rocks. Ryugu is a carbon-rich asteroid, similar to ancient meteorites known as CI chondrites, characterized by high carbon content and traces of water. It formed in a region of the solar system characterized by low temperatures, with water and carbon dioxide existing as ice. The temperature on Ryugu is believed to have almost never risen above 50 degrees Celsius.
Djerfisherite typically forms in very reduced environments found in enstatite chondrites, which are located in the warmer, inner regions of the solar system. Its presence in Ryugu presents a puzzling anomaly. The discovery suggests that the formation history of Ryugu and similar asteroids was more complex than previously thought.
"The occurrence of djerfisherite in Ryugu is like the discovery of a tropical seed in Arctic ice—it suggests either an unexpected local environment or long-distance transport in the early solar system," said Masaaki Miyahara, the first author of the study. "Its presence indicates that materials with different formation histories may have mixed in the early stages of the evolution of the solar system, or that Ryugu was in local, chemically heterogeneous conditions that were not previously recognized."
The scientists propose two hypotheses for the presence of djerfisherite in Ryugu. One possibility is that it came with other particles from another part of the solar system, implying that materials from different regions mixed during the formation of Ryugu's parent body. The other hypothesis is that local hot zones arose in the depths of the asteroid, allowing djerfisherite to form under conditions previously not thought possible on Ryugu.
"Ultimately, our goal is to reconstruct the early mixing processes and thermal developments that shaped small bodies like Ryugu, thereby improving our understanding of planet formation and material transport in the early solar system," Miyahara added.
Ryugu, a space rock approximately 800 meters wide and weighing 496 million tons, originated from a parent body formed about 1.8 to 2.9 billion years after the birth of our solar system. The asteroid's samples, brought back to Earth in December 2020, were the subject of intense study by researchers worldwide. The discovery of djerfisherite in these samples challenges the previously held belief that Ryugu's composition was largely uniform and formed entirely under low-temperature conditions.
The research team was not searching for djerfisherite when they discovered it. They were studying what changes could occur on Ryugu grain surfaces due to contact with Earth's atmosphere using microscope technology. When they examined grain number 15 from a sample plate, they discovered the unexpected mineral.
"The discovery indicates that Ryugu's past was likely much more complex than scientists had imagined," Miyahara explained. "If djerfisherite did not form on Ryugu under known conditions, then different materials must have mixed in a way that we have not yet fully understood."
The work was published in the journal Meteoritics & Planetary Science. It underscores the importance of sample-return missions like Hayabusa2 in providing direct evidence that can challenge and refine our understanding of planetary formation.
Written with the help of a news-analysis system.