NASA’s Asteroid Samples Reveal Bennu Had Water-Rich Past, May Have Originated From Small Ocean World

The dirt and rocks that NASA collected indicate that asteroid Bennu is from a hydrothermally active body with possibly even subsurface lakes or oceans.

Mineral Composition of Asteroid Bennu

On September 8, 2016, the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) spacecraft was launched to an asteroid known as Bennu. Formerly called 1999 RQ36, the space rock is an asteroid as large as 0.3 miles (500 meters) wide dwelling in near-Earth space.

The spacecraft took a sample from rocks and dust on Bennu's surface on October 20, 2020, which was delivered to Earth on September 24, 2023. With the drop-off in a capsule via Earth's atmosphere, OSIRIS-REx is forwarded for a new mission to explore asteroid Apophis.

Asteroid Bennu is thought to be a piece of some larger asteroid, which had been broken due to a further-out collision. Based on telescope observations, it was predicted that Bennu had minerals altered by water.

From the data collected, astronomers assumed that the parent body of Bennu had accreted ice, which melted after its formation about 4.5 billion years ago.

Just recently, a team of scientists conducted an investigation of a sample from Bennu brought back to Earth by OSIRIS-REx. Their work was detailed in a paper entitled "Asteroid (101955) Bennu in the laboratory: Properties of the sample collected by OSIRIS-REx."

Led by Dante S. Lauretta of the University of Arizona, the team examined a sample of 14.9 from the 121.6 grams of the return samples brought to Earth. The study sample consisted of aggregates of fine and coarse particles, as well as stones up to 3.5 cm wide.

Different types of aqueously altered minerals were identified by Lauretta and his colleagues, and they included carbonates, smectite, serpentine, magnetite, phosphates, and sulfides. The identified minerals occur either as discrete particles or as coats on other materials.

The researchers identified that most of the minerals are formed through the silicate material reaction with water, mainly alkaline fluids.

The researchers also discovered some evidence of fluid flow. Some small fractures are filled with evidence of phyllosilicate, which are vein-like in rocks. Images captured by OSIRIS-REx also show meter-long-veins in boulders, thought to be minerals that precipitated once water evaporated.

Insight into Asteroid Bennu's Origins

The team also found magnesium-sodium phosphate, which is interesting because it only forms when water becomes saturated with carbonates- a process that tells researchers pools of water persisted on Bennu's parent body for quite some time.

It could be that Bennu's parent body turned into a "muddy ball" due to the melting of ice, says University of Manchester cosmochemist Rhian Jones. One possibility is that heat from radioactive decay within the body started melting ices soon after that body formed-with the heating and aqueous alteration persisting for millions of years.

Meanwhile, geochemist Lucy McGee of the University of Adelaide suggested that there may have been several episodes of heating and fluid alteration, particularly in producing the veins.

According to the researchers, the watery past of Bennu mirrors processes taking place on Earth and icy moons in the Solar System. Future analyses of the sample could help put a timeline on the hydrothermal processes that took place on Bennu's parent body.