Evidence of water discovered on asteroid Bennu: NASA

Scientists have discovered evidence of water on asteroid Bennu, using data from NASA's OSIRIS-REx spacecraft which recently reached the rocky cosmic body to unravel its mysteries.

From August through early December this year, the spacecraft aimed three of its science instruments toward Bennu and began making the mission's first observations of the asteroid.

During this period, the spacecraft travelled the last 2.2 million km of its outbound journey to arrive at a spot 19 km from Bennu on December 3.

Data obtained from the spacecraft's two spectrometers reveal the presence of molecules that contain oxygen and hydrogen atoms bonded together, known as "hydroxyls."

The team suspects that these hydroxyl groups exist globally across the asteroid in water-bearing clay minerals, meaning that at some point, the rocky material interacted with water.

While Bennu itself is too small to have ever hosted liquid water, the finding indicates that liquid water was present at some time on Bennu's parent body, a much larger asteroid.

"This finding may provide an important link between what we think happened in space with asteroids like Bennu and what we see in the meteorites that scientists study in the lab," said Ellen Howell, senior research scientist at University of Arizona's Lunar and Planetary Laboratory (LPL) in the US.

"It is very exciting to see these hydrated minerals distributed across Bennu's surface, because it suggests they are an intrinsic part of Bennu's composition, not just sprinkled on its surface by an impactor," said Howell.

"The presence of hydrated minerals across the asteroid confirms that Bennu, a remnant from early in the formation of the solar system, is an excellent specimen for the OSIRIS-REx mission to study the composition of primitive volatiles and organics," said Amy Simon, OVIRS Deputy Instrument Scientist at NASA Goddard Space Flight Center.

Data obtained from the OSIRIS-REx Camera Suite (OCAMS) corroborate ground-based radar observations of Bennu and confirm that the original model -- developed in 2013 by OSIRIS-REx Science Team Chief Michael Nolan, now based at LPL, and collaborators -- closely predicted the asteroid's actual shape.

Bennu's diameter, rotation rate, inclination and overall shape presented almost exactly as projected.

"Radar observations don't give us any information about colours or brightness of the object, so it is really interesting to see the asteroid up close through the eyes of OSIRIS-REx," Nolan said.

"As we are getting more details, we are figuring out where the craters and boulders are, and we were very pleasantly surprised that virtually every little bump we saw in our radar image back then is actually really there," he said.

The mission team used this ground-based Bennu model when designing the OSIRIS-REx mission.

The accuracy of the model means that the mission, spacecraft, and planned observations were appropriately designed for the tasks ahead at Bennu.

"Our initial data show that the team picked the right asteroid as the target of the OSIRIS-REx mission. We have not discovered any insurmountable issues at Bennu so far," said Dante Lauretta, OSIRIS-REx principal investigator.

"The spacecraft is healthy and the science instruments are working better than required. It is time now for our adventure to begin," Lauretta said.