Did asteroid Bennu originate from an ancient ocean world?

Four years ago, a NASA spacecraft landed on near-Earth asteroid Bennu.

The minivan-sized OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) spacecraft was sent there to collect dust and rock samples and returned with 121.6 grams of pristine samples from Bennu last September, following a seven-year roundtrip voyage through space.

It is the most extensive sample of an asteroid brought back to our planet.

Based on the findings, a scientific paper published on June 26 in the monthly peer-reviewed journal Meteoritics & Planetary Science revealed an exciting discovery — Bennu, a rare carbonaceous asteroid thought to be four and a half billion years old, may have broken off from an ancient, small, primitive ocean world.

"OSIRIS-REx gave us exactly what we hoped: a large pristine asteroid sample rich in nitrogen and carbon from a formerly wet world," said Jason Dworkin, the paper co-author, in a statement on June 26.

According to scientists, Bennu, situated more than 160 million kilometres from Earth and with a body shaped like an acorn that developed in the early stages of our solar system, may contain important information about the beginnings of life on Earth.

The OSIRIS-REx Sample Analysis Team, which has been studying the samples to uncover any secrets they may hold about the asteroid's makeup, initially suggested last October that the asteroid's rock and dust are rich in carbon and include clay minerals that hold water.

Dworkin mentioned then that the sample contains almost 5 percent carbon by weight, representing one of the highest carbon concentrations discovered in an asteroid.

Surprise finding

In the most recent analysis, the researchers say the sample contains not only carbon, nitrogen, and organic compounds, all crucial for supporting life, but also magnesium-sodium phosphate. The team was surprised by this finding, as it was not observed in the spacecraft's remote sensing data from Bennu.

The latest discovery led researchers to believe the asteroid may have come from an ancient ocean world, which no longer exists.

"The presence and state of phosphates, along with other elements and compounds on Bennu, suggest a watery past for the asteroid," said Dante Lauretta, co-lead author of the paper and principal investigator for OSIRIS-REx at the University of Arizona, Tucson. "Bennu potentially could have once been part of a wetter world. However, this hypothesis requires further investigation."

The sample mainly comprises clay minerals like serpentine, similar to the rocks found at mid-ocean ridges. This kind of rock forms when material from the Earth's mantle meets water, NASA says, and besides forming clay, this interaction also creates various minerals like carbonates, iron oxides, and iron sulphides.

The most unexpected find is the presence of phosphates that dissolve in water, which are components of the biochemistry of all known life on Earth, it added.

The magnesium-sodium phosphate discovered on the Bennu asteroid sample is unique because it is very pure and has more prominent grains than the samples that come from other asteroids, according to the space agency. This differentiates it from a similar phosphate found in the Ryugu asteroid sample that Japan's Hayabusa2 mission brought in in 2020.

Ancient samples

Associate Professor Nick Timms from Curtin's School of Earth and Planetary Sciences, a member of the OSIRIS-REx Sample Analysis Team, has highlighted the unique pristine condition of the material collected from Bennu.

"Analyses show Bennu is among the most chemically primitive materials known, similar in composition to the visible surface of the sun," Timms said. "This indicates Bennu has undergone different processes to the planets, and these processes changed the abundance of particular elements relative to the sun."

Even though Bennu might have had contact with water in the past, it is still a chemically basic asteroid with elemental compositions that closely match those of the Sun, NASA said. "The sample we returned is the largest reservoir of unaltered asteroid material on Earth right now," said Lauretta.

This means the samples give scientists a better peek into the initial stages of our solar system, more than four and a half billion years in the past, as these rocks have preserved their original form, without undergoing any melting or re-solidification since their formation, unlike meteorites that have fallen to Earth, confirming their ancient roots.

Elements of life

Plus, carbon and nitrogen, which the team confirmed are abundant in the samples from Bennu, play a crucial role in determining the origins of the asteroid’s materials and the chemical processes that led to the formation of complex molecules, possibly creating the conditions necessary for life on Earth.

"These findings underscore the importance of collecting and studying material from asteroids like Bennu — especially low-density material that would typically burn up upon entering Earth's atmosphere," said Lauretta. "This material holds the key to unravelling the intricate processes of solar system formation and the prebiotic chemistry that could have contributed to life emerging on Earth."

After the OSIRIS-REx spacecraft returned to Earth last year, specimens were distributed to research labs internationally. In the coming months, NASA's Johnson Space Center in Houston will be sending portions of the Bennu sample to numerous labs in the US and across the globe, according to the agency.

"The Bennu samples are tantalisingly beautiful extraterrestrial rocks," said Harold Connolly, co-lead author on the paper and OSIRIS-REx mission sample scientist at Rowan University in Glassboro, New Jersey.

"Each week, analysis by the OSIRIS-REx Sample Analysis Team provides new and sometimes surprising findings that are helping place important constraints on the origin and evolution of Earth-like planets."