By James Ashworth
Around 4.6 billion years ago, vast clouds of dust began to accumulate to form the solar system.
Ancient asteroids such as Bennu contain the remnants of this formative moment, allowing scientists to understand what happened long before the Earth existed. But the asteroid appears to be missing some key elements, and scientists now think they know why.
Some of the most basic building blocks of the solar system are missing from one of its most dangerous asteroids.
Bennu has been the subject of intense scientific research over the past few years. Not only is the asteroid revealing more about the early years of the solar system, but it’s also one of the most likely to hit Earth. While the 0.06% risk of a collision is small, scientists are keeping a watchful eye on this space rock.
As part of ongoing research into Bennu, it was recently visited by NASA’s OSIRIS-REx mission. The robotic probe retrieved around 120 grams of pristine material from the rock which formed at the birth of the solar system. It is now being studied by scientists all over the world.
Professor Sara Russell and Dr Ashley King, two of our space researchers, has been involved in this project. He’s part of a new paper which reveals that Bennu appears to contain less of the solar system’s early solid materials, known as chondrules and refractory inclusions, than expected.
“Chondrules are grains of silica-based materials, while refractory inclusions often contain elements like calcium and aluminium,” Ashley explains. “The inclusions were the first solid materials to form in the solar system 4.6 billion years ago, with chondrules following a few million years later.”
“As Bennu only contains these materials in very small amounts, it tells us it must have come from an area of the solar system where they weren’t common. We think that Bennu’s parent body may have come from a part of the solar system from which we don’t get many meteorites.”
“Our detailed work on Bennu also shows that it formed from a unique array of dust, organic material and ices,” adds Sara. “These components can give us clues as to the environments in which all the planets were born, and how they changed across the solar system.”
The findings of the study were published in Nature Geoscience.
How did Bennu form?
The rocks that would eventually become Bennu formed more than 4.6 billion years ago. A large cloud of dust and other matter gradually collapsed under its own weight, forming the Sun, the planets, and everything else we know in the solar system today.
Bennu would have initially been part of a larger parent body that formed far from the Sun. Previous research has suggested that it contained water that reacted to form a variety of different salts, amino acids and other organic materials.
“We knew that all kinds of reactions were taking place in Bennu, but we’re now able to make a much more detailed study of the minerals in the samples returned by OSIRIS-REx,” Ashley says. “We’re now really starting to tie down the exact conditions under which they formed.”
One mineral the scientists were particularly interested in is iron sulphide. The exact composition of iron sulphide is related to the temperature the mineral formed at, allowing researchers to reconstruct the conditions during Bennu’s early history.
“While we often think of space as very cold, these reactions were happening at a temperature of around 25ºC,” explains Ashley. “Collisions, radioactive elements and the pressure of Bennu’s forming parent body could all have helped to raise the temperature inside the asteroid.”
As temperatures rose, the ice within Bennu’s parent body would have melted. This water began to spread throughout the rock, causing chemical changes over a long period of time.
“For a long time, people have argued that the melting ice would have reacted with the minerals near it, and wouldn’t have moved that far,” Ashley says. “However, in the Bennu samples, we see that the fluid isn’t staying static over time, but continuously changing as it spreads through the asteroid and reacts with its minerals.”
“While there’s more research to be done to confirm this, the presence of veins in the rock adds further evidence that the fluids would have been moving around.”
“We have one of the best meteorite collections in the world at the Natural History Museum and, since most meteorites come from asteroids, we already know a lot about the processes that occur on them,” Sara adds. “Being able to analyse material brought to Earth from an asteroid, however, has been a real game changer.”
“While OSIRIS-REx was at Bennu, we could see huge veins on the surface of the asteroid. Now we can relate these images beamed back from space to the sample we have in our labs.”
How different are asteroids and comets?
The presence of ice and certain volatile substances in Bennu is also blurring the line between rocky asteroids and icy comets. Historically, these two types of bodies have been treated separately but recent research suggests they might be part of a spectrum.
“Bennu is almost an intermediate between comets and asteroids,” Ashley says. “Comets form a tail behind them when they enter the solar system, as the materials in them sublimate when they get closer to the Sun.”
“While Bennu’s rocky composition is more asteroid-like, OSIRIS-REx saw small particles being ejected from the surface when it first arrived. This behaviour is more similar to a comet than an asteroid, so studying more examples could help to clarify exactly how they are related.”
One object that might shed light on this relationship is the near-Earth asteroid Phaethon. It has a dust tail much larger than Bennu’s, so much so that it is sometimes described as a ‘rock comet’. The Earth passes through Phaethon’s tail every December, causing the Geminid meteor shower.
A Japanese Space Agency mission, known as DESTINY+, hopes to visit Phaethon to find out more about it. By flying past the asteroid, DESTINY+ aims to get a closer look at Phaethon’s surface and analyse the dust released by it.
This mission is currently scheduled to launch in 2028 and should reach the asteroid a couple of years later. In the meantime, research on Bennu continues to reveal more about the bodies Earth shares the solar system with.