First measurements of "Osiris Rex": Asteroid Bennu had contact with water

He woke up on the morning of December 3, grins Dante Lauretta, and found an appointment on his calendar ten years ago. The planetary scientist works at the University of Arizona and is the chief scientist of the NASA probe "Osiris Rex". And she arrived at her destination on 3 December, the asteroid "Bennu".

Lauretta gushes about the celestial body measuring about 500 meters as a "fascinating little world". At the Annual Meeting of the American Geophysical Union (AGU) in Washington, he has now presented together with colleagues first photos and measurements from there.

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Two spectrometers aboard "Osiris Rex", the instruments "Ovirs" and "Otes" have therefore provided evidence that clay minerals on the entire surface of the asteroid have come into contact with water.

Only minimal gravity

However, according to the researchers, the celestial body itself is too small to ever have liquid water - after all, Bennu's gravity is only one hundredth of a thousand times that of Earth. However, according to the measurements presented, there is no doubt that the rock has come into contact with water in the past.

How does it fit together?

Bennu, as Lauretta describes it, was once supposed to have been part of a larger celestial body. There was probably ice cream there - and that had melted eventually. So the rock material had come into contact with the water. In a catastrophic impact the larger celestial body was then destroyed, probably about a billion years ago.

From the remnants later Bennu was created, who pulls since then as a kind of flying boulders heap its course in the solar system. A pile of boulders with a total mass of more than 60 million tons, mind you. The asteroid also regularly crosses the Earth's orbit - and could also be dangerous to our planet in the distant future.

So there is at least the low probability of impact in the year 2135. Also to estimate this risk, the measurements of "Osiris Rex" serve. They are just starting to get started.

Search for organic connections

We are looking for example for organic compounds. These can be found, for example, in certain meteorites that have fallen from the asteroid belt to earth. These so-called carbonaceous chondrites come from a certain subgenus of asteroids to which also Bennu belongs.

An important intermediate goal has already been achieved, says Lauretta: "Very early in the mission, we know that Bennu has the material we want to bring back." Using a kind of cosmic vacuum cleaner, the probe is to collect fine rock crumbs from the asteroid surface - and bring it to Earth in 2023. The amount should be anywhere between 60 and 2000 grams.

However, the collection may be more difficult than planned: Earlier radar measurements with radio telescopes on Earth had suggested that there is only one larger boulder at Bennu's South Pole, about ten feet tall. The rest of the surface seemed dusty.

In reality, however, the rock measures 50 meters. And more chunks of the ten-meter class, there are hundreds. "The landscape is a bit rougher than we thought," says Lauretta. Sampling is complicated because of this - and the team has to choose a spot as free of boulders as possible. That's why you want to spend more than a year now.