Imagine stumbling onto something that existed long before the solar system was even formed! As incredible as it sounds, that is precisely what cosmochemistry researcher and physicist Olga Pravdivtseva and her team discovered while examining the Allende meteorite. The team detected presolar grains, which are traces of stardust that originated in interstellar space before the Sun was formed. However, this is not the first time scientists have discovered presolar grains inside a meteorite. Just a few weeks earlier, the oldest known material was found, and it is thought to be seven billion years old.
Researchers at the Washington University in St. Louis conducted the study of the Allende meteorite, and the area where they found the presolar grains contradicts our knowledge of long-traveled interstellar materials.
After traveling through space for billions of years, the mysterious space rock, dubbed as the “Allende meteorite,” crashed into the Earth and rained down over the Mexican state of Chihuahua in 1969. Over half a century later, Pravdivtseva and a team of researchers at the Washington University in St. Louis have discovered presolar grains made of silicon carbide, or SiC, inside the meteorite. What is even more surprising is that it was found within the inclusion known as “Curious Marie,” so named in honor of Marie Curie. Curious Marie is a CAI, or a calcium–aluminum-rich inclusion, which means this part of the meteorite was formed separately from the rest of the rock.
CAIs are also some of the oldest solid objects found in the Solar System, and they date back approximately 4.6 billion years when the Solar System is thought to have formed. When gas clouds condensed and formed the Sun along with the planets around it, the CAIs endured temperatures as extreme as 1,000 °C. Interestingly, our current understanding dictates that presolar grains that are composed of SiC (silicon carbide) crystals are not supposed to survive such hot conditions. That is what makes this discovery even more fascinating.
When heating up a small sample of the meteorite’s CAI inclusion, the researchers detected noble gas signatures, which revealed the presence of SiC of silicon carbide inside.
Pravdivtseva and her team conducted various experiments, one of which involved heatinga 20-microgram sample of the Curious Marie inclusion and then analyzing the noble gases that were released. The unique isotopic signatures present in the noble gases reveal critical information about the CAI, including where and how it originated and whether or not it contains presolar grains made of SiC. The researchers have yet to determine how SiC from a different star came to exist inside such a primordial solid. Still, its very existence is proof that we might have to rethink our understanding of the chemistry of the Solar System’s formation.
Though rare, presolar grains are not entirely unheard of. In fact, merely a few weeks before Pravdivtseva and her team announced their discovery, another group of scientists found the oldest known material on Earth, and it is significantly older than the Solar System.
In January of 2020, a study was published in the scientific journal Proceedings of the National Academy of Sciences, and it revealed the discovery of the oldest material ever found on Earth. In this case, the presolar grains were brought to Earth by the Murchison meteorite, which crashed into Australia in 1969. The Field Museum in Chicago obtained 52 kilograms of the meteorite and spent significant time studying it. The researchers were able to find silicon carbide inside the meteorite, and after conducting an extensive range of tests on these microscopic grains, they were able to determine its age. According to their findings, these presolar grains are older than the Solar System by at least a couple billion years.