Chicago teens search the bottom of Lake Michigan for a meteorite.
On February 6, 2017, a minivan-sized meteorite sonic-boomed through the atmosphere at dawn, crashing into Lake Michigan, about 10 miles off the coast of Sheboygan, Wisconsin. That’s where members of the Adler Planetarium’s Aquarius Project would begin their meteorite hunt.
The Aquarius Project got its start after an Adler astronomer saw the meteorite fall. In the morning he sent an email to colleagues involved in the Adler’s student outreach program, Far Horizons.
A teen-driven meteorite recovery, they decided, might be possible. But they’d need help. They dubbed the effort the Aquarius Project and enlisted a NASA planetary scientist, a Shedd Aquarium biologist, and the Field Museum’s meteoritics curator—UChicago cosmochemist Philipp Heck. One of Heck’s graduate students, Jennika Greer, was instrumental in teaching the teens about meteorites: what they look like, how to find them, and why they matter to astronomers.
Meteorites are invaluable for scientific research, because they contain the oldest materials of our solar system—providing a record of the conditions and chemical compositions of the system’s infancy. A recovered meteorite can also be used to piece together the current composition of its birthplace—usually the asteroid belt for Earth-bound meteorites, but occasionally the moon or Mars.
The majority of these coveted space rocks plunge into water, and almost all remain there. Only one has been recovered using ROVs (remotely operated vehicles): the Nautilus expedition sought fragments of a meteorite that crashed off the coast of Washington State in March 2018. Nautilus, which retrieved a single two-millimeter-wide meteorite fragment, was run by scientists from NASA, NOAA, and several universities and institutes. That was almost a year after the Aquarius meteorite fell into Lake Michigan, so when the Far Horizons summer interns embarked on their mission, they had no experience and no example to follow. But what they lacked in expertise, they made up for in numbers.
To figure out how to retrieve meteorite pieces 260 feet under water, the Far Horizons students teamed up with the Field’s Youth Council and the Shedd’s underwater robotics program to design and build an underwater sled to dredge the lake bed. Over three summers, more than 600 Chicago Public School students invented and adapted such a sled, eventually dubbed Starfall, and the retrieval arm for their Trident ROV, provided by a National Geographic initiative.
Starfall’s meteorite-retrieving power comes from wheels bearing rare-earth magnets. It also has collection bays for the magnet harvest, a ballast for orientation, and drop and GoPro cameras. Flexible cage-like contraptions taken from a gardening tool called a Nut Wizard gather less-magnetic pieces like fallen acorns off a lawn. The sled is almost completely built from repurposed parts, including material salvaged from retired Adler exhibits and the Shedd’s kitchen renovation. The ROV retrieval arm was made using a 3D printer and is also outfitted with a powerful magnet.
Greer wasn’t involved with the engineering—that was student-driven—but she taught the team that most meteorites contain some amount of iron-nickel metal and would therefore be attracted by magnets. She also helped them create artificial meteorites made of cement and metal ball bearings to test their sled in the lake.
In July 2018 and 2019, the Aquarius teens hitched a ride out to the crash site on the University of Wisconsin–Milwaukee’s research vessel Neeskay. It wasn’t all smooth sailing; their first trip out, the magnetic sled got stuck to the ship’s metal floor. After each haul, the team collected everything attached to and contained in the sled and ROV, even the finest grains, and brought it back to the lab.
Greer is especially involved in the sorting and analyzing stage and is still working through the collection from Summer 2019. “The majority of what we get is these iron slag droplets,” she says, easily spotted and discarded because they’re spherical and rusty. If a rock has a black glassy layer, that could be a fusion crust, formed when meteorites blaze through the atmosphere.
“Based on the radar, we think that [the Aquarius meteorite] is an ordinary chondrite”—a meteorite with “round blobs” inside that were once molten. Sometimes Greer inspects the interior by cracking the rock in half. If it’s white, that’s likely crushed quartz, not what they expect to find in their meteorite. If the rock is too hard to break—“like, denting the chisel”—then it’s probably more slag.
If Greer finds a promising candidate, Field Museum researchers will use spectroscopy for chemical analysis. If that reveals minerals common to meteorites, they’ll examine the rock with a scanning election microscope to determine elemental composition. “The last stage will be to take it to the SIMS—secondary ionization mass spectrometer,” says Greer, “to do isotopic work.” So far Greer has ruled out all the candidates she’s seen. “That’s my role,” she says, “to kill dreams.”
The Aquarius Project may never find a meteorite—and with the Field Museum closed this spring, analysis is on hold—but the program has already paid for itself, so to speak. The fireball fell in an unexplored area of Lake Michigan, and when the team visited the crash site, they mapped the lake bed with sonar and magnetometers, which measure magnetic fields. They correctly predicted the lake bed was clay, but they discovered it was covered by several inches of soft, dark sediment that billowed up when disturbed by the sled. The bed was also carpeted with invasive quagga mussels. These revelations provided new data for the Shedd’s biologist and informed future adjustments to the equipment.
The Aquarius Project had a long lead-up; the teens had to first find the strewn field, learn about meteorites, build the equipment, and test it out. More than a year passed between the crash and the first collection, during which time the fragments might have been moved by currents or buried by sediment. The odds were already against the team. But now the technique has been developed and the equipment outfitted. When another fireball falls close to home, the next crew of Aquarius Project meteorite hunters will be ready.