Decoding Bennu: UK Scientists Unpacking the First Results from NASA’s Historic Asteroid Mission

The Bennu asteroid travelled billions of miles through space, carrying secrets from the dawn of our Solar System. Now, its dust has landed in the hands of scientists at the University of Manchester, who are decoding its cosmic mystery. 

From ancient water traces to the building blocks of life, their research is revealing how asteroids like Bennu may have shaped Earth’s past — and possibly, the origins of life itself. We spoke with planetary scientist Dr Rhian Jones to uncover the latest findings and what they mean for our understanding of space, time, and everything in between.

NASA’s Bennu Mission: A Time Capsule from the Early Solar System

NASA’s OSIRIS-REx mission was launched in 2016 to study asteroid Bennu, a carbon-rich near-Earth object believed to contain clues about the early Solar System. After arriving at Bennu in 2018, the spacecraft spent nearly two years mapping its surface before successfully collecting samples in 2020. 

The material — estimated to be around 4.5 billion years old — returned to Earth in September 2023, providing scientists with a pristine glimpse into the building blocks of planets. Early analyses have confirmed the presence of water-bearing minerals, carbon-based compounds, and traces of volatile elements, supporting theories that asteroids like Bennu may have played a role in delivering the ingredients for life to Earth. 

Now, researchers worldwide, including a team at the University of Manchester, are analysing these samples to unlock even deeper insights into planetary formation and the distribution of organic molecules in the early Solar System.

What role did the University of Manchester play in the analysis of the Bennu samples?

Several researchers at the University of Manchester have analysed the Bennu samples. Using a scanning electron microscope, we have looked in detail at the minerals that make up Bennu material. 

We studied what minerals are present, the different compositions of the minerals, and how they fit together in the rock. Together, these observations give us clues to understanding the conditions where the rock formed, for example the temperature it was heated to, and the fact that water was present in ancient times. 

This work contributed to the paper in Nature by McCoy, Russell and co-authors, on the presence of salts produced from evaporation of briny water.  

Also at Manchester, we have been analysing tiny pieces of Bennu material to measure the isotopes of xenon. Xenon is a noble gas, which means it doesn’t react in chemical reactions. But the amounts of the different isotopes can tell us a lot about the formation of the rock, including the gas that was trapped in the asteroid when the original particles stuck together, gas that was brought into the asteroid from outside the Solar System, and gas from the solar wind that was trapped in the rock over time. 

How do these findings from Bennu compare to previous studies of other asteroids or meteorites, such as from the asteroid Ryugu, in terms of organic compound diversity and abundance?

Material from Bennu is fundamentally very similar to material from Ryugu, and both of these asteroids are closely related to meteorites called carbonaceous chondrites. 

Specifically, they resemble the Ivuna-like, or “CI” group of carbonaceous chondrites which contain many water-bearing minerals, minerals produced from reactions with water, and a wide range of organic compounds. However, there are some important differences. For example, in the Bennu material we find quite a few mineral grains that have escaped reaction with water, whereas these are rare in CI chondrites. Also, Bennu material is much more nitrogen-rich than Ryugu and CI chondrites.  

Even though we have known about the CI chondrite meteorites for a long time, there were always some important open questions.

CI chondrites are very rare because they break apart easily when they travel through Earth’s atmosphere and when they hit the ground. Also, several of the CI chondrites in the world’s collections fell many years ago: since that time, they have reacted with humid air, so that some of the important original properties of the rocks have changed. 

Organic compounds and mineral salts can be affected easily by these reactions, making it hard to learn details about the original material from space. Getting samples directly from asteroids Ryugu and Bennu and preserving them in an inert atmosphere in laboratories from the moment they landed on the Earth, gives us an excellent opportunity to study the uncompromised organic material. 

Bennu and Ryugu materials contain thousands of organic compounds, including amino acids, and nucleobases found in DNA and RNA. This shows that a lot of organic chemistry reactions took place on asteroids in the early days of the Solar System. Also, it shows that complex organic molecules, such as those needed for life, were already present in the Solar System at the time the Earth was forming. 

The Nature Astronomy paper by Glavin, Dworkin and co-authors also investigated an essential property of organic molecules in Bennu material, which is the “chirality” or “handedness” of molecules like amino acids. On Earth, life favours the left-handed version of organic molecules, and measurements of organic compounds in CI chondrites showed that there are more left-handed than right-handed molecules. However, organic material in Bennu shows an equal split between left-handed and right-handed molecules, which argues against suggestions that left-handed life arose because the organic building blocks were predominantly left-handed.

What are the next steps in the research following these discoveries from the Bennu samples?

In Manchester and in many other laboratories worldwide, we are continuing with work to characterise the minerals in Bennu. Even though all the Bennu material is essentially similar in many ways, the individual small pieces show a surprising range of different minerals, and different effects of alteration by water. 

The Bennu material is a mixture of pieces of rock grabbed from the surface of asteroid Bennu, but within this sample we have pieces that came from slightly different parts of the asteroid at the time when it was altered. It will take a lot of work to fully understand the range of mineral properties and how these subtle differences can be explained in terms of varying local conditions. 

Researchers are also measuring the isotope properties of Bennu material, including isotopes of hydrogen, nitrogen, oxygen, carbon, and elements such as titanium and chromium. This work will help us further understand how Bennu relates to carbonaceous chondrites and other meteorites. 

The Bennu asteroid probably formed originally from small particles of dust and ice in the cold region beyond Jupiter. Isotope studies will add important insights into our overall models of how the Solar System formed, what materials made the rocky planets, including the Earth, and how water got added to the rocky planets. Isotopes will also be used to measure the age of different processes that the rock has experienced, for example, to determine the time at which liquid water was present on the Bennu asteroid.

In Manchester, as well as in other laboratories, we will continue to work on measuring noble gases, including neon and argon, as well as halogen elements (chlorine, bromine and iodine). These elements tell us more about the original materials and the space environment at the surface of Bennu. An additional measurement will be to measure Xe isotopes that come from the radioactive decay of iodine, which is another way to measure the age of the sample. 

How do the discoveries from Bennu influence our understanding of the distribution of water and organic materials in the early solar system? How do the organic compounds found in Bennu’s samples support theories about the origins of life on Earth?

Studies of meteorites have already provided our basic framework for understanding the distribution of water and organic materials in the early Solar System. By comparing the light spectra reflected off meteorites with the light spectra of asteroids (measured with telescopes), we know how water-rich asteroids are distributed in today’s Solar System. 

The asteroid Bennu was targeted for sample return precisely because it was known to contain water-bearing minerals and to be carbon-rich, based on its light spectrum. However, the present arrangement of water- and carbon-rich asteroids is unlikely to be the same as the original distribution when the Solar System was first forming. 

Asteroids like Bennu originally formed much further away from the Sun, compared with where Bennu orbits today, and that such distant asteroids have since been flung around the Solar System, particularly in its early history. The importance of studying Bennu and related meteorite materials is that we can build a picture of where water and organic materials were located at the time when the planets were growing. Much of this work depends on precise analyses of isotope compositions. Knowing that Bennu is a pristine sample makes it particularly important so that we can better interpret the meteorite analyses. 

The habitability of a planetary body means that it must have the right conditions, including the possibility of having liquid water. The Bennu and Ryugu samples, as well as meteorites, show us that asteroids once had these conditions, although the water was only present as liquid for a short time soon after asteroids formed. However, knowing that water ice, water-bearing minerals, and organic compounds could have been transported around the Solar System and delivered to planetary bodies and icy moons raises the possibility that the ingredients needed for life could have found their way onto bodies that had favourable conditions for habitable environments. For the early Earth, organic materials arriving at the surface within pieces of asteroids and comets could have provided the ingredients needed for life to begin.