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7 billion-year-old stardust is the oldest stuff on Earth

An electron microscope image of an 8-micrometer-wide silicon carbide grain. (Janaína N Ávila)
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Microscopic grains of dead stars are the oldest known material on the planet — older than the moon, Earth and the solar system itself. By examining chemical clues in a meteorite’s mineral dust, researchers have determined the most ancient grains are 7 billion years old — about half as old as the universe. Rocks don’t get much more classic than this.

The researchers studied minerals in the Murchison meteorite, a large space rock that disintegrated in 1969 above cow pastures in Murchison, Australia. Dairy farmers collected the fragments and sold kilograms of the meteorite to museums and universities.

This paper shows that scientific collections “include materials that have existed, in essentially their current form, for the better part of the life of the universe,” said Gregory Herzog, an expert in extraterrestrial chemistry at Rutgers University who was not a part of the research team.

“We’ve used this really old sample, the oldest solid samples available to science, to try to learn something about the history of our galaxy,” said Philipp Heck, a meteorite expert at the Field Museum of Natural History in Chicago. Heck and an international team of cosmochemists published the new study Monday in the Proceedings of the National Academy of Sciences.

Heck met an Australian professor who, as a student, dug through cow manure to hunt for meteorite fragments. “Pre-solar grains don’t care about” dung, Heck said. “They’re tough.” Years later, scientists have taken advantage of that toughness. They mashed part of the Murchison meteorite into powder and bathed the powder in acid. The chemical attack destroyed everything but the stardust grains, which are made of an exceptionally hard mineral called silicon carbide.

Silicon carbide is so strong manufacturers use a synthetic version in bulletproof armor. Though natural silicon carbide is rare on Earth, stars make the mineral during their dying gasps.

At the end of their lives, stars swell and release hot gas. When that cools, silicon carbide and other solid materials condense out of the gas. Tarry organic goo, newly formed alongside the grains, clumped the matter together into a form Heck likened to granola clusters. As clusters, they may have been able to better weather the supernova shock waves when the stars explode. Eventually, those clumps entered our solar neighborhood and became part of the rock that crashed into Australia.

While the space granola floated through the cosmos, it was bombarded with cosmic rays. Every so often, a direct hit from a cosmic ray shattered an atom within the silicon carbide, turning silicon into other elements like neon and helium.

“These hits are pretty constant over time, so we can just count the products from those hits and determine how long they were flying in space,” Heck said. The study authors measured the amount of neon in the grains using an instrument called a mass spectrometer at ETH Zurich, a technology university in Switzerland. 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.

“This is hard, hard work,” said Neyda Abreu, a planetary scientist at Pennsylvania State University at DuBois. Abreu, who was not involved with this study, added: “You’re counting a signature that’s incredibly tiny, of a gas.”

Of the 40 grains the researchers examined, the most ancient, at 7 billion years old, are 2.5 billion years older than Earth. The majority were 4.6 billion to 4.9 billion years old — not as extreme but still hundreds of millions of years older than the solar system.

The unusual concentration of grains of about the same age suggests a “baby boom” of stars, Heck said. Some astronomical studies of starlight suggest a surge in star formation in the galaxy about 7 billion or so years ago. As these boomer stars reached the end of their 2-billion-year lifetimes, the stardust they sloughed off could be responsible for the spike that Heck detected.

Studying matter like these grains can complement observations of stellar radiation as a way to “understand large-scale processes” in our galaxy, Abreu said. She anticipates more revelations in the future, she said, from missions like OSIRIS-REx, a NASA spacecraft scheduled to deliver pieces of the asteroid Bennu to Utah in 2023.

For now, meteorites are “the only way that we have access to these materials,” Abreu said. A fallen grain, as part of our galactic history, is the closest thing to a sample return from a star.

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