Fifty years ago, a fireball streaked across the skies of Victoria, Australia, scattered into three fragments, and crash-landed, spraying fragments over 13 square kilometers (5 square miles). Now, scientists have discovered stardust trapped inside the meteorite, time-stamping the interstellar grains to 5-7 billion years ago.
"This is one of the most exciting studies I've worked on," said lead author Philipp Heck, a curator at the Field Museum and an associate professor at the University of Chicago. "These are the oldest solid materials ever found, and they tell us about how stars formed in our galaxy."
Compared to a star, our lives are minuscule – specks of sand in the cosmic timeframe of millions to billions of years. When stars die, their particles float out into space to eventually form new stars, planets, moons, and meteorites. The stardust, called presolar grains-minerals, are only found in about 5 percent of meteorites on Earth, each mote coming in at a mighty 1/100th the size of a period on this page.
Pieces of the Murchison meteorite, as it is called, were sent to museums all over the world, with the Field Museum receiving the largest chunk. To isolate the presolar grains – so named for originating before our star – bits of the meteorite were crushed into powder and separated, giving the final product a “rotten peanut butter” smell. The material was then dissolved with acid until only the interstellar grains were left for the team to analyze.
To age the presolar grains, the team used exposure age data since dating of interstellar dust directly is not possible. "We counted the atoms produced in the grains that formed by interactions with cosmic-rays," Heck told IFLscience. "In particular we counted atoms of helium and neon that formed by these interactions. We think we know how many are produced per unit of time and thus can calculate an age by just counting how many atoms of each species are present. I compare this with putting out a bucket in a rainstorm. Assuming the rainfall is constant, the amount of water that accumulates in the bucket tells you how long it was exposed."
Most of the grains were between 4.6 to 4.9 billion years old, but some were even older. This makes the interstellar grains even older than the planet they crash-landed on – Earth at 4.5 billion years old. The results are published in the journal PNAS.
"We also found that there are many more young grains than expected," said Heck. "We attribute this to a baby boom in star formation that produced stars 7 billion years ago which started to puff out dust 4.9 billion years ago, 'only' 300 million years before the start of the solar system. This is how we think the “young” grains formed."
The space material is evidence added to a long-standing debate on whether star formation is constant, created at a steady rate, or if it ebbs and flows over time. The grains from the Murchison meteorite support the theory that the birth of stars can happen in episodic bursts.
"But thanks to these grains, we now have direct evidence for a period of enhanced star formation in our galaxy 7 billion years ago with samples from meteorites. This is one of the key findings of our study," said Heck. The grains also stick together in clusters, "like granola," in a process "no one thought was possible at that scale."
"I was surprised to find that the grains travelled through interstellar space as large clusters (larger than 200 micrometers), probably held together by some organic goo, analog to but obviously smaller than granola clusters held together by sugar!
"It's so exciting to look at the history of our galaxy," added Heck. "Stardust is the oldest material to reach Earth, and from it, we can learn about our parent stars, the origin of the carbon in our bodies, the origin of the oxygen we breathe. With stardust, we can trace that material back to the time before the Sun."