D: By studying rocks from Earth, meteorites, and the rocks the Apollo astronauts brought back from the moon, scientists know that our solar system is about four point six billion years old.
Y: Yes Don, but in 2019 an international team of researchers reported that they found tiny grains of dust in a meteorite that fell in Australia in 1969 that are even older than the solar system, and may be seven billion years old.
D: But Yaël, where could these dust grains have even come from?
Y: They came from beyond the solar system, out in the space between the stars. Astronomers know there are tiny dust grains in interstellar space, and know their chemical composition, because of the way they scatter and reflect starlight. The researchers recognized interstellar dust grains made of silicon carbide in the Australian meteorite from their odd abundances of isotopes.
D: So, given that they recognized these tiny grains, how did they ever figure out their ages?
Y: It wasn’t easy. Researchers usually date old rocks based on the slow decay of the radioactive elements they contain into other substances. They couldn’t do that with the dust grains, because their strange isotope abundances made it impossible to determine how much of the other substances to expect. Instead, they studied the effects of the grains’ bombardment by high energy cosmic rays in deep space. They surmised that this bombardment would have produced small amounts of helium and neon in the samples. The longer the grains had been exposed to space radiation, the more helium and neon there would be. That’s how the researchers arrived at an estimate of seven billion years.
