Meteorite contained oldest material on Earth

Scientists say a meteorite that landed in Australia half a century ago contained the oldest solid material ever found on Earth – stardust that formed five to seven billion years ago.

That’s exciting – “one of the most exciting studies I’ve worked on” – says Philipp Heck, because these presolar grains, formed before the Sun was born, “tell us about how stars formed in our galaxy”.

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Scanning electron micrograph of a dated presolar silicon carbide grain. The grain is about eight micrometres in its longest dimension. Credit: Janaína N. Ávila

Heck, a curator at the Field Museum and associate professor at the University of Chicago, US, is lead author of a paper describing the findings in the journal Proceedings of the National Academy of Sciences.

Interstellar dust particles are tiny and rare – found only in about 5% of meteorites that reach Earth. They also cannot be dated by astronomical observation or radioactive decay, due to analytical limitations and unknown initial isotopic compositions, making it hard to get information about their age and longevity.

Heck and colleagues found a way, however, using 40 large presolar silicon carbide grains that had been isolated from the Murchison CM2 meteorite found near Murchison, in the Australian state of Victoria, in 1969.

The isolation process was somewhat basic, involving crushing fragments of the meteorite into a smelly paste then dissolving this with acid until only the presolar grains remained.

The current team then developed a way to determine 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,” says Heck. 

“Some of these cosmic rays interact with the matter and form new elements. And the longer they get exposed, the more those elements form.” 

The researchers learned, they say, that some of the grains are the oldest ever discovered, based on how many cosmic rays they’d soaked up; most had to be 4.6 to 4.9 billion years old, and some were older than 5.5 billion years. The Sun is 4.6 billion years old.  

But the age of the presolar grains wasn’t the end of the discovery, Heck says.

“We have more young grains that we expected. 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.”

Heck says the finding will likely add more fuel to the debate about whether or not new stars form at a steady rate, or if there are highs and lows in the number of new stars over time. 

“Some people think that the star formation rate of the galaxy is constant,” he says. “But thanks to these grains, we now have direct evidence for a period of enhanced star formation in our galaxy seven billion years ago with samples from meteorites. 

“This is one of the key findings of our study.”

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