The Solar System is built out of the ashes of long-dead stars, which provide all the elements heavier than helium that compose our worlds. Yet this stardust is not evenly mixed. Some of Earth's material has been identified as coming from red giant stars, but the same components are rarer on Mars or in the asteroid belt, and probably the outer planets too. A new paper provides an explanation.
The cloud from which the Solar System formed included not just raw elements, but grains of dust formed in and around other stars, representing about 3 percent of the Solar System's dust. Remarkably, more than four billion years later there are tests that can recognize these grains based on the ratios of isotopes of the elements that make them up, and the type of star that made them.
Professor Maria Schönbächler of ETH Zurich is studying samples of palladium to distinguish their origins. Palladium is a transition metal that had a brief moment of glory almost 30 years ago when it was wrongly thought it held the key to cold fusion, which if true would probably have solved the world's clean energy problem. Instead it is used to break down car exhaust. Its volatility lies between two groups of elements that show different distribution patterns through the Solar System, making it perfect to test how these patterns arise.
As stars that once made hydrogen into helium start converting the helium to carbon and oxygen they swell to become a form of swollen red giant known as AGB stars. In their cores a process known as slow neutron capture produces heavy metals, palladium included. The rest of the universe's palladium comes from more dramatic events – neutron star collisions and supernova explosions.
In Nature Astronomy, Schönbächler and co-authors report their findings and try to explain why less stardust palladium is found in the meteorites they studied compared to metals others had studied. "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" co-author Dr Mattias Ek of the University of Bristol said in a statement.
Meteorites from the asteroid belt, or those knocked off Mars when larger objects struck its surface, have less of this red giant material than Earth's crust. Schönbächler has an answer to that too. "When the planets formed, temperatures closer to the Sun were very high," she said. Close to the Sun many grains evaporated, but those from red giants were larger, and therefore more robust, than the smaller particles from supernovas.
Consequently the innermost planets ended up with a lot of red giant dust, whereas further out, including Mars, there was more of a mix in origins.