Seven-billion-year-old stardust in meteorite 'oldest solid material found'

Katie Ramirez
January 14, 2020

Scientists recently identified the oldest material on Earth: stardust that's 7 billion years old, tucked away in a massive, rocky meteorite that struck our planet half a century ago.

"This is one of the most exciting studies I've worked on, "said cosmochemist Philipp Heck of the Field Museum of Natural History and the University of Chicago".

They are made of silicon carbide, the first mineral formed when a star cools.

While it's actually not unheard of for meteorites to contain grains of material that predate the Solar System - they're called "presolar grains" - they are rare, and hard to identify because the bits of material are so small, and deeply embedded in the rock.

It came down in 1969 in Murchison, Victoria state, and scientists from Chicago's Field Museum have possessed a piece of it for five decades.

This "rotten-peanut-butter-meteorite paste" was then dissolved with acid, until only the presolar grains remained.

Once the presolar grains were isolated, the researchers figured out from what types of stars they came and how old they were by measuring their exposure to cosmic rays.

"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", said Heck, whose research focuses on pioneering new ways to understand astrophysical questions by studying meteorites.

Some of these rays interact with the matter they encounter and form new elements. "And the longer they get exposed, the more those elements form", Heck said in the press release.

Think of it this way: Imagine putting a bucket outside during a rainstorm.

"We have more young grains than we expected", said Professor Heck.

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The tiny dust grains were found in the Murchison meteorite, which landed in Australia over 50 years.

The authors acknowledge that their methodology, which uses neon isotopes to age grains, "suffers from relatively large uncertainties".

These presolar grains are very small and rare - they're only found in about 5% of meteorites that have fallen to Earth.

The researchers reckon parts of the grains started off in a star that formed seven billion years ago, during a time when part of the Milky Way was experiencing higher levels of star formation than today.

The star dust represented time capsules in front of the solar system.

While some astronomical models assume stars form at a constant rate, Dr Heck and his colleagues' work shows that isn't the case. This suggests stars are born in bursts, rather than at a constant rate.

"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", said Heck. This is one of the main conclusions of our study. The researchers in their paper explained, "Presolar grains are the oldest datable solid samples available and provide invaluable insight into the presolar chronology of our galaxy".

The ages of these grains of stardust isn't only important in themselves, but also because of what they tell us about the evolution of our galaxy.

"Once learning about this, how do you want to study anything else?" said Greer. "It's awesome; it's the most interesting thing in the world".

"I always wanted to do astronomy with geological samples I can hold in my hand", said Heck. Stardust is the remnant material left behind after the death of a star in a supernova. That spectrometer is the only one on the planet sensitive enough to detect the trace amounts of neon gas trapped in the stardust, he said.

As clusters, they may have been able to better weather the supernova shock waves when the stars explode.

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