News brief: OSIRIS-REx finds water on Bennu

OSIRIS-REx team members held a press briefing today at the 2018 American Geophysical Union meeting, and announced that the mission has already found water on asteroid Bennu. The water is locked in clay minerals, and was detected with the spacecraft’s OVIRS and OTES instruments. The signature was strong enough to be detected while OSIRIS-REx was still approaching Bennu, which was a bit of a surprise, according to NASA Goddard's Amy Simon.

Bennu was chosen specifically because it was suspected to have both water and organic materials — important building blocks for life — so today’s announcement checks off an important mission milestone. Organics have not yet been found.

Bennu, like asteroid Ryugu, is very rocky. Principal Investigator Dante Lauretta says there are both tip-of-the-iceberg boulders that extend deep beneath Bennu, and others that are merely resting on the surface. This will make collecting a sample in 2021 challenging. It’s still early to think about sample sites, but one possibility is basins that look like impact craters and are filled with fine-grained material. Another finding from the briefing was that ground-based radar models of Bennu were very accurate.

Finally, I asked the OSIRIS-REx team what they know about Bennu's mass thus far. Erin Morton, the mission's communications lead, said these figures were derived after the first three passes of the spacecraft over Bennu's north pole:

• Mass determination from 3 flybys: 7.34x10¹⁰(±0.15)kg
• Volume = 0.06171×10⁹ (±0.0004) m³
• Density = 1190 (±24) kg/m³
• Bennu Gravitational Parameter = 4.9 (±0.1) m³/s²

For more on this story:

• NASA press release: NASA’s Newly Arrived OSIRIS-REx Spacecraft Already Discovers Water on Asteroid
• University of Arizona OSIRIS-REx website
• Youtube recording of AGU press briefing
• Press briefing materials (PDF)