Japanese researchers have discovered the first micrometeorites known to land on Earth. No larger than droplets of fog, the spherical, iron-rich particles arrived 240 million years ago, 50 million years before the previous record-holding space dust.

“These are the the oldest fossil micrometeorites I’ve ever heard of, and the preservation is fantastic. They look exactly like their modern equivalents,” said geologist Susan Taylor of the U.S. Army Corps of engineers, who wasn’t involved in the work, published in Geology May 4. “If we can figure out where these things came from, they can help inform us about the history of the solar system.”

Meteorites and micrometeorites come from comets and asteroids, many as old as the Solar System itself. Although larger space rocks are more popular, they're exceedingly rare. The overwhelming majority of extraterrestrial material is dust, of which some 30,000 tons falls from space each year..

About 90 percent vaporizes while passing through the Earth’s atmosphere, producing the sparks seen during meteor showers. Of what makes it to the ground, a small fraction gets stuck in mud, clay and other sediment that becomes fossilized.

By finding and comparing rare collections that span the geologic record, researchers can make guesses about cosmic conditions that Earth experienced throughout its history. Trouble is, the farther back in time geologists go, the harder it is to find well-preserved micrometeorites.

“I gave up trying to find them years ago,” said Taylor. “These aren’t tough particles. They’re fragile. Just a little acid etches out their glass, causing them to disintegrate. It’s amazing we find ones this old at all.”

Geologist Tetsuji Onoue of Kagoshima University discovered the new specimens in ancient shale and chert rock from Ajiro Island, at the southern tip of Japan.

To get to the roughly 300 micrometeorites, Onoue’s team crushed and sieved the rock, cleaned it with ultrasonic waves, then ran a magnet over the material. Under an electron microscope, they found 10-micron-wide spheres that somehow survived descent through Earth’s atmosphere, chemical weathering and 240 million years of punishing fossilization.

The samples are not representative sample of ancient space dust – everything but the iron-containing spheres dissolved away – but Taylor said they still offer a unique lens into cosmic history.

“We spend a lot of money and time sending things out into space, which is great, but here on Earth we have tens of thousands of tons of extraterrestrial material arriving each year,” she said. “If we looked for more of them, we could get more information about comets and asteroids throughout time without having to visit them.”

Image: Iron-rich micrometeorites recovered from Japan's Ajiro Island in Japan. (T. Onoue/Geology)

Citation: "Composition and accretion rate of fossil micrometeorites recovered in Middle Triassic deep-sea deposits." Tetsuji Onoue, Tomoki Nakamura, Takeshi Haranosono and Chika Yasuda. Geology, published online May 4. DOI: 10.1130/G31866.1

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