In the early morning hours of February 6, 2017, more than 500 people in 12 states and one Canadian province reported sightings of a bright green fireball. When weather radar data revealed the unmistakable trajectory of meteors in Lake Michigan, the hunt was on.
Researchers and community scientists, including teenagers from the Chicago area, quickly mobilized, but were unable to recover any large pieces of the extraterrestrial visitor. However, they did not return empty-handed: the collaboration revealed many micrometeorites.
Scientists presented the findings this week at the Meteorite Society’s 85th annual meeting in Glasgow, Scotland.
Tiny emissaries from space
Micrometeorites, which are about 0.01 to 2 millimeters in diameter, are relatively common. Researchers have estimated that about 60 tons of space dust falls on the Earth’s surface every day. “This dust is everywhere,” said Maitrei Bose, an astrochemist at Arizona State University who was not involved in the study. The trick, of course, is to find it—the vast majority of dust on our planet is garden-type terrestrial dust, not extraterrestrial material. (Antarctica and Greenland, both relatively untouched places, are good places to search, but even searches in urban settings have yielded micrometeorites.)
After the 2017 fireball event, the Project Aquarius scientific community came together in the Chicago area to find the meteorites responsible. The crowd grew to include team members associated with the Adler Planetarium, the Shedd Aquarium, the Field Museum of Natural History, and NASA. Under the guidance of scientists and educators, a group of teenagers led the design and manufacture of an underwater sled that could be towed along the bottom of Lake Michigan. The coffee-table-sized sled, dubbed the Starfall, contained powerful magnets that collected iron meteorites.
In 2018 and 2019, members of the Aquarius Project team enlisted the help of the University of Wisconsin-Milwaukee research vessel R/V Neeskay— launch Starfall into operation. During the ship’s cruise on Lake Michigan, the sled was lowered approximately 200 feet below the surface and towed along the bottom of the lake. Not surprisingly, the sled’s magnets picked up all sorts of rusty metal debris, but they also snagged centimeter-scale objects that looked suspiciously like meteorites.
In the end, most of the candidate meteorites turned out to be terrestrial rocks or pieces of iron slag, but the lake sediments that lurked there were what the team believed to be micrometeorites.
Spectroscopy reveals the secrets of micrometeorites
After each candidate micrometeorite was sealed with epoxy, Maria Valdez, a cosmochemist at the Robert A. Pritzker Center for Meteorites and Polar Studies at the Field Museum in Chicago, and her colleagues polished them by hand to reveal their interior. The researchers then pointed a green laser at each candidate micrometeorite and measured the different wavelengths of light scattered by the objects. This method, a type of spectroscopy, allows the chemical composition to be determined, Valdes said. “Spectral patterns are distinct chemical fingerprints of certain minerals,” she said.
Five of the six candidate micrometeorites were composed primarily of olivine or pyroxene, the team found. This is strong evidence that these objects are indeed micrometeorites, even though they are the most common subtype, dominated by silicate minerals, Valdes said. But interestingly, the researchers found that the micrometeorites differ in texture: one was clearly finer than its brothers. The researchers suggested that this find said something about the origin of these objects. “They don’t all seem to come from the same parent body,” Valdez said. Some of these micrometeorites may have been delivered to the 2017 event, but others likely were not, the team concluded.
In the future, Valdez and her team hope to analyze the oxygen isotopes present in these micrometeorites. Such a study would shed light on the type of Solar System object that initially ejected these bits of cosmic dust, Bowes said.
And if that’s not enough to keep researchers busy, Valdez recently received a whole new batch of micrometeorite candidates to study. “I received about 50 more candidates,” she said.
—Kateryna Korniy (@KatherineKornei), scientific writer