Boffins think they have found the oldest known substance on Earth, dust grains that were formed around five to seven billion years ago - before the Solar System had even formed.
“This is one of the most exciting studies I’ve worked on,” said Philipp Heck, lead author of the paper published in the Proceedings of the National Academy of Sciences on Monday and an associate professor of geophysical sciences at the University of Chicago. “These are the oldest solid materials ever found, and they tell us about how stars formed in our galaxy.”
The tiny dust grains were found in the Murchison meteorite, which landed in Australia over 50 years ago. The research team analysed samples of the meteorite by grinding it up into a fine powder to create a chemical paste that can be dissolved with acid to leave “presolar” grains behind.
“It starts with crushing fragments of the meteorite down into a powder,” said Jennika Greer, a graduate student at the University of Chicago and a co-author of the study. “Once all the pieces are segregated, it’s a kind of paste, and it has a pungent characteristic—it smells like rotten peanut butter.”
These presolar particles were forged before the Sun was formed in environments with completely different chemical compositions. After inserting the samples into a mass spectrometer, the researchers could study the concentrations of different isotopes.
Isotopes like neon-21 are created when the meteorite was bombarded with galactic cosmic rays. The concentration of certain isotopes provides an estimation of how long ago the rock was zapped by the rays.
“Some of these cosmic rays interact with the matter and form new elements. And the longer they get exposed, the more those elements form,” Heck explained. “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.”
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Enough neon-21 had accumulated in the Murchison meteorite that the oldest parts of it were found to be older than 5.5 billion years old. Most of the grains, however, were around 4.6 to 4.9 billion years old, a time that predates the formation of the Sun 4.6 billion years ago.
Presolar grains are formed when material made in stars is shed and thrown off from dying stars, where they go on to mix and mingle with interstellar material. The oldest grains in the Murchison meteorite, therefore, were created in an ancient star that predates 5.5 billion years ago.
The researchers reckon parts of the grains started off in a star that formed seven billion years ago, during a time when part of the Milky Way was experiencing higher levels of star formation than today.
“Some people think that the star formation rate of the galaxy is constant,” Heck said. “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.” ®
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