Samples from the Bennu asteroid, which is currently more than 200 million miles from Earth, and Murchison meteorite, which fell to Earth in 1969, both contain molecules that build proteins necessary for life. New research has found that the molecules may have formed through the same process but originated far from one another in the solar system. Using a new technique to look at isotopes, or slight variations in the mass of the molecular compounds' atoms, a team including researchers at Penn State studied the molecules, called amino acids, from a sample of Bennu, delivered in 2023 by NASA's OSIRIS-REx mission, and from a sample of Murchison. The Bennu sample, direct from space and without terrestrial contamination, gave insight into where these molecules might have formed.
According to the researchers, who published their findings in Nature Communications Earth & Environment, the wide range of isotope values measured within the amino acids in the Bennu asteroid and Murchison meteorite samples were surprising and suggest that carbon and nitrogen atoms within the molecules originated in different parts of the solar system.
"Amino acids are the building blocks of proteins and peptides, but they are also found in carbon-rich meteorites and asteroid samples, so they can provide a direct link between space chemistry and the origins of life on Earth," said Ophélie Mcintosh, former postdoctoral researcher in Penn State's Department of Geosciences and lead author on the paper. "Most studies focus on the distribution and quantification of amino acids, so our question about how they're formed is quite new. We used very unique isotope instruments, including one that exists only at Penn State, to help answer those questions."
To understand how these molecules were formed, researchers measured the isotopes, finding clues to the chemicals the isotope came from and how it ended up in the molecule under study.
"Isotope ratios provide a measurable tag that can link molecules to specific reservoirs or processes," said Christopher House, professor of geosciences and co-author on the paper. "For our study, isotopes provided clues to an older history for specific atoms in a molecule."
The team previously looked at glycine, the simplest and most abundant amino acid, and found that the glycine on Bennu may have formed in a harsh, cold environment - which goes against the prevailing idea that the building blocks of life formed under warm and wet conditions. This time, they focused on the amino acid beta-alanine. Beta-alanine is not one of the 20 amino acids that form proteins, but it is naturally produced in the human body and can combine with other amino acids to make peptides that can act as molecular enzymes. It is also one of the most abundant amino acids found on asteroid and meteorite samples and can provide important context about prebiotic organic compounds.
"Beta-alanine is a prominent example of the amazingly diverse collection of amino acid-like molecules found in space samples," said Katherine Freeman, Evan Pugh University Professor, Dr. Hubert Barnes and Dr. Mary Barnes Professor of Geosciences, and the principal investigator on the project. "It can help us understand why there are so many different types of these compounds and where and how they were made. Ultimately, this helps us understand ways that space organics potentially shaped the unique chemistry in life."
According to the researchers, most isotopic analyses take an average of the isotopes - in this case carbon isotopes - in the molecule. To study the Bennu sample, the researchers developed a new technique to look at the individual carbon isotopes within the molecule.
"This intramolecular information provides a much more detailed isotopic fingerprint," Mcintosh said. "Instead of simply asking whether a molecule is enriched or depleted in a particular isotope, we can investigate how that isotope is distributed within the molecule and use this pattern to distinguish between possible formation pathways."
Two pieces of equipment make these measurements possible. The pico-CSIA, only at Penn State, takes measurements of isotopic averages of incredibly small amounts of material, which is necessary when working with small, rare samples, the researchers said. And a modified GC-Orbitrap measures the isotope compositions at different positions within the molecule.
"Being able to analyze the isotope values for each carbon and nitrogen within the beta-alanine molecule is a dream for geochemistry," House said. "And here it is coupled to the ability to study a pristine asteroid sample returned to Earth by NASA."
The researchers also measured beta-alanine in a sample from the Murchison meteorite to see how the intramolecular isotope measurements compared with the sample from Bennu.
"When you compare the Bennu and Murchison samples, the carbon isotope average is so close that when you factor in margin of error, they're basically the same," Mcintosh said. "But when you look at intramolecular carbon, they're completely different."
Mcintosh explained that out of the three carbons in beta-alanine, one measured much higher in subatomic particles called neutrons while the other two were very depleted in both samples, but the specific values were distinctly different. In fact, the enriched Bennu carbon measured as the highest ever recorded isotope anomaly in an organic molecule from space, highlighting the extreme difference between enriched and depleted carbons in the samples.
"I was genuinely gobsmacked by the enormous isotope difference between two neighboring atom positions within the same molecule," Freeman said. "The preservation of such startling differences is an indicator of the primordial nature of these molecules. We are working with molecules formed at the very dawn of the solar system, possibly even before the formation of Earth."
