With the return of an Asteroid Bennu sample in 2023 as part of NASA’s OSIRIS-REx mission, researchers from the University of Arizona have made significant discoveries toward understanding the origins of life and the earliest history of our solar system.

The most recent studies include findings from the subjects of mineralogy and organics, but all point to the presence of life’s building blocks in Bennu’s samples.

Asteroid Bennu today is characterized as a carbon-rich, rubble pile asteroid created through impacts, according to UA assistant professor Jessica Barnes. However, according to Barnes, Bennu’s parent body was likely created about 4.5 billion years ago through an accretion of ice, dust and organic matter. 

According to Barnes, because it was created early in our solar system’s history, the parent body accreted with short-lived radionuclides — isotopes that decayed fast and could produce heat. This heat melted the ice on the parent body but not the rocky material, and so the fluid this made started to interact with the accreted material, like the dust, which altered and formed new minerals. 

This process likely occurred within the first 10 million years of our solar system’s history. 

Early on, the parent body was also catastrophically disrupted and then re-accreted together. According to Barnes, within the last billion years, scientists think it was disrupted again, ultimately creating the near-Earth asteroid known as Bennu.

Researchers have discovered minerals never seen before in meteorite samples or samples returned from other asteroids.

“Those minerals include things like sodium carbonates [and] phosphates. We’re seeing salt, like sodium chloride [and] potassium chloride, and we’re seeing those things for the first time in a material that are analogs of Bennu,” Barnes said.

While meteorites have similar bulk chemical compositions to those found on Bennu, to current knowledge, these don’t contain the same suite of minerals as those in the Bennu samples, according to Barnes. Researchers also have not seen this mineral abundance in extraterrestrial materials before.

Researchers questioned why these minerals weren’t seen in meteorites. 

The difference was that only in the last four years have researchers had samples returned from asteroids, samples that were preserved, according to Barnes. Researchers found that when meteorite samples interacted with Earth’s biosphere, while this interaction might add salts, the meteorites also appeared to have lost some salts — like the evaporite minerals. 

“Some meteorites are so highly reactive that shortly after arriving on Earth, they start altering to different forms of minerals,” said Tom Zega, professor of planetary science at UA. 

“We’ve made assumptions based on how asteroids form and evolve their chemistry, how they accreted, when they accreted, [and] we’ve made these predictions based on meteorites, but some of those predictions might be faulty because those meteorites inherently fall to Earth through the atmosphere,” Barnes said.

The return samples also indicate an abundance of the type of mineral called sheet silicates or phyllosilicates. These minerals have water, one of the key ingredients to life, as a part of their crystal structures, according to Zega.

“The fact that we find them [water-bearing silicates] in major abundance in asteroid Bennu suggests that at some point in its history, the asteroid itself had an active hydrothermal system, much like we have here on Earth,” Zega said.

Likewise, through meteors, meteorites and comets, some researchers believe that Earth got its water or that life could have been seeded through these objects delivering these precursor compounds, according to Zega 

The samples also provide insight into the fluid circulating on Bennu’s parent body. 

Researchers originally thought that it was just water-rich or aqueous fluids circulating on parent bodies, and they knew that Bennu accreted water ice due to the hydrated clays in the Bennu samples, according to Barnes. 

What was missing in the previous meteorite collection was knowledge of what happens after the circulation of aqueous fluid.

“Through evaporation or precipitation of minerals or even freezing, you can then form a brine, which is a salty fluid, and from that, you can precipitate the sodium carbonates and magnesium phosphates, other types of salts that we now see in the Bennu samples,” she said.

According to the Nature Astronomy paper on this ancient brine, “Brines […] are environments in which life could have evolved or might persist in the Solar System.”

But it’s not just these occurrences of salt minerals in Bennu samples. Researchers are seeing the organic makeup as well.

According to the Nature Astronomy paper on the organic makeup of Bennu Samples, “Bennu samples are volatile rich, with more carbon, nitrogen and ammonia than samples from asteroid Ryugu and most meteorites. […]  We detected amino acids (including 14 of the 20 used in terrestrial biology), amines, formaldehyde, carboxylic acids, polycyclic aromatic hydrocarbons and N-heterocycles (including all five nucleobases found in DNA and RNA).”

“We have higher ammonia abundances and isotopically different nitrogen in the Bennu samples than compared to other astromaterials, meteorite samples or [Asteroid] Ryugu samples,” Barnes said.

Researchers are seeing evidence for these minerals like phosphates and sodium potassium carbonates, and gas like ammonia, on bodies that have astrobiological significance — targets for potentially habitable environments, according to Barnes. 

“These studies are tying, and provide important groundwork for future exploration of potentially habitable worlds,” Barnes said. “These are all the [abiotic] ingredients that, if you put them in the right environment […] they can lead to, you know, biology.”

Barnes believes this science might get us closer to understanding what caused Earth to start with certain ingredients and evolve to have life.

Going forward, future studies will delve further into detailed analytical work, like examining some of these materials at the atomic level to piece together their chemical origins, according to Zega.

Research is still ongoing for the Bennu samples, but these new scientific inquiries, discoveries and additions to basic research wouldn’t have been possible without sample return missions themselves, Barnes said. Furthermore, preserving Bennu samples provides materials for future generations to study.

“The samples that come back are the gifts that keep giving,” Barnes said. 

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