Asteroid Bennu sample shows more signs of a watery past

Bennu is a roughly 0.3-mile-wide (500 meters) asteroid that orbits in near-Earth space. Scientists suspect it's a chunk of a larger asteroid that broke off due to a collision farther out. Telescope observations and data collected by NASA's OSIRIS-REx spacecraft showed that Bennu has minerals that have been altered by water. Hence, scientists suspect the asteroid's parent body accreted ice that subsequently melted after it formed around 4.5 billion years ago...

The team found various varieties of the aqueously altered minerals, including serpentine, smectite, carbonates, magnetite, sulfides, and phosphates. The minerals are present as individual particles and as crusts coating other materials.

Cosmochemist Rhian Jones of the University of Manchester, who is a member of the Sample Analysis Team, suspects Bennu's parent body became a "muddy ball" over time, when the ices 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 boulders, also thought to be minerals that precipitated once water evaporated.

The study team also found magnesium-sodium phosphate. Lauretta says this type of phosphate is intriguing because it only forms when water has become saturated with carbonates, suggesting that pools of water persisted on Bennu's parent body for an extended time.