In a few days, NASA will bounce its OSIRIS-REx probe off the asteroid Bennu. The mission will collect a sample from the asteroid and return it to Earth for a more in-depth study, one of the first missions of its kind.
That returning sample will help us understand not just asteroids, but the early days of the Solar System’s existence. However, this is not the only mission of OSIRIS-REx.
The spacecraft arrived in Bennu’s orbit in December 2018 and has since used its suite of tools to learn as much as it can about the asteroid ahead of their long-planned encounter.
And boy, did he ever make it. Six separate documents have just hit the newspapers Science is Advances in science detailing Bennu̵
“The spacecraft has been observing the asteroid for nearly two years,” said astronomer Joshua Emery of Northern Arizona University and a member of the OSIRIS-REx science team. “Bennu turned out to be a small and fascinating asteroid and has reserved many surprises for us.”
Bennu is what is known as a “ pile of rubble ” 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, and at least some of the material is back together. In Bennu’s case, the shape he formed is a rough diamond, with a pronounced crest at the equator.
Now, for the first time, we have a detailed 3D digital map of the asteroid’s terrain, led by Michael Daly of York University. This reveals that the equatorial crest is not alone: other much thinner crests extend from pole to pole, indicating that although the asteroid is made of rubble, it does have some internal cohesion.
In recent years, we’ve had hints of other weird things going on at the Diamond B (i.e., Bennu).
Last year, we discovered that Bennu was ejecting material from its surface, some of which fell back and others appeared to enter stable orbit. And scientists have found evidence of carbonaceous material that suggested the presence of water at some point in Bennu’s mysterious past.
A new global spectral investigation of the asteroid in the infrared and near-infrared, led by Amy Simon of NASA-Goddard, has confirmed the presence of organic and carbon-carrying materials spread across the surface of Bennu – the first concrete detection these things in an asteroid near Earth. This is consistent with the hypothesis that asteroids and meteorites could have carried at least some of the ingredients for life on Earth.
There was also water
But the asteroid’s carbon content has a more detailed story to tell. A careful spectral study revealed luminous veins of carbonate material traversing a series of boulders.
This, according to a team of scientists led by NASA-Goddard’s Hannah Kaplan, is consistent with carbonates found in “aqueously altered carbonaceous chondrite meteorites” – carbonates that formed through interactions with water.
Some of these veins are one meter long and several centimeters thick. This, the researchers say, is evidence that water once flowed freely over rocks, an asteroid-scale hydrothermal system that was once present on the parent body and that Bennu was born.
“The flow of fluid over Bennu’s parent body would have taken place over distances of miles from thousands to millions of years,” the researchers wrote in their paper.
Multispectral images of the surface revealed that Bennu is exposed to the elements unevenly in an analysis conducted by Daniella DellaGiustina of the University of Arizona. With false-colored visible light images of the asteroid, the team found that some regions were exposed to weathering phenomena such as cosmic rays and solar wind for longer than others, suggesting processes – such as impact events – that exhibit fresh material at different times. .
The region of the nightingale crater where the probe will retrieve a sample is cooler material, which means it will provide a cleaner look at things from the early Solar System, when Bennu is thought to have formed.
And there is more. A study on temperature changes conducted by Ben Rozitis of the Open University found something interesting about the Bennu boulders. They are divided into two types: stronger and less porous and weaker and more porous. The strongest boulders are those that have carbonate veins, suggesting that interaction with water may eventually produce stronger rock as the liquid seeps material into the holes.
But even the faintest boulders are interesting. They are unlikely to survive entering Earth’s atmosphere, as they would overheat and explode, meaning they are likely a type of space rock that we haven’t had the opportunity to study closely before.
Finally, let’s go back to those aforementioned ejected rocks. We still don’t know exactly how they are ejected from the asteroid, but the way they fly up and down is a surprisingly useful tool for probing the interior of the asteroid.
“It was kind of like someone was on the surface of the asteroid and throwing these marbles 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 gravitational field measurements made by the OSIRIS-REx in orbit, the team was able to compile an internal density profile of the asteroid, as denser regions create a stronger local gravitational field.
And they found something surprising. They thought the asteroid would have roughly the same density all along the way; but it looks denser on the surface. The least dense regions are the equatorial crest and the core of the asteroid, as if it had a large vacuum inside.
Since the asteroid’s rotation is accelerating over time, this means that, eventually, it is likely to spin on its own.
However, it is a long way into the future. For now, the asteroid will have to settle for a kiss from a probe on the crater. And these new analyzes have provided researchers with a framework within which to interpret the close-up study of that sample, when it finally makes its way to Earth.
The six documents, published in Science is Advances in science, can be found here, here, here, here, here and here.