Scientists from the Field Museum of Natural History in Chicago discovered presolar grains in their sample from the Murchison meteorite and said that it was the oldest ever discovered. Some grains were believed to be more than 5.5 billion years old - before the Sun was created.

By Ian Horswill

Posted on January 14, 2020

Scientists looking at a meteorite believe they have discovered the oldest material known to exist on Earth.

The researchers at the Field Museum of Natural History in Chicago, Illinois, found dust grains within the large meteorite that fell to Earth near Murchison, Victoria, in Australia, in 1969. Its age could be as much as 5.5 billion years old.

The oldest of the dust grains were formed in stars that roared to life long before our Solar System was born.

A team of researchers has described the result in the journal Proceedings of the National Academy of Sciences.

When stars die, they pitch the particles that formed into space. Those particles, or space dust, eventually form new stars, planets, moons and meteorites. In the Murchison meteorite that was observed falling to Earth, stardust that formed 5 to 7 billion years ago has been discovered. That was before the Sun was born.

“They’re solid samples of stars, real stardust,” said lead author Philipp Heck, a curator at Chicago’s Field Museum and associate professor at the University of Chicago.

The stardust trapped in meteorites remain unchanged for billions of years, making them time capsules of the time before the solar system. The presolar grains for the study were isolated from the Murchison meteorite about 30 years ago at the University of Chicago.

oldest material, earth stardust

“It starts with crushing fragments of the meteorite down into a powder,” said co-author Jennika Greer, from 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 “rotten-peanut-butter-meteorite paste” was then dissolved with acid, until only the presolar grains remained.

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

Once the presolar grains were isolated, the researchers tried to find 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,” said 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.

“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. 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 researchers discovered some of the presolar grains in their sample were the oldest ever discovered based on how many cosmic rays they had 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. For context, the Sun is 4.6 billion years old and Earth is 4.5 billion.

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

Heck and his colleagues look forward to 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,” he said.

Heck noted that there are lifetimes’ worth of questions left to answer about presolar grains and the early Solar System.

“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. “It’s so exciting to look at the history of our galaxy. Stardust is the oldest material to reach Earth, and from it, we can learn about our parent stars, the origin of the carbon in our bodies, the origin of the oxygen we breathe. With stardust, we can trace that material back to the time before the Sun.”

“It’s the next best thing to being able to take a sample directly from a star,” added Greer.