New analyses of the samples taken from asteroid Bennu by NASA's OSIRIS-REx have revealed new insights into its origin - and The University of Manchester's scientists have played a key role.
A series of three new papers published this week in Nature Astronomy and Nature Geoscience, reveal that Bennu is a mix of dust formed in our solar system, organic matter from interstellar space and stardust that predates the solar system itself. The asteroid is thought to have formed from fragments of a larger parent asteroid destroyed by a collision in the asteroid belt between the orbits of Mars and Jupiter.
In the first paper, co-led by researchers at the University of Arizona and NASA's Johnson Space Center, published in the journal Nature Astronomy, Manchester researchers studied the gases trapped inside Bennu's samples - in particular xenon, which is a very rare gas. Their measurements showed that Bennu's gases resembled those found in some of the most primitive meteorites found on earth and materials returned from asteroid Ryugu by Japan's Hayabusa2 mission.
When combined with other elemental and isotopic analyses, the results suggest that Bennu's parent body contained material from a range of origins, close to the Sun, far from the Sun, and even some grains from beyond our solar system.
The findings also show that while much of the materials in the parent asteroid had been affected by water and heat, some of the material had escaped various chemical processes and retained its original chemical signatures. Some even survived the extremely energetic collision that broke it apart and formed Bennu.
The studies also show that while some of Bennu's original material survived unchanged, similarly, much of it was transformed by reactions with water. Minerals in its parent asteroid likely formed, dissolved, and re-formed over time, with up to 80% of Bennu's material now made up of water-bearing minerals.
These findings were reported in a second paper the paper published in Nature Geoscience co-led by the University of Arizona and the Smithsonian's National Museum of Natural History, and included contributions from Professor Rhian Jones at The University of Manchester.
In the third paper, co-led by Lindsay Keller at NASA's Johnson Space Center and Michelle Thompson of Purdue University, also published in Nature Geoscience, researchers found microscopic craters and tiny splashes of once-molten rock - known as impact melts - on the sample surfaces - signs that the asteroid was bombarded by micrometeorites. These impacts, together with the effects of solar wind, are known as space weathering and occurred because Bennu has no atmosphere to protect it.
Lindsay Keller at NASA's Johnson Space Center, said: "The surface weathering at Bennu is happening a lot faster than conventional wisdom would have it, and the impact melt mechanism appears to dominate, contrary to what we originally thought.
"Space weathering is an important process that affects all asteroids, and with returned samples, we can tease out the properties controlling it and use that data and extrapolate it to explain the surface and evolution of asteroid bodies that we haven't visited."
As leftovers from the formation of planets 4.5 billion years ago, asteroids like Bennu provide a valuable record of solar system history. Unlike meteorites that fall to Earth, which often burn up or are altered in the atmosphere, Bennu's pristine samples give scientists a rare opportunity to study untouched material.
The project brings together researchers from NASA, universities and research centres around the world - including the UK, the United States, Japan and Canada - to study Bennu's samples and unlock new insights into the origins of the solar system.
