7-Billion-Year-Old Stardust Found in Meteorite

(CN) – Scientists announced Monday the discovery of the oldest solid material found on Earth – a meteorite formed of stardust from 5 to 7 billion years ago before our own sun was born.

Dust-rich outflows of evolved stars similar to the pictured Egg Nebula are plausible sources of the large presolar silicon carbide grains found in meteorites like Murchison. (Image courtesy of NASA)

In a paper published in the Proceedings of the National Academy of Sciences, researchers described the significance of the material found in the meteorite that crashed in Australia 50 years ago.

“This is one of the most exciting studies I’ve worked on,” said Philipp Heck, lead author and associate professor at the University of Chicago. “These are the oldest solid materials ever found, and they tell us about how stars formed in our galaxy.”

Called presolar grains-minerals, Heck said the materials found in the meteorite that fell to Earth in 1969 are “solid samples of stars.” The stardust, which date back to before the formation of our solar system, were trapped in the meteorite, allowing the scientists an accurate glimpse into the past.

Presolar grains rarely make it to Earth, found in only about 5% of meteorites that fall on our planet. The researchers said the grains that do make it to Earth are tiny, with “a hundred of the biggest ones” fitting into a period at the end of a sentence.

To accurately date the grains, scientists first crushed small parts of the meteorite, named the Murchison meteorite after the town it landed in, into a powder.

“Once all the pieces are segregated, it’s kind of a paste, and it has a pungent characteristic – it smells like rotten peanut butter,” said Jennika Greer, co-author and graduate student at the Field Museum.

The paste-like substance is then dissolved in acid until only presolar grains were left.

“It’s like burning down the haystack to find the needle,” Heck said.

Once the scientists were able to identify the presolar grains, they could then study them to determine what kind of stars they came from and their age.

“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,” 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.

“I compare this with putting out a bucket in a rainstorm. Assuming the rainfall is constant, the amount of water that accumulates in the bucket tells you how long it was exposed,” he said.

The amount of cosmic rays soaked up by the grains tells scientists how old they are. Some grains from the Murchison meteorite were discovered to be more than 5.5 billion years old, older than our sun at 4.6 billion years old.

Heck said they also discovered grains dating back 4.6 to 4.9 billion years ago, adding that the new information can help scientists uncover major astronomical events from that time period.

“We have more young grains that we expected,” says Heck. “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.

“Some people think that the star formation rate of the galaxy is constant,” Heck said. “But thanks to these grains, we now have direct evidence for a period of enhanced star formation in our galaxy 7 billion years ago with samples from meteorites. This is one of the key findings of our study.”

Heck said he hopes the newly revealed data will assist other colleagues in determining the nature of our galaxy’s formation.

“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,” he said.

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