Around 4.5 billion years ago, an interstellar molecular cloud collapsed. At its centre, the Sun was formed; around that, a disc of gas and dust appeared, out of which the earth and the other planets would form. This thoroughly mixed interstellar material included exotic grains of dust: "Stardust that had formed around other suns," explains Maria Schoenbaechler, a professor at the Institute of Geochemistry and Petrology at ETH Zurich. These dust grains only made up a small percentage of the entire dust mass and were distributed unevenly throughout the disc. "The stardust was like salt and pepper," the geochemist says. As the planets formed, each one ended up with its own mix.
Thanks to extremely precise measurement techniques, researchers are nowadays able to detect the stardust that was present at the birth of our solar system. They examine specific chemical elements and measure the abundance of different isotopes - the different atomic flavours of a given element, which all share the same number of protons in their nuclei but vary in the number of neutrons. "The variable proportions of these isotopes act like a fingerprint," Schönbächler says: "Stardust has really extreme, unique fingerprints - and because it was spread unevenly through the protoplanetary disc, each planet and each asteroid got its own fingerprint when it was formed."
Studying palladium in meteorites
Over the past ten years, researchers studying rocks from the Earth and meteorites have been able to demonstrate these so-?called isotopic anomalies for more and more elements. Schoenbaechler and her group have been looking at meteorites that were originally part of asteroid cores that were destroyed a long time ago, with a focus on the element palladium.
Other teams had already investigated neighbouring elements in the periodic table, such as molybdenum and ruthenium, so Schoenbaechler's team could predict what their palladium results would show. But their laboratory measurements did not confirm the predictions. "The meteorites contained far smaller palladium anomalies than expected," says Mattias Ek, postdoc at the University of Bristol who made the isotope measurements during his doctoral research at ETH.
Now the researchers have come up with a new model to explain these results, as they report in the journal Nature Astronomy. They argue that stardust consisted mainly of material that was produced in red giant stars. These are aging stars that expand because they have exhausted the fuel in their core. Our sun, too, will become a red giant four or five billion years from now.
In these stars heavy elements such as molybdenum and palladium were produced by what is known at the slow neutron capture process. "Palladium is slightly more volatile than the other elements measured. As a result, less of it condensed into dust around these stars, and therefore there is less palladium from stardust in the meteorites we studied" Ek says.
The ETH researchers also have a plausible explanation for another stardust puzzle: the higher abundance of material from red giants on Earth compared to Mars or Vesta or other asteroids further out in the solar system. This outer region saw an accumulation of material from supernova explosions.
"When the planets formed, temperatures closer to the Sun were very high," Schoenbaechler explains. This caused unstable grains of dust, for instance those with an icy crust, to evaporate. The interstellar material contained more of this kind of dust that was destroyed close to the Sun, whereas stardust from red giants was less prone to destruction and hence concentrated there. It is conceivable that dust originating in supernova explosions also evaporates more easily, since it is somewhat smaller. "This allows us to explain why the Earth has the largest enrichment of stardust from red giant stars compared to other bodies in the solar system" Schoenbaechler says.
Ek M, Hunt AC, Lugaro M, Schoenbaechler M: The origin of s-process isotope heterogeneity in the solar protoplanetary disk, Nature Astronomy (2019), doi: 10.1038/s41550-019-0948-z