Scientists have identified the oldest solid material on Earth in tiny grains of microscopic dust which were forged in a distant star somewhere between 5-7 billion years ago.
In fact, according to their work which was published in the PNAS magazine, this material predates our solar system for a few hundred million years.
This ancient stardust was hidden in a massive meteorite that reached our planet half a century ago.
This ancient stardust was hidden in a massive meteorite that reached our planet half a century ago. The Murchison meteorite, which was a huge 100-kilogram C2 chunk of rock, exploded on September 28, 1969, near Murchison, Victoria (Australia).
Stardust before our Sun
After analysing dozens of pre-solar silicon carbide grains from the Murchison meteorite, scientists discovered the oldest known material on Earth. This stardust existed long before our solar system was born.
"These are the oldest solid materials ever found, and they tell us about how stars formed in our galaxy,” said Philipp Heck, curator at the Chicago Field Museum and lead author of the study.
Their discovery is hugely important, as presolar grains are very rare and can barely be found in just five per cent of the meteorites that have fallen to the Earth.
They are uncommon and difficult to identify because the fragments of material are really small and deeply embedded in the rock.
As Heck stressed, presolar grains had never been found on Earth's rocks because plate tectonics, volcanism and other planetary processes warmed and transformed all the dust that could have accumulated during the formation of our planet.
Most presolar grains usually measure approximately 1 micron in length or are even smaller, but the grains analysed by the scientists in this study were much larger, with a length of 2 to 30 microns.
“We call them ‘boulders’,” Heck noted, “we can see them with an optical microscope.”
Crushing the meteorite
To get these presolar grains, scientists crushed fragments of the meteorite down to powder.
"Once all the pieces are segregated, it forms a kind of paste, and it has a pungent characteristic - it smells like rotten peanut butter,” explains Jennika Greer, from the University of Chicago
This paste was subsequently mixed in acid, leaving just the stardust behind.
"It's like burning down the haystack to find the needle," added Philipp Heck.
To determine the age of the grains, the researchers measured how long they were exposed to cosmic rays in space.
These rays are high-energy particles that travel through our galaxy and penetrate solid matter, while some of them interact with the matter they find and form new elements. The longer the exposure time, the more elements are formed.
Researchers used a particular form of the neon element - Ne-21 - to date the beans. Measuring how many of the new elements are present tells them how long the grain was exposed to cosmic rays, which in turn reveals how old it is.
“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,” said Heck.
“There was a time before the start of the solar system when more stars formed than normal.”
According to the researchers’ findings, this period of star formation was about 7,000 million years ago.
"Some people think that the star formation rate of the galaxy is constant," concluded Heck.
"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."
Reference: Philipp R. Heck, Jennika Greer, Levke Kööp, Reto Trappitsch, Frank Gyngard, Henner Busemann, Colin Maden, Janaína N. Ávila, Andrew M. Davis, Rainer Wieler. Lifetimes of interstellar dust from cosmic ray exposure ages of presolar silicon carbide. Proceedings of the National Academy of Sciences, Jan. 13, 2020; DOI: 10.1073/pnas.1904573117