NASA has revealed it has discovered water inside clays that make up the Bennu asteroid.

The asteroid is a remnant from early in the formation of the solar system.

During the OSIRIS-REx mission's approach phase, between mid-August and early December, the spacecraft traveled 2.2 million km on its journey from Earth.

During this time, the science team aimed three of the spacecraft's instruments towards Bennu and began making the first scientific observations of the asteroid.

This showed the presence of molecules that contain oxygen and hydrogen atoms bonded together, known as 'hydroxyls.'

The team suspects that theseexist across the asteroid in water-bearing clay minerals: meaning that at some point, Bennu's rocky material interacted with water.

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

This artist's concept shows the Origins Spectral Interpretation Resource Identification Security - Regolith Explorer (OSIRIS-REx) spacecraft contacting the asteroid Bennu | Image: NASA's Goddard Space Flight Centre

Amy Simon is a deputy instrument scientist at NASA's Goddard Space Flight Centre.

"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.

"When samples of this material are returned by the mission to Earth in 2023, scientists will receive a treasure trove of new information about the history and evolution of our solar system."

Bennu's surface material is a mix of very rocky, boulder-filled regions and a few relatively smooth regions that lack boulders.

However, the quantity of boulders on the surface is higher than expected.

The team will make further observations at closer ranges, to more accurately assess where a sample can be taken on Bennu to be returned to Earth.

The mission currently is performing a preliminary survey of the asteroid, flying the spacecraft in passes over Bennu's north pole, equator, and south pole at ranges as close as 7 km.

This is to better determine the asteroid's mass.

The mission scientists and engineers need to know the mass of the asteroid to design the spacecraft's insertion into orbit, because mass affects the asteroid's gravitational pull on the spacecraft.