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Bennu
This mosaic image of the asteroid Bennu is composed of 12 PolyCam images collected on Dec. 2 by the OSIRIS-REx spacecraft from a range of 15 miles. A prominent boulder can be seen at lower right. (NASA / Goddard / University of Arizona Photo)

Just one week after the OSIRIS-REx spacecraft’s official arrival at the asteroid Bennu, the mission’s scientists have announced a significant find: Water appears to be locked inside the diamond-shaped mini-world’s clay minerals.

Two scientific instruments — known as the OSIRIS-REx Visible and Infrared Spectrometer or OVIRS, and the OSIRIS-REx Thermal Emission Spectrometer or OTES — registered the readings during the probe’s approach phase, which started in mid-August. The findings were shared today during the American Geophysical Union’s fall meeting in Washington, D.C.

Spectral measurements revealed the presence of molecules with bonded hydrogen and oxygen atoms, or hydroxyls. Scientists suspect that these hydroxyl groups are contained in clays that interacted with water long ago.

The quarter-mile-wide asteroid is too small to host liquid water, but researchers surmise that liquid water was present on Bennu’s parent body — perhaps a much larger asteroid — before it broke up.

“This is really big news. This is a great surprise,” Amy Simon, OVIRS deputy instrument scientist at NASA’s Goddard Space Flight Center, said today during an AGU news briefing.

In a news release, Simon said that the presence of hydrated minerals “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,” she said.

The mission’s name is an acronym standing for “Origins, Spectral Interpretation, Resource Identification, Security – Regolith Explorer,” reflecting the twin objectives of analyzing the makeup of an ancient asteroid and figuring out how to deflect potentially hazardous space rocks.

OSIRIS-REx was launched in 2016 and caught up with Bennu on Dec. 3 after a 1.4 million-mile trip. Because of changes in orbital positions since the launch, the near-Earth asteroid is currently about 73.6 million miles from our planet.

In the wake of its arrival, the spacecraft is gearing up for a series of surveys that will bring it well within a mile of the asteroid’s rock-strewn surface. One of the most intriguing surface features is a prominent boulder sticking up near Bennu’s south pole. Images from the spacecraft’s OSIRIS-REx Camera Suite indicate that the boulder is roughly 164 feet high and 180 feet wide, which is bigger than expected.

More than a year’s worth of surveys will set the stage for sample collection in 2020 and the start of the return trip in 2021. OSIRIS-REx is designed to drop off a capsule containing up to 4.4 pounds of material as it zooms past Earth in 2023, with landing targeted for the Utah Test and Training Range.

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