In just a few days, NASA is going to bounce its probe OSIRIS-REx off asteroid Bennu. The mission will collect a sample from the asteroid, and return it to Earth for closer study – one of the first missions of its kind.
That return sample will help us to understand not just asteroids, but the earliest days of the Solar System’s existence. However, that is not the sole mission of OSIRIS-REx.
The probe arrived in Bennu orbit in December of 2018, and since that time has been using its suite of instruments to learn as much as it can about the asteroid before their long-planned meet-up.
And boy, has it ever. Six separate papers have just dropped in the journals Science and Science Advances detailing Bennu’s physical properties, and how they reveal a surprisingly complex history.
“The spacecraft has been observing the asteroid for nearly two years now,” said astronomer Joshua Emery of Northern Arizona University and a member of the OSIRIS-REx science team. “Bennu has turned out to be a fascinating small asteroid and has given us many surprises.”
Bennu is what is known as a ‘rubble pile‘ asteroid, which is exactly what it sounds like – a relatively loose, low-density conglomerate of rock, thought to have formed when a larger object broke apart, and at least some of the material came back together. In the case of Bennu, the shape it formed is a rough diamond, with a pronounced ridge at the equator.
Now, for the first time, we have a detailed 3D digital terrain map of the asteroid, led by Michael Daly of York University. This reveals that the equatorial ridge isn’t alone – other, much more subtle ridges extend from pole to pole, indicating that, although the asteroid is made of rubble, it does have some internal cohesiveness.
Over the past few years, we’ve had hints of other strange things afoot at the Diamond B (that is, Bennu).
Last year, we found that Bennu was ejecting material from its surface, some of which fell back down, and some of which seemed to enter stable orbit. And scientists found evidence of carbonaceous material that hinted at the presence of water sometime in Bennu’s mysterious past.
A new global spectral survey of the asteroid in infrared and near-infrared, led by Amy Simon of NASA-Goddard, has confirmed the presence of carbon-bearing and organic materials, widespread across the surface of Bennu – the first concrete detection of such things in a near-Earth asteroid. This is consistent with hypotheses that asteroids and meteorites could have carried at least some of the ingredients for life to Earth.
There was once water, too
But the asteroid’s carbon content has a more detailed story to tell. A close spectral study has revealed bright veins of carbonate material running through a number of boulders.
This, according to a team of scientists led by Hannah Kaplan of NASA-Goddard, is consistent with carbonates found in “aqueously altered carbonaceous chondrite meteorites” – carbonates that formed through interactions with water.
Some of these veins are metre-length and several centimetres thick. This, the researchers say, is evidence that water once flowed freely over the rocks, an asteroid-scale hydrothermal system that was once present on the parent body that went on to later birth Bennu.
“Fluid flow on Bennu’s parent body would have taken place over distances of kilometres for thousands to millions of years,” the researchers wrote in their paper.
Multispectral images of the surface revealed that Bennu is unevenly weathered in an analysis led by Daniella DellaGiustina of the University of Arizona. By false-colouring visible-light images of the asteroid, the team found that some regions have been exposed to weathering phenomena such as cosmic rays and solar wind longer than others, suggesting processes – such as impact events – that expose fresh material at different times.
The Nightingale crater region where the probe is going to retrieve a sample is fresher material, which means it will provide a cleaner look at stuff from the early Solar System, when Bennu is thought to have formed.
And there’s more. A study of temperature changes led by Ben Rozitis of the Open University found something interesting about the boulders on Bennu. They fall into two types – stronger and less porous, and weaker and more porous. The stronger boulders are the ones that have carbonate veins, suggesting that interacting with water may ultimately produce stronger rock as liquid seeps material into the holes.
But the weaker boulders are interesting too. They would be unlikely to survive entry into Earth’s atmosphere, as they’d heat up and explode – which means that they’re likely a type of space rock we’ve not had the opportunity to study up close before.
Finally, we get back to those aforementioned ejected rocks. We still don’t know exactly how they’re getting kicked off the asteroid, but the way they fly up and come back down is a surprisingly useful tool for probing the asteroid’s interior.
“It was a little like someone was on the surface of the asteroid and throwing these marbles up so they could be tracked,” said study leader Daniel Scheeres of the University of Colorado Boulder. “Our colleagues could infer the gravity field in the trajectories those particles took.”
When combined with gravity field measurements taken by the orbiting OSIRIS-REx, the team was able to compile an interior density profile of the asteroid, since denser regions create a stronger local gravity field.
And they found something surprising. They thought that the asteroid would roughly have the same density all the way through; but it seems more dense at the surface. The least dense regions are the equatorial ridge and the core of the asteroid – as though it has a large void inside.
Since the asteroid’s rotation is accelerating over time, this means that, eventually, it’s likely to spin itself apart.
That’s a long way into the future, though. For now, the asteroid will have to content itself with a kiss from a probe on the crater. And these new analyses have given researchers a framework within which to interpret the close study of that sample, when it finally makes its way to Earth.