Overturning Warm Water Assumptions (Image Credits: Pixabay)
Analysis of microscopic grains from the 4.6-billion-year-old Bennu asteroid has revealed that essential amino acids likely originated in the cold, irradiated outer reaches of the early solar system.[1][2]
Overturning Warm Water Assumptions
Scientists long assumed amino acids on asteroids arose mainly through reactions in liquid water, but Bennu samples tell a different story. A Penn State-led team examined a teaspoon-sized portion of material returned by NASA’s OSIRIS-REx mission in 2023. Their findings, published this week in the Proceedings of the National Academy of Sciences, point to formation in primordial ices exposed to intense radiation.[3]
The study centered on glycine, the simplest amino acid and a foundational molecule for proteins. Glycine appeared in Bennu alongside 18 other amino acids, all confirmed as extraterrestrial through elevated carbon-13 and nitrogen-15 levels. These isotopic signatures suggested processes far removed from the mild, watery conditions previously favored.[4]
Precision Isotope Probing Unlocks Secrets
Custom instruments at Penn State enabled measurements on scant organic traces in the samples. Researchers quantified carbon and nitrogen isotopes, revealing patterns inconsistent with traditional Strecker synthesis, which requires hydrogen cyanide, ammonia, and aldehydes reacting in water. Instead, data supported radical reactions in frozen mixtures of water, methanol, hydrogen cyanide, and ammonia under ultraviolet or cosmic ray bombardment.[1]
This icy radiation chemistry occurred before Bennu’s parent body accreted, preserving the molecules through later alterations. The approach highlighted technological advances, as co-lead author Allison Baczynski noted: “Without advances in technology and investment in specialized instrumentation, we would have never made this discovery.”[5]
Stark Contrasts with Murchison Meteorite
Bennu’s profile diverged sharply from the Murchison meteorite, which fell in Australia in 1969. Murchison’s amino acids matched aqueous origins on its parent body, with glycine showing carbon-13 enrichment in the alpha carbon position. Bennu glycine displayed uniform intramolecular carbon isotopes, aligning with ice-based formation.
| Amino Acid | Bennu δ¹³C (‰) | Murchison δ¹³C (‰) |
|---|---|---|
| Glycine (molecular avg.) | +21 ± 6 | +21 ± 3 |
| Glycine Cα | +15 ± 4 | +51 ± 13 |
| Glycine COOH | +28 ± 13 | -9 ± 15 |
| β-Alanine | +12 ± 6 | +8 ± 4 |
Nitrogen isotopes further distinguished the two: Bennu values reached +277‰ in D-glutamic acid, far exceeding Murchison’s +78‰ for glycine. These differences implied parent bodies from distinct solar system zones – one inner and watery, the other outer and frozen.[2]
- Bennu: High δ¹⁵N (+170 to +277‰), icy primordial synthesis.
- Murchison: Moderate δ¹⁵N, post-accretion aqueous processing.
- Shared molecular-average δ¹³C, but intramolecular contrasts reveal pathways.
- Bennu aldehydes/ketones depleted in ¹³C (-19 to -1‰ vs. Murchison’s broader range).
Mysteries in Mirror Images and Future Probes
An enigma emerged in glutamic acid’s enantiomers, the left- and right-handed forms typically sharing isotopes. In Bennu, D- and L-forms differed by 87‰ in nitrogen-15, defying expectations. Co-lead author Ophélie McIntosh remarked that such patterns suggest Bennu and Murchison hailed from “chemically distinct regions of the solar system.”[4]
The team plans broader meteorite surveys to map this diversity. Multiple pathways expand prospects for prebiotic chemistry across space, hinting life’s seeds scattered widely.
Key Takeaways
- Amino acids formed via ice radiation, not just water, broadening habitable conditions.
- Bennu preserves primordial signatures, unlike altered meteorites.
- Enantiomer isotope splits pose new puzzles for organic evolution.
These revelations from Bennu underscore the early solar system’s chemical versatility, potentially seeding Earth and beyond with life’s precursors. What pathways might future samples illuminate? Share your thoughts in the comments.
