Bennu Amino Acids May Have Formed in Ice

A tiny amount of rock collected from asteroid Bennu contains amino acids, a remarkable finding in itself: it shows that some of the ingredients required for life were already being “cooked” long before Earth could support life.

A fresh analysis now suggests that some of these amino acids were not created in sheltered pools of warm water. Instead, they may have formed in ice, exposed to radiation, amid the deep freeze of the early solar system.

Researchers led by Penn State have found evidence that Bennu’s amino acids – glycine in particular – bear an isotopic signature consistent with a colder, more severe origin than scientists had often expected.

A different recipe for amino acids

For many years, the prevailing explanation has been that amino acids developed within asteroids when liquid water was available, enabling particular chemical reactions to take place.

That interpretation remains valid for many meteorites. However, the researchers say Bennu appears to have taken a different route.

“Our results flip the script on how we have typically thought amino acids formed in asteroids,” said Allison Baczynski, an assistant research professor of geosciences at Penn State and co-lead author on the paper.

“It now looks like there are many conditions where these building blocks of life can form, not just when there’s warm liquid water. Our analysis showed that there’s much more diversity in the pathways and conditions in which these amino acids can be formed.”

The crucial message is not that chemistry involving warm water is incorrect. Rather, it may represent just one possibility among a much wider range.

Bennu’s chemistry indicates that these molecules may also be produced in frozen settings, provided the necessary ingredients are present alongside radiation capable of driving reactions.

Why glycine gets so much attention

The researchers examined glycine in detail, as it is the simplest amino acid. Small, widespread and chemically adaptable, it is precisely the sort of molecule expected to emerge early while nature experiments with prebiotic chemistry.

Glycine also serves as a chemical clue. Its presence in comets or asteroids supports the possibility that some raw materials for life formed in space before arriving on the young Earth through impacts.

This does not imply that life itself originated in space. It does, however, indicate that early Earth may have received supplies of useful components – parts that biology could subsequently combine into more complex forms.

To establish how glycine on Bennu formed, the team studied isotopes, which are slight differences in atomic mass that work as chemical fingerprints.

Such fingerprints can retain evidence of the surroundings and reactions responsible for making a molecule.

Reading isotopic fingerprints in dust

Using a sample smaller than a teaspoon, the researchers relied on customised instruments built to identify isotopic patterns in exceptionally tiny quantities of organic matter.

This is highly significant in practical terms because samples from space are valuable, while the relevant compounds may occur at extremely low concentrations.

“Here at Penn State, we have modified instrumentation that allows us to make isotopic measurements on really low abundances of organic compounds like glycine,” Baczynski said.

“Without advances in technology and investment in specialized instrumentation, we would have never made this discovery.”

The results supported a scenario in which Bennu’s glycine formed in radiation-exposed ice. That is consistent with regions of the early solar system where cold conditions prevailed, even though radioactive elements and energetic processes could still propel chemistry onwards.

Amino acids in space rocks

To interpret Bennu’s results, the team compared its amino acids with those in the Murchison meteorite, a well-known carbon-rich meteorite that landed in Australia in 1969 and has been examined for decades.

Murchison has long provided a foundation for studying amino acids in space rocks. Its chemistry frequently aligns with models involving liquid water and comparatively gentle conditions on its parent body.

Bennu did not follow the same pattern. The isotopic “accent” of its amino acids differed, indicating that its parent body developed in another chemical region of the solar system.

“One of the reasons why amino acids are so important is because we think that they played a big role in how life started on Earth,” said Ophélie McIntosh, a postdoctoral researcher at Penn State and co-lead author on the paper.

“What’s a real surprise is that the amino acids in Bennu show a much different isotopic pattern than those in Murchison, and these results suggest that Bennu and Murchison’s parent bodies likely originated in chemically distinct regions of the solar system.”

This distinction is especially important. If separate areas of the early solar system generated amino acids through different processes, a rock’s birthplace could affect the type of prebiotic chemistry it contains.

Put another way, amino acids made in space may not have a single universal origin story.

Mirror-image molecules that don’t match

The research also introduces a puzzling new question. Many amino acids exist in two mirror-image versions, comparable to left and right hands.

Although chemically almost identical, these forms can behave very differently in biology. Life on Earth, for instance, generally favours particular amino-acid “handedness”.

A longstanding belief has held that when both mirror forms of an amino acid are present, their isotopic signatures should usually be alike because they arose under identical conditions.

Yet Bennu produced an unexpected result: two mirror-image forms of glutamic acid showed very different nitrogen isotopic values.

The study therefore poses an evident question: how can “left” and “right” versions emerge with what appear to be different histories?

The team does not yet offer an explanation, but the finding is exactly the kind of evidence that could make scientists reconsider how these molecules develop and change in space environments.

More samples, more surprises

Rather than neatly resolving the story, the researchers effectively conclude that the puzzle has become larger.

Bennu points to one set of amino-acid-forming conditions, while Murchison points to another. The solar system probably accommodated many more.

“We have more questions now than answers,” Baczynski said.

“We hope that we can continue to analyze a range of different meteorites to look at their amino acids. We want to know if they continue to look like Murchison and Bennu, or maybe there is even more diversity in the conditions and pathways that can create the building blocks of life.”

Bennu therefore does more than place one additional point on the map. It suggests that the map itself may be considerably larger than previously thought.

If amino acids can arise in both warm-water environments and icy, irradiated settings, the early solar system may have been a much more inventive chemistry laboratory than the traditional account suggests.

Image Credit: NASA