Scientists say oldest solid material found

14 Janvier, 2020, 09:36 | Auteur: Therese Cote
  • Oldest material on Earth discovered

This in turn informs them how old it is.

When stars die after millions or billions of years, their particles end up in outer space - eventually forming new stars as well as moons or meteorites.

Philipp Heck, curator of meteorites at Chicago's Field Museum which has a piece of the meteorite in its collection, examined the pre-solar grains.

"These are the oldest solid materials ever found, and they tell us about how stars formed in our galaxy".

A team of researchers has described the result in the journal Proceedings of the National Academy of Sciences.

"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", Heck said.

Scientists previously had found a presolar grain in the Murchison meteorite that was about 5.5 billion years old, until now the oldest-known solid material on Earth.

Presolar grains for this study were isolated from the Murchison meteorite about 30 years ago at the University of Chicago.

"It starts with crushing fragments of the meteorite down into a powder", said co-author Jennika Greer, from the Field Museum and the University of Chicago. A large number of microscopic grains of a mineral called silicon carbide from inside the meteorite were identified as interstellar - and therefore presolar - by 1990, but a precise age has been harder to pin down.

To date the material, the researchers used a unique technique to measure the effects of cosmic rays hitting the grains.

"Some of these cosmic rays interact with the matter and form new elements", Dr Heck said. So the longer a sample is exposed, the more secondary elements are formed.

By measuring how numerous new cosmic-ray produced elements are present in a pre-solar grain, scientists can tell how long it was exposed to cosmic rays, telling them how old it is.

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

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In 1969, a 4.6-billion-year-old meteorite struck Murchison, Australia.

When a cosmic ray -a stream of high energy particles, mainly protons and alpha particles - penetrates a presolar grain it occasionally splits one of its carbon atoms into fragments.

These ages correspond to the moment when the first stars began to break up, and since that type of star lived for two to 2.5 billion years, the stardust can be as old as seven billion years.

Heck said about 70,000 meteorites are now known to science, and of those, "at most 5% of these contain presolar grains".

Based on how many cosmic rays they had soaked up, most of the grains had to be 4.6 to 4.9 billion years old, and some grains were older than 5.5 billion years.

Astronomers have long debated whether new stars form at steady rate or in accelerated bursts. According to the researchers, this finding suggests that our galaxy went through a period of intense star formation around that time.

For Dr Heck, this latest research is one of the most exciting studies he's worked on, but he's sure it won't be the last.

Heck notes that this isn't the only unexpected thing his team found.

Numerous grains recovered were between 4.6 and 4.9 billion years old, while others were older than 5.5 billion years. "No one thought this was possible at that scale", Philipp Heck explained.

Heck and his colleagues look forward to all of these discoveries furthering our knowledge of our galaxy.

"These hits are pretty constant over time, so we can just count the products from those hits and determine how long they were flying in space", Heck said. "We hope this will be picked up and studied so that people can use this as input for models of the whole galactic life cycle", Dr Heck said. "It contains some of the oldest condensates in the solar system and also presolar materials". "Once learning about this, how do you want to study anything else?"

Murchison is really interesting because it's not being cooked up that much in the asteroid from which it came from, hence more of these delicate grains survived than would have in an asteroid with more geological activity going on.

"I always wanted to do astronomy with geological samples I can hold in my hand", says Heck. "With stardust, we can trace that material back to the time before the Sun".



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