Unexpected Mineral Discovery in Ryugu Asteroid Grain Challenges Solar System Formation Theories

A surprising discovery within a grain of the Ryugu asteroid is prompting scientists to rethink established theories about the formation of our solar system. Researchers at Hiroshima University have identified the mineral djerfisherite, a potassium-containing iron-nickel sulfide, in a sample returned by the Hayabusa2 mission. This finding,published in the journal *Meteoritics & Planetary Science* on May 28,2025,challenges the prevailing understanding of asteroid composition and the conditions present during the early solar system.

The Unexpected Find: Djerfisherite in Ryugu

Djerfisherite is typically found in highly reduced environments, such as those associated with enstatite chondrites, a type of meteorite believed to have formed in the inner solar system. Its presence in a Ryugu grain, which is composed of materials similar to CI chondrites (carbon-rich meteorites that have undergone extensive aqueous alteration), is entirely unexpected. According to Associate Professor Masaaki Miyahara, the led author of the study, this discovery is akin to “finding a tropical seed in Arctic ice,” suggesting either an unusual local environment or long-distance transport in the early solar system.

Did You Know? The Hayabusa2 mission successfully delivered pristine samples from asteroid Ryugu to Earth on December 6, 2020, marking a major milestone in asteroid exploration.

Implications for Understanding Asteroid Formation

The discovery of djerfisherite raises significant questions about the uniformity of Ryugu’s composition and the processes that shaped primitive asteroids. It 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. This finding challenges the notion that Ryugu is compositionally uniform and opens new questions about the complexity of primitive asteroids.

Two Competing Hypotheses

The research team has proposed two hypotheses to explain the presence of djerfisherite in the Ryugu grain:

• External Origin: The djerfisherite arrived from another source during the formation of Ryugu’s parent body.
• Internal Formation: The djerfisherite formed intrinsically when the temperature of Ryugu was raised to above 350 ℃.

Preliminary evidence suggests that the internal formation hypothesis is more likely to be true. Though, further research is needed to confirm this.

Ryugu’s Origins and composition

Ryugu is believed to be part of a larger parent body that formed between 1.8 to 2.9 million years after the beginning of the Solar System. This parent body likely originated in the outer region of the solar system, where water and carbon dioxide existed in the form of ice. Heat generated by the decay of radioactive elements caused the ice to melt around 3 million years after its formation,with temperatures remaining below approximately 50℃.

Pro tip: Studying asteroid samples provides valuable insights into the early solar system, as asteroids are remnants from that era.

In contrast, the parent bodies of enstatite chondrites, which are known to contain djerfisherite, are believed to have formed in the inner region of the solar system. Thermodynamic calculations indicate that djerfisherite in enstatite chondrites formed directly from high-temperature gas. Hydrothermal synthesis experiments have also shown that djerfisherite can form through reactions between potassium-bearing fluids and Fe-Ni sulfides at temperatures above 350 ℃.

Future Research and the Quest for Answers

The next steps in this research involve conducting isotopic studies of the Ryugu grains, including the one containing djerfisherite, to determine their origins. by analyzing the isotopic composition of the mineral, scientists hope to gain a better understanding of its formation history and the processes that led to its presence in the Ryugu grain.The ultimate 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 other surprising discoveries might be hidden within asteroid samples? How will this new information reshape our models of the early solar system?

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