Meteorite contains the oldest material on Earth: 7-billion-year-old stardust

James Marshall
January 14, 2020

Witnesses saw a fireball streak through the sky and break into three pieces just before 11 a.m. local time, followed by an audible tremor in the area.

This is a fragment of the Murchison meteorite.

Eventually, these grains were carried to Earth in a meteorite.

A fragment of the Murchison meteorite that fell to Earth on September 28, 1969, in Victoria, Australia. "It contains some of the oldest condensates in the solar system and also presolar materials".

For a new study published in the journal Proceedings of the National Academy of Sciences, scientists analyzed presolar grains from one of these meteorite fragments. A group of researchers led by Philipp Heck at the Field Museum in Chicago examinded what are known as "presolar grains", or minerals formed before the sun was born.

Another meteorite that was recently added to the Field Museum's collection, the Aguas Zarcas from Costa Rica, or "cosmic mudball meteorite", was said to smell like cooked Brussels sprouts. As nearly a side note to the main research questions, in examining the way that the minerals in the grains interacted with cosmic rays, the researchers also learned that presolar grains often float through space stuck together in large clusters, "like granola", says Heck.

Researchers said Monday that new techniques have allowed them to identify the oldest solid material ever found on earth.

Scanning electron microscope image of the 6.2-billion-year-old grain of silicon carbide from the Murchison meteorite.

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

"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. The new study is evidence of the latter. The researchers used a particular form (isotope) of the element neon - Ne-21 - to date the grains.

Once the presolar grains were isolated, the team figured out from what types of stars they came and how old they were. Those cosmic rays react with rock to form new elements that accumulate over time. Many of them had tales to tell about the meteorite's distinctive aroma. "But rarely there is an interaction, [and] one of those protons can hit an atom in the grain". 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 silicon can be split into helium and neon", Heck says.

"With this study, we directly determined the lifetimes of star dust".

The discovery could give us an insight into how stars form, scientists believe, based on tiny grains dating from before the Sun formed.

The authors acknowledge that their methodology, which uses neon isotopes to age grains, "suffers from relatively large uncertainties". Inset: SiC grain with ~8 micrometers in its longest dimension. These rays are high-energy particles that travel through our galaxy and penetrate solid matter.

"I compare this with putting out a bucket in a rainstorm".

Over time, the material in these planetary nebulae cools and condenses into grains of dust and minerals. And the older the material, or the smaller the grain, the higher the uncertainty in the dating estimate.

Most of these grains were on the younger side, though ⁠- just four million to 300 million years older than the sun. So it's a hard estimate.

"And then it took about two to two-and-a-half billion years for those stars to become dust producing", Heck explained.

Heck and colleagues also hypothesize that the majority of the grains in the study could have formed during a period of active star formation about 7 billion years ago, which would have produced large amounts of dust roughly 4.6 to 4.9 billion years ago-the same age as most of the grains. A fallen grain, as part of our galactic history, is the closest thing to a sample return from a star. Billions of years later, a chunk of that asteroid crashed into Australia.

The discovery of microscopic presolar grains, or stardust, in the Murchison meteorite is a rarity with only five per cent of meteorites found on Earth containing them.

Other reports by Click Lancashire

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