A surprise in NASA’s asteroid rocks hints Bennu came from an ocean world

What the team found was water-soluble magnesium-sodium phosphate in mottled stones — a mineral no one expected because it didn’t show up in any of the data the spacecraft collected when it was at the asteroid Bennu. Phosphate compounds are key for all known life, forming the backbone of DNA.

The new study’s findings, published in Meteoritics & Planetary Science, are part and parcel with the “trickster asteroid,” nicknamed as such for baffling scientists every step of the OSIRIS-Rex mission.

“The presence and state of phosphates, along with other elements and compounds on Bennu, suggest a watery past for the asteroid,” said principal investigator Dante Lauretta in a statement.

This Bennu particle, about a millimeter wide, reveals a bright crust of phosphate under a microscope.
Credit: Lauretta & Connolly et al. (2024) Meteoritics & Planetary Science

NASA’s $800 million OSIRIS-Rex mission, short for Origins, Spectral Interpretation, Resource Identification, and Security Regolith Explorer, launched in 2016. The robotic spacecraft completed its 4-billion-mile trip when it dropped the capsule from 63,000 miles above Earth onto a patch of Utah desert last year. It’s the first U.S. mission to grab a sample of an asteroid. These are the most significant space souvenirs NASA has obtained since the Apollo moon rocks, gathered between 1969 and 1972.

NASA selected Bennu for the mission because it has a very remote chance of hitting Earth in the coming centuries. Learning about the asteroid could be helpful in future efforts to redirect it.

But the team also chose Bennu because it is chock-full of carbon, meaning it might contain the chemical origins of life. Some of its mineral fragments could be older than the 4.5 billion-year-old solar system. These grains of stardust could have come from dying stars or supernovas that eventually led to the formation of the sun and planets.

NASA has made the asteroid Bennu samples collected during the OSIRIS-Rex mission available to scientists around the world for research.
Credit: NASA / Erika Blumenfeld and Joseph Aebersold

All forms of Earth life have specific chemicals in their makeup, such as amino acids and sugars. Scientists have known that asteroids hold molecules believed to be the precursors to these chemicals. That’s why many suspect space rocks were responsible for bringing them to the planet through collisions in ancient cosmic history. By studying the Bennu samples, they hope to gain more insight into how these ingredients could have evolved.

“What I want to know is how do you go from a simple carbon molecule, like methane, which is a natural gas, to something like amino acids, which make our proteins, or nucleic acid, which makes up our genetic material,” Lauretta said last year.

His dream discovery would be evidence of amino acids starting to link together through chemical bonds to form a chain, known as a peptide, signaling protein evolution.

Bennu is the shape of a droplet and made of gravel and boulders barely held together by their own microgravity.
Credit: NASA Goddard Space Flight Center / University of Arizona

The mission succeeded in bringing home about a half-cup of crushed rocks and dirt. So far researchers haven’t been disappointed with their bounty.

The sample is rich in nitrogen and carbon, essential ingredients for life. The team’s early analysis has found lots of clay minerals, particularly serpentine. This is similar to the type of rocks found at mid-ocean ridges on Earth, where geologists think the recipe for life may have begun for our planet.

The magnesium–sodium phosphate in the Bennu sample resembles sodium phosphates on Enceladus. This Saturn moon is wrapped in a saltwater ocean under ice and known to shoot enormous geysers into space. Similar phosphate-enriched fluids are found in Earth’s soda lakes, such as Last Chance and Goodenough in Canada.

The OSIRIS-Rex sample return capsule after it landed on Earth.
Credit: NASA / Keegan Barber

In the new OSIRIS-Rex paper, scientists suggest a “possible link” between Bennu and Enceladus, but this idea would require more investigation to prove. The research on the sample has only begun to scratch the surface.

“These findings underscore the importance of collecting and studying material from asteroids like Bennu,” Lauretta said, “especially low-density material that would typically burn up (if it were to enter) Earth’s atmosphere.”