Noble gas and halogen compositions of OSIRIS-REx samples and carbonaceous chondrites

The Earth's environment is habitable because of the presence of water and other volatile species, such as the gases that make up the atmosphere and the carbon-based molecules that make up living things. It is thought these volatiles were incorporated into asteroids and comets, which later brought them to the Earth. One type of asteroid is thought to have played an important role; such asteroids are the sources of a type of meteorites called "carbonaceous chondrites". We aim to understand more about how the Earth acquired water and volatile species by studying samples from such asteroids, which were a waypoint in the journey of volatile elements to the Earth.

It's an ideal time to study this because the NASA mission OSIRIS-REx is about to return a sample from one such asteroid, called Bennu. Exposure to the Earth's environment can affect meteorite samples, and we can't know which asteroid a meteorite came from. Returned samples address these problems. In addition, today's asteroids may have been modified over 4.5 billion years of solar system evolution since material accreted to the Earth. We will work on samples from Bennu alongside meteorite samples so that we can understand the range of carbonaceous material, whether it has been modified over the lifetime of the solar system, and how it has been affected by exposure to the Earth's environment.

We have developed special instruments and techniques dedicated to analysing the volatile elements xenon, chlorine, bromine and iodine; we measure the halogens as noble gases after irradiating samples in a nuclear reactor, and as a consequence we also measure concentrations of potassium, calcium, uranium and barium. Xenon makes up less than one part in a million of the Earth's atmosphere, but by studying the proportions of xenon atoms of different mass ("isotopes") in samples from different parts of the solar system, we can understand how the composition of xenon varied across the solar system and how the xenon in those bodies relates to the Earth's atmosphere and internal reservoirs. Chlorine, bromine and iodine ("the halogens") have similar chemical reactions to each other but the ways they behave in the environment varies. The halogens also record how they were incorporated into asteroids and what sort of asteroid delivered them to the Earth. And if we can understand how the Earth's xenon and halogens were brought to the Earth, then we will have understood how other "volatiles" reached the Earth too, such as the water and the carbon-rich material on which life depends.

In order to achieve our aims, our objectives are:

*To measure the compositions of samples returned by the OSIRIS-REx mission.

*To understand how and why these compositions are different from carbonaceous chondrites (and how and why the compositions of carbonaceous chondrites vary).

*Making use of the radioactive decay of an iodine isotope to a xenon isotope, and of a potassium isotope to an isotope of argon, to understand when material was processed and what effect that had on the composition we measure.

*To study samples that we know have experienced heating during their lifetime on their parent asteroid to understand how this changes the xenon and halogen compositions. This will allow us to understand whether this has had an effect on samples from Bennu.

We will also develop new techniques that will have applications in areas of Earth science, and maintain a unique UK analytical facility for participation in future sample return missions.

Once we have achieved our objectives, at the end of this project we will have a much clearer idea about the pathway volatile elements took from the cloud of material from which the solar system formed to the planets we are familiar with today. Our work will contribute to a detailed picture of the sources of material that made the Earth habitable.