On Earth: Stardust from 7.5 Billion Years Ago -BB

As objects go, Earth is getting on in years. But it is definitely not ancient by cosmic standards. The very oldest native planetary material we can examine are tiny zircon crystals that formed here about 4.4 billion years ago. But the universe has been around for approximately 13.8 billion years, and has been building stars and forging heavy elements since it was some 200 million years old. 

That means that element-rich matter has been polluting interstellar space for a long, long while. Some of that matter, especially from older stars and supernova, has condensed and frozen into microscopic grains of silicate and carbon compounds. Eventually those stardust grains might find themselves pulled into the gravity-well of new-forming stars, and planets. But the vigor and turmoil of star and planet formation can erase the evidence of the age of those building blocks.

Some of the most ancient crumbs of building material we can study are those inside certain kinds of meteorites that still conveniently fall to Earth. Stardust swept up as the solar system coalesced.

Now a study by Heck et al., reported in the Proceedings of the National Academy of Sciences in the U.S., seems to have located interstellar grains that are as old as 7.5 billion years. Pre-dating our entire solar system by some 3 billion years. 

The investigation took material from the famous Murchison meteorite that fell and scattered across parts of Australia (including the town of Murchison) back in 1969. This meteorite is a carbonaceous chondrite, a supremely ‚primitive‘ object, rich in carbon compounds. It represents material that glommed together 4.5 billion years ago out of interstellar stuff – never being processed inside proto-planetary interiors. 

By crushing a sample and dissolving it in acid, the researchers were able to isolate so-called pre-solar or interstellar grains – hardy little crumbs, barely several micrometers across. 

But how do you estimate their ages? It turns out that the universe provides, in a manner, a clock. Cosmic rays are constantly zipping through space, and when these energetic particles (like protons and alpha-particles) collide with the atomic nuclei in a grain the nuclear damage leaves an isotopic signature. The longer the grain has been floating around the more of those nuclear changes accumulate.

With a lot of additional detective work to account for different initial compositions from different stellar sources of elements, and all the potential traumas of heating and bashing around that the grains might have endured, it’s possible to use the cosmic ray signature to come up with an estimate of how long the grains were hanging around in interstellar space.

In this case, using the accumulation of Neon-21 in particular, most of the grains in Murchison seem to be less than a few hundred million years old (commensurate with the amount of time they were originally adrift before getting stuck in the Murchison body). But at least 8% are astonishingly old, reaching back to approximately 7.5 billion years, some two billion years older than any previous samples.

The range of grain ages is also consistent with the idea that some 7.5 billion years ago there was a lot more star formation going on, not just in the Milky Way but across the universe.

It’s a remarkable result, and one that demonstrates that sometimes the secrets of the cosmos can literally fall in our backyard.

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