A team of researchers from the United States, Switzerland and Australia has found 4.6 to 7-billion-year-old presolar grains of silicon carbide (SiC) in the Murchison meteorite, a large C2 chondrite that fell in Australia in 1969.
“These are the oldest solid materials ever found, and they tell us about how stars formed in our Galaxy,” said Dr. Philipp Heck, a curator at the Field Museum of Natural History and a researcher in the Chicago Center for Cosmochemistry and the Department of the Geophysical Sciences at the University of Chicago.
The presolar grains Dr. Heck and colleagues examined formed before our Sun was born.
“They’re solid samples of stars, real stardust. These bits of stardust became trapped in meteorites where they remained unchanged for billions of years, making them time capsules of the time before the Solar System,” Dr. Heck said.
But presolar grains are very rare, found only in about 5% of meteorites that have fallen to Earth.
“It starts with crushing fragments of the meteorite down into a powder,” said Jennika Greer, a graduate student at the Field Museum and the University of Chicago.
“Once all the pieces are segregated, it’s a kind of paste, and it has a pungent characteristic — it smells like rotten peanut butter.”
“This paste was then dissolved with acid, until only the presolar grains remained,” Dr. Heck added.
Once the presolar grains were isolated, the team figured out from what types of stars they came and how old they were.
“We used exposure age data, which basically measures their exposure to cosmic rays, which are high-energy particles that fly through our Galaxy and penetrate solid matter,” Dr. Heck said.
“Some of these cosmic rays interact with the matter and form new elements. And the longer they get exposed, the more those elements form.”
“By measuring how many of these new cosmic-ray produced elements are present in a presolar grain, we can tell how long it was exposed to cosmic rays, which tells us how old it is.”
The scientists learned that some of the presolar grains in their sample were the oldest ever discovered-based on how many cosmic rays they’d soaked up, most of the grains had to be 4.6 to 4.9 billion years old, and some grains were even older than 5.5 billion years.
Since presolar grains are formed when a star dies, they can tell us about the history of stars. And 7 billion years ago, there was apparently a bumper crop of new stars forming — a sort of astral baby boom.
“We have more young grains that we expected,” Dr. Heck said.
“Our hypothesis is that the majority of those grains, which are 4.9 to 4.6 billion years old, formed in an episode of enhanced star formation. There was a time before the start of the Solar System when more stars formed than normal.”
The team looks forward to all of these discoveries furthering our knowledge of our Galaxy.
“With this study, we have directly determined the lifetimes of stardust. We hope this will be picked up and studied so that people can use this as input for models of the whole galactic life cycle,” Dr. Heck said.
The findings appear today in the Proceedings of the National Academy of Sciences.
Philipp R. Heck et al. Lifetimes of interstellar dust from cosmic ray exposure ages of presolar silicon carbide. PNAS, published online January 13, 2020; doi: 10.1073/pnas.1904573117
This article is based on text provided by the Field Museum of Natural History.