A waypoint in the journey from interstellar space to atmosphere.
Article written by Jamie Gilmour.
Minute quantities of xenon and iodine trapped in nanometre size grains of diamond found in meteorites tell a story about how the elements that make up the Earth’s atmosphere, oceans and ecosystem arrived, and allow us to precisely date events on asteroids 4.5 billion years ago.
The atoms that make up the solid Earth and its atmosphere, oceans and lifeforms were, with the exception of hydrogen and helium, almost entirely made in generations of stars that were born, lived and died before our solar system formed. But what routes did these atoms follow to get here from their points of origin? We can help to map one such route by focusing on the elements xenon and iodine.
Xenon and iodine are both considered volatile elements. This means that they tend to be most concentrated in the Earth’s atmosphere and oceans. Xenon makes up about 90 billionths of the air we breathe. Iodine is incorporated into biological material in the oceans, which “rains” out onto the ocean floor to give the sediments very high iodine concentrations. But how did Earth acquire iodine and xenon in the first place? Planets grew in a disk of dust and gas surrounding the early sun, but planets like the Earth were formed from the dust and volatile iodine and xenon were part of the gas.
Meteorites provide a snapshot of the processes that unfolded as planets grew in the early solar system – they are relics of a stage when the solid material that would make the Earth and other planets was distributed among millions of bodies of about 100 km diameter. When we look at these samples, we find that the xenon is trapped in organic material. “Organic” means the material is mostly made from carbon and hydrogen. On Earth organic materials such as oil and coal are mostly products of life, whereas organic material in meteorites wasn’t produced by life. (It may have helped life get started, but that’s another story.) Whatever process trapped xenon into organic material is what ultimately led to the xenon in Earth’s atmosphere today, so if we can understand that process we can understand more about the origin of planetary atmospheres. We’ve previously shown that there is iodine trapped in this organic material, as well as xenon – see our Genesis paper. The presence of xenon and iodine in this material also links these trace elements to the history of the material that life-on-Earth is built from.
Our new work looks at another carbon rich material found in meteorites – nanodiamonds. These grains are a few millionths of a millimetre across and have the same crystal structure as diamonds. Here’s how we know that at least some of these grains formed before our solar system. The atoms around us were made in many different stars before our solar system formed. They were forged by different mechanisms depending on the type of star. The products from all these stars were mixed together before the solar system formed. However, a small fraction of the xenon from just one source was incorporated into these nanodiamonds and preserved through the mixing process. If they had been formed in our solar system they couldn’t have retained this “one source” material, so they must be presolar.
We showed before that some of these grains also retain xenon trapped before the last input of freshly made atoms; the products of atom-making processes are all mixed together in this xenon, but they are in slightly different proportions from what we find in our solar system; some material from one particular process must have been added to our solar system’s source region after this xenon had been trapped in nanodiamonds. In this paper we show that this xenon in nanodiamonds is trapped alongside iodine, just like the xenon that is trapped in organics.
How can this have happened? This is our proposal. We suggest that the trapping process was not an isolated event in the history our solar system, but something that has probably happened many times before in the lifetime of the galaxy. The underlying process that trapped xenon and iodine from well-mixed gas into the solid dust was the same, but it happened several times. Some organic material from an early example of this process was transformed into nanodiamonds by a supernova shockwave. After that, new material made in stars was added to the galaxy, but couldn’t mix with what was trapped in the nanodiamonds. For this reason nanodiamonds retain a fossil “mixed xenon” signature from the time before the latest addition. Xenon that had gained the new material was trapped into organics by later example of the same process. Both nanodiamonds and new organics got incorporated into meteorites, and took their xenon and iodine with them.
Our work also provides an opportunity to understand when asteroids were processed in the early solar system. This is because there is a clock that relates xenon and iodine. In the early solar system a radioactive isotope of iodine (iodine-129) decayed to an isotope of xenon (xenon-129) with a half life of 16.1 million years. By measuring how much of the decay product has been retained by the nanodiamonds we can figure out when xenon was lost from them after they were trapped on asteroids. Our work shows that they managed to start retaining xenon only after about 30 million years of solar system history had elapsed, and that some was still being lost even 50 million years later than that. Even after the Earth formed asteroids were being heated up and losing volatile elements, we think this was most likely because of impacts on their surface as the debris of planet formation was ground down into dust and cleared out of the solar system. But that is also another story, one we are still working on.
Full citation: Gilmour, J.D., Holland, G., Verchovsky, A. B., Fisenko, A.V., Crowther, S.A. and Turner, G. (2016) Xenon and iodine reveal multiple distince exotic xenon components in Efremovka “nanodiamonds”. Geochimica et Cosmochimica Acta, 117, 778-93. doi 10.1016/j.gca.2015.12.028.
Publisher’s website for open access copy of this article: http://www.sciencedirect.com/science/article/pii/S0016703715007309.
This work was funded by the Science and Technology Facilities Council Grants ST/M001253/1 and ST/J001643/1.