Asteroid samples returned to Earth by the Japanese Hayabusa2 spacecraft contain organic compounds that challenge prevailing theories on the origins of such molecules in our solar system, according to new research published this week in the journal Science.
Key Findings From Asteroid Ryugu Samples
The Hayabusa2 spacecraft collected subsurface material from asteroid Ryugu and returned it to Earth in 2020. Analysis of the pristine samples by an international team revealed the presence of various organic compounds, including some that have rarely been found in extraterrestrial materials before.
Some of the key findings include:
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Detection of polycyclic aromatic hydrocarbons (PAHs) containing aliphatic groups (side chains), which are very rare in extraterrestrial samples. Some of these compounds have never been reported in meteorites or asteroid samples previously.
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The organic matter is rich in deuterium, indicating it formed in very cold conditions below -250°C. This suggests the compounds were created in cold interstellar molecular clouds, prior to the formation of the solar system.
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The minerals and organic compounds show signs of exposure to water at some point, demonstrating asteroid Ryugu experienced aqueous alteration early in its history when the solar system was still forming.
Implications for Theories on Origins of Organic Compounds
The rare PAHs and their deuterium-rich composition challenges the prevailing assumption that most organic compounds in primitive asteroids were formed by chemical reactions in the hot inner solar system near the protosun.
“These results reopen the debate about where and how complex organics formed in the early solar system,” said lead author Dr. Queenie Chan from the University of Hong Kong.
There are two leading hypotheses:
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Organic synthesis in the hot inner solar system during its formation.
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Delivery from cold interstellar molecular clouds.
The Ryugu samples provide some of the first solid evidence for the second theory.
“It seems that at least some extraterrestrial organics actually formed prior to the solar system, and were later incorporated into planetesimals like Ryugu,” explained co-author Dr. Ashley King from the Carnegie Institution for Science.
Next Steps for Research
More analyses are underway to better understand the origins and chemistry of Ryugu’s organics.
There are also plans to compare these findings with samples from other asteroids like Bennu, to determine if the organics share common interstellar sources or formation conditions.
“Furtherisotopic analyses could help clarify their exact origins, and how widely these exotic organics are distributed in the solar system,” Chan said.
Additionally, some researchers think the presence of complex organics increases astrobiological potential for asteroids like Ryugu.
They recommend including astrobiology objectives in upcoming sample return missions targeting carbon-rich asteroids. This could uncover more clues about abiotic organic synthesis in space, and help narrow down sources of prebiotic compounds that may have seeded early Earth.
Table Summary of Key Discoveries in Ryugu Samples
| Organic Compound | Properties | Interpretation |
|---|---|---|
| Deuterium-rich PAHs with aliphatic side chains | Very rare in meteorites Formed at < -250°C |
Interstellar origin Formed prior to solar system |
| Amino acids | Some mirror earthly biology | Formed by aqueous alteration early in solar system |
| Complex macromolecular organics | Ancient age based on isotope studies | Interstellar sources incorporated into the asteroid |
The rare and exotic organics found on Ryugu open up new questions about the timeline and mechanisms for abiotic synthesis of complex prebiotic compounds in space. As Chan summarized:
“These findings will reshape our ideas about the chemistry that preceded life.”
Conclusion
The exceptional organic diversity in the returned Ryugu samples challenges prevailing theories about where such compounds originated, indicating interstellar cold molecular clouds as a source. Researchers also found evidence the material experienced early aqueous processing, and contains organics with biological relevance.
As one of the most pristine astromaterials ever analyzed, Ryugu’s cargo of exotic carbon chemistry contributes vital clues about abiotic synthesis pathways crucial to the origins of life. While further work is needed, these discoveries reopen exciting debates about the formation of complex prebiotic compounds in interstellar space and the early solar system.
Upcoming analyses and sample return missions targeting asteroids and comets will uncover more pieces in this cosmic puzzle – bringing us closer to understanding our beginnings, and life’s beginnings throughout the universe.
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