Crews process NASA's OSIRIS-REx asteroid Bennu sample return capsule in a clean room in 2023. Credit: NASA.
Bennu is an asteroid about 500 meters (1,640 feet) wide that orbits in near-Earth space. Scientists suspect that it is a piece of a larger asteroid that broke off due to a more distant collision. Telescope observations and data collected by NASA's OSIRIS-REx spacecraft showed that Bennu has minerals that have been altered by water. Therefore, scientists suspect that the parent body of the asteroid accumulated ice that subsequently melted after forming about 4.5 billion years ago.
Now, a team of researchers has completed a preliminary analysis of a sample of Bennu that the probe brought back to Earth last fall. They published their findings in the journal Meteorites and planetary scienceThe results point, in fact, to a watery past reminiscent of the processes taking place on Earth and on the icy moons of the solar system.
The group examined 14.9 of the 121.6 grams of the total material brought back to Earth. The study sample included aggregates of fine and coarse particles, and stones up to 3.5 cm wide. What they found is that most of the minerals are those produced when silicate materials interact with water, especially alkaline fluids.
“Think carbonated water with lots of carbon dioxide,” says lead author and planetary scientist Dante Lauretta of the University of Arizona, who is also the principal investigator for OSIRIS-REx.
Related: Bennu material shows signs of building blocks needed for life
A muddy asteroid called Bennu
The team found several varieties of water-altered minerals, including serpentine, smectite, carbonates, magnetite, sulfides and phosphates. The minerals are present as individual particles and as crusts covering other materials.
Cosmochemist Rhian Jones of the University of Manchester, who is a member of the sample analysis team, suspects that Bennu's parent body turned into a “mudball” over time as the ice melted.
The study team also found evidence of fluid flow. In particular, some of the phyllosilicates had filled tiny fractures that look like veins in the rocks. Images taken by OSIRIS-REx also show meter-long veins in the rocks, which are also thought to be minerals that precipitated out once the water evaporated.
The study team also found magnesium-sodium phosphate. Lauretta says this type of phosphate is intriguing because it only forms when water becomes saturated with carbonates, suggesting that pools of water persisted on Bennu's parent body for a long time.
“Depending on the ratio of ice to rock, there could have been an ocean or an ice-covered lake, like the icy satellites in the outer solar system,” he says. The researchers note that sodium phosphate has been detected in ice grains in plumes of smoke rising from the subsurface ocean on Saturn’s moon Enceladus.
Related: Bennu sample has rocks unlike any meteorite found so far
The team also found that some of the particles have irregular shapes reminiscent of cauliflower. Similar rocks found in southern Italy could have formed in areas that were exposed to shallow seas, they note. Jones says the magnesium-sodium phosphate indicates a “very, very evolved fluid system,” similar to the salty brines found in evaporating lakes on Earth’s surface.
Phyllosilicate deposits are also found at hydrothermal vents on mid-ocean ridges, where plumes of hot seawater emerge. Some of the particles in the Bennu sample have angular or sharp edges that could have formed in a similar high-pressure environment, the researchers note.
Bennu's parent body
Future analyses of the sample could also help trace a timeline of hydrothermal processes that took place on Bennu’s parent body. One possibility is that heat from radioactive decay within that body began melting ice soon after it formed, and that the heat and aqueous alteration persisted for millions of years.
Geochemist Lucy McGee of the University of Adelaide, who was not involved in the sample analysis, says she is curious about the possibility that there may have been multiple episodes of heating and fluid alteration, particularly in producing the veins.
“Here in Australia, we have ancient rocks that have overlays of different processes that occurred over time,” he says.
Such heat could have been generated by impacts or by the gravity of larger bodies churning the asteroid's interior.
Lauretta says he also looks forward to future analyses that will look at the ages of phyllosilicates in the sample to get a sense of when fluid alteration occurred on Bennu's parent body. Other efforts include finding methods to determine whether Bennu's phyllosilicates formed under higher-pressure conditions, such as at hydrothermal vents on Earth.
For now, Bennu joins the list of previously discovered vents where hydrothermal activity produces spectacular features such as the icy geysers on Enceladus, the Lost City Hydrothermal Field in the Atlantic Ocean and soda lakes on Earth.
“What we're seeing is that these water systems are quite common throughout the solar system,” Lauretta says.
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