The oldest solid material on Earth has just been identified , and the amazing thing is that it predates our solar system by at least a few hundred million years. According to the scientists, who just published their work in the journal PNAS , the tiny grains of microscopic dust were forged into a distant star somewhere between 5-7 billion years. For perspective, our Sun is only 4.6 billion years old.
This ancient stardust, or pre-solar grains, was hidden in a massive, rocky meteorite that struck our planet half a century ago with an asteroid that produced the Murchison meteorite, a massive 100-kilogram C2 chondrite rock that fell on September 28, 1969, near Murchison, Victoria (Australia).
Stardust before our Sun
After analyzing dozens of pre-solar grains of silicon carbide from the Murchison meteorite, they have discovered the oldest known material on Earth. This stardust, caused by particles thrown into space when stars die, came to life long before our solar system was born.
“They are solid samples of stars, stardust,” said Philipp Heck, curator of the Field Museum in Chicago and lead author of the study. “They are the oldest solid materials ever found, and they tell us how stars formed in our galaxy.”
However, this discovery is important, because presolar grains are very rare; only 5% of the meteorites that have fallen to Earth are found. They are rare and difficult to identify because the material fragments are really small and deeply embedded in the rock.
Although stardust is abundant in the universe, no pre-solar grains have ever been found in rocks on Earth. This is because plate tectonics, volcanism and other planetary processes heated and transformed all the pre-solar dust that may have accumulated during the formation of our planet, Heck clarified.
Most presolar grains are typically about 1 micron in length, or even smaller, but the grains analyzed by the scientists in this study were much larger, ranging in length from 2 to 30 microns. “We call them ‘boulders,'” Heck said. “We can see them with an optical microscope.”
Crushing the meteor
“Once all the pieces are segregated, it becomes a kind of paste and has a pungent characteristic: it smells like rotten peanut butter, ” says Jennika Greer of the University of Chicago. This off-white paste dissolved in acid, leaving only stardust.
“It’s like burning the haystack to find the needle,” commented 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. Some of these rays interact with the matter they encounter and form new elements. The longer the exposure time, the more elements are formed. The researchers used a particular form (isotope) of the element neon – Ne-21 – to date the grains.
Measuring how many of the new elements are present tells scientists how long the grain was exposed to cosmic rays. Which in turn reveals how old he is.
“Our hypothesis is that most of these grains, which are between 4.9 and 4.6 billion years old, formed in an episode of enhanced star formation,” Heck said. “There was a time before the start of the solar system when more stars were formed than normal.”
This period of star formation would have been about 7 billion years ago, according to the team’s findings. “Some think that the star formation rate of the galaxy is constant, but thanks to these grains, we now have direct evidence of a period of enhanced star formation in our galaxy 7 billion years ago with meteorite samples. This is one of the key findings of our study, “concluded Heck.
Referencia: 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
