In the early morning hours of 6 February 2017, more than 500 people across 12 states and one Canadian province reported sightings of a bright green fireball. When weather radar data revealed the unmistakable trajectory of meteors into Lake Michigan, the hunt was on.
Researchers and community scientists, including Chicago area teenagers, were quick to mobilize, but they failed to recover any large chunks of the extraterrestrial visitor. However, they didn’t return empty-handed: The collaboration unearthed a slew of micrometeorites.
Scientists presented these findings this week at the 85th Annual Meeting of the Meteoritical Society in Glasgow, Scotland.
Tiny Emissaries from Space
Micrometeorites, which measure between roughly 0.01 and 2 millimeters in diameter, are relatively commonplace. Researchers have estimated that about 60 tons of cosmic dust rain down on Earth’s surface each day. “This dust is everywhere,” said Maitrayee Bose, a cosmochemist at Arizona State University who was not involved in the research. The trick, of course, is finding it—the vast majority of the dust on our planet is garden-variety Earthly dust, not extraterrestrial material. (Antarctica and Greenland, both relatively pristine places, are good places to look, but even searches in urban settings have yielded micrometeorites.)
After the 2017 fireball event, a community science effort—the Aquarius Project—came together in the Chicago area to search for the culprit meteorites. The crowd grew to include team members associated with the Adler Planetarium, the Shedd Aquarium, the Field Museum of Natural History, and NASA. Guided by scientists and educators, a group of teenagers spearheaded the design and building of a submersible sled that could be towed along the bottom of Lake Michigan. The coffee table–sized sled, dubbed “Starfall,” incorporated powerful magnets to pick up iron meteorites.
In 2018 and 2019, Aquarius Project team members enlisted the help of a University of Wisconsin-Milwaukee research ship—R/V Neeskay—to put Starfall to work. While the ship cruised on Lake Michigan, the sled was lowered to roughly 60 meters below the surface and towed along the lake bottom. Not surprisingly, the sled’s magnets picked up all sorts of rusted metal debris, but they also snagged some centimeter-scale objects that looked suspiciously like meteorites.
Ultimately, most of the candidate meteorites ended up being terrestrial rocks or bits of iron slag, but hiding within the lake sediments scooped up were what the team thought could be micrometeorites.
Spectroscopy Reveals Micrometeorites’ Secrets
After encasing each candidate micrometeorite in epoxy, Maria Valdes, a cosmochemist at the Robert A. Pritzker Center for Meteoritics and Polar Studies at the Field Museum in Chicago, and colleagues hand polished them to reveal their interiors. The researchers then directed a green laser at each candidate micrometeorite and measured the different wavelengths of light the objects scattered. This technique, a type of spectroscopy, reveals chemical composition, said Valdes. “The spectral patterns are distinct chemical fingerprints of particular minerals,” she said.
Five out of the six candidate micrometeorites were composed largely of olivine or pyroxene, the team found. That’s strong evidence that these objects really are micrometeorites, albeit the most common subtype dominated by silicate minerals, said Valdes. But intriguingly, the researchers found that the micrometeorites differed in texture: One was decidedly finer grained than its brethren. That finding said something about these objects’ origin, the researchers suggested. “They don’t seem to all come from the same parent body,” said Valdes. Some of these micrometeorites might have been delivered by the 2017 event, but others probably weren’t, the team concluded.
In the future, Valdes and her team hope to analyze the isotopes of oxygen present in these micrometeorites. Such an investigation would shed light on the type of solar system object that originally shed these bits of cosmic dust, said Bose.
And in case that’s not enough to keep the researchers busy, Valdes recently received a whole new batch of candidate micrometeorites to investigate. “I’ve gotten about 50 more candidates,” she said.
—Katherine Kornei (@KatherineKornei), Science Writer