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Bennu isn’t a cold, dead rock after all. NASA’s OSIRIS-REx spacecraft has spotted the asteroid spewing out dust and rocks on 11 separate occasions over a period of just a few weeks, which was completely unexpected.

For the most part, Bennu is as we predicted before OSIRIS-REx arrived there in December: it is shaped like a spinning top and rotates once every 4.3 hours. It isn’t very dense and its surface is one of the darkest in the solar system.

But the particles flying off its surface are something new. They range in size from a few centimetres to tens of centimetres. At least four appear to have ended up in orbit around Bennu, forming what are essentially miniature moons around the asteroid. Bennu itself orbits the sun mostly between Earth and Mars.

“We have had spacecraft around other asteroids, and nothing like this was ever reported,” says Andrew Rivkin at Johns Hopkins University in Maryland. “The question is, why is this asteroid different?”

One major difference is that, unlike the other asteroids we’ve been to, Bennu is full of hydrated minerals, which have water locked into their molecular structure. If it also has ice under its rocky surface, heat from the sun could be turning into gas and blowing rocks away, Rivkin says. That would mean that Bennu didn’t form where it is now, because it is too warm there to maintain much ice for long.

Asteroid mystery

For now, it will remain a mystery. “Every time we go to an asteroid, we find we don’t know them as well as we thought we did,” Rivkin says.

Efforts to learn more about Bennu may be hampered by another surprise: its surface is covered in large boulders, which could make it more difficult for NASA’s OSIRIS-REx spacecraft to grab a sample from its surface.

Instead of smooth areas coated in dust, the surface has thousands of small boulders and more than 200 that are in excess of 10 metres across. We only expected one rock this big from images taken before the spacecraft entered orbit.

The mission was designed to pick up samples from dusty regions, not rocky ones. The team has found only a few areas that will work, but they are confident they will still be able to get a sample. “We have a very capable spacecraft and we can adapt,” says OSIRIS-REx project scientist Jason Dworkin at NASA’s Goddard Space Flight Center in Maryland. “It is a solvable situation.”

Choosing exactly where to take a sample will be a hard choice, because the surface appears to vary considerably. “Some areas look smooth and shiny, and some look dull and bumpy,” says Dworkin.

The spacecraft spotted individual boulders containing variations in brightness of up to 33 per cent. It isn’t clear yet if this is due to an actual texture difference, a result of compositional differences on the surface or simply a trick of the light, says Dworkin. NASA is still working on chemical analysis of Bennu’s surface, which would provide firmer answers.

If the diversity is really there, it may mean that Bennu’s surface has some pristine areas that are remnants of the original, larger rock it originally chipped off from, as well as some relatively fresh spots that have undergone recent activity.

The observations, presented today at the Lunar and Planetary Science Conference in The Woodlands, Texas, led Dworkin and his colleagues to suggest that Bennu is probably between about 100 million and one billion years old, which is older than we expected, and its surface may contain rocks that are even older than that.

We won’t know for sure where Bennu’s diversity comes from until OSIRIS-REx brings its sample back to Earth in 2023, Dworkin says. Because Bennu is a relic from the age of planet formation in the early solar system, these samples may help us understand how planets like Earth formed and where Earth got its water and complex chemistry.

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