Scientists have identified the oldest material ever found on Earth inside a space rock that crashed into Australia more than a half-century ago.
The meteorite is filled with tiny grains of ancient stardust that predate the birth of the Sun and our solar system by more than two billion years, according to a study published on Monday in Proceedings of the National Academy of Sciences.
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The “presolar” grains were dated with the help of NASA’s Voyager 1 probe, which collected necessary new data when it crossed into interstellar space in 2012. The stardust is an unprecedented time capsule from an era of our galaxy before the solar system had coalesced into existence.
“It is something I can never get used to because it’s so fascinating,” said lead author Philipp Heck, a curator at the Field Museum and associate professor at the University of Chicago, in a call. “Just to have a rock in the lab and extract minerals and learn something about the history of the galaxy—I think it is just amazing that nature made such samples available to us.”
The grains measure just a few microns across (for reference, a human hair is about 100 microns in width). They were delivered to Earth by the Murchison meteorite, a huge chunk of extraterrestrial rock that fell in pieces near the Australian town of Murchison in 1969. The impact has become a major part of the town’s identity, but locals still donated most of its 220-pound mass to the Field Museum in Chicago.
“The people of Murchison, this rural community, collected the meteorite and were aware that it was something important to science, and made most of it available to science,” Heck said. “Without that, we wouldn’t have this study and many other studies.”
The grains of stardust inside the meteorite were formed by dying stars billions of years ago. At the end of their lifetimes, stars start to blow some of their heavier elements out into the surrounding environment, where the material cools into dust grains.
“Once the dust grain forms, it gets pushed away by the radiation pressure from the star into the interstellar medium and travels in the galaxy,” explained Heck. “The forming solar system 4.6 billion years ago incorporated that presolar dust into objects that were forming at that time, like Earth, the Sun, asteroids, comets—everything that formed in the solar system contained these presolar grains.”
Though early Earth was initially riddled with the grains, geological processes such as internal heating, plate tectonics, and volcanism have obliterated them. However, the grains survive on inside about five percent of asteroids and comets in our solar system. Multiple meteorites have carried presolar grains to our planet, though the Murchison rock is the only one that preserves grains large enough to provide age estimates.
Some 30 years ago, University of Chicago scientists extracted these stardust grains by crushing a sample of the Murchison meteorite into powder and using chemical agents to pare it down to minute specks of silicon carbide.
“There are other types of presolar grains that we know of, but silicon carbide is the most robust and retentive one,” Heck said. “It’s almost as hard as diamond. The crystal is very tight, essentially.”
Scientists already knew that the grains predated the solar system, but the new study has produced far more accurate age estimates and pushed the timeline of grain formation much further back. Heck and his colleagues were able to achieve this breakthrough in part because of NASA’s Voyager 1 probe.
“We now have data from a spacecraft that left our solar system and measured the cosmic rays outside,” Heck said. “Those rays are key to our dating method.”
When these grains were floating through the interstellar medium billions of years ago, they were constantly exposed to the cosmic rays, which are high-energy particles that pervade the galaxy. As the rays smashed into the grains, they triggered the creation of neon isotopes inside the stardust.
This process enables scientists to provide age estimates for the grains, as the older samples show longer cosmic ray exposure and thus higher neon isotope levels. Heck and his colleagues used a mass spectrometer to measure the neon isotope in the grains to zero in on their ages.
“We could use the cosmic ray data that was measured by Voyager to determine what type of cosmic rays, and how many cosmic rays of each energy, the grains were exposed to more than 4.6 billion years ago,” Heck explained. “We didn’t have that data 10 years ago.”
Not only did the researchers find that some of the grains date back about seven billion years, making them by far the most ancient material available on our planet, they also discovered that about two thirds of the grains dated to about 4.6 to 4.9 billion years ago.
This hints at a “baby boom in star formation,” Heck said. The Milky Way may have produced a bumper crop of stars that burned out and died during a period of about 300 million years, right before the solar system formed.
The lives and deaths of those bygone stars are now written in the grains trapped within the Murchison meteorite, which serendipitously ended up on Earth. The results corroborate theories that the Milky Way may have a stellar boom and bust cycle, rather than a constant rate of star formation.
As mind-boggling as all of this is, Heck and his colleagues expect many more discoveries to emerge from these presolar grains in the future. The team plans to continue extracting and dating the stardust, plus they will now be able to incorporate cosmic ray data obtained by Voyager 2, which followed its twin to interstellar space in 2018.
“It is a career-spanning project but I’m training my students in it as well, so hopefully we’ll get more ages in the next years and decades,” Heck said.
