On the night of 23 April 2019, the peace of the rain forest near Aguas Zarcas, Costa Rica, was shattered by a brilliant streak of light in the sky, a loud blast, and a hailstorm of rocks—the remains of a small asteroid that exploded in the atmosphere. One fragment punched a grapefruit-sized hole in the roof of a house, and a second smacked into a doghouse.
Within days, the silence was broken again as scientists and collectors from around the world began scouring the landscape for fragments, which were worth more than gold—both literally and scientifically.
One team of geologists proposed that the Aguas Zarcas meteorite demonstrates that at least one class of asteroids may be busier than expected, with small pebbles hopping around their surfaces like kernels of popcorn in the microwave—action already observed on the surface of the asteroid Bennu.
“Asteroids were pretty active in the early solar system, but today, most people think they’re dead,” said Xin Yang, a graduate student at the University of Chicago and lead author of the study. “This tells us that carbonaceous asteroids may not be dead; they just have a low level of activity.”
The study began shortly after the Field Museum in Chicago received a 1.9-kilogram fragment of the meteorite, which is classified as a carbonaceous chondrite. It consists of small chunks of material embedded in a matrix of finer-grained rock.
“These meteorites are very rare,” said Philipp Heck, curator of meteoritics and polar studies at the museum and a study coauthor. “They contain primitive material that formed in the early solar system and is still preserved today. Aguas Zarcas is a type that’s very pristine, primordial stuff. It contains presolar stardust, some of the earliest mineral crystals in the solar system, and prebiotic organic molecules. The organics gave it an interesting smell—a little tarry, a little sweet—almost like vanilla.”
Small Pebbles Resist Destruction
Yang, who also works at the Field Museum with Heck, began processing a 79-gram fragment of the meteorite, which had been subjected to a computed tomography (CT) scan to map its interior to study its mineralogy. The specimen had been collected before any rain could fall on it and then stored in nitrogen, limiting terrestrial contamination.
The first step was to subject it to an alternate cooling-warming cycle in liquid nitrogen and water, which eventually pulverized the rock. After more than 100 cycles, though, pebble-sized inclusions known as chondrules remained unbroken. They also proved tough to destroy with a mortar and pestle. “We thought that was interesting, so we said, ‘let’s not break them down,’” said Heck.
They did an additional CT scan and looked at the original scans again, which revealed that the odd chondrules were flattened, like pancakes, instead of spherical, as chondrules (including many that were found in the same chunk of Aguas Zarcas) normally appear. (Chondrules are spherical because they form as small beads of molten rock cool and solidify in the zero-gravity environment of space. They then are incorporated into an asteroid as it coalesces, so they predate their host asteroid.)
In addition, the scans showed that all the flattened oddballs had been aligned in the same direction inside the meteorite, indicating they all got squished when another space rock slammed into the parent asteroid and then later were scattered, allowing them to mix with undeformed chondrules.
Heck said the findings reminded him of Bennu, the asteroid visited by OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) from 2018 to 2021. In addition to collecting a few dozen grams of surface material for return to Earth next year, the craft discovered pebbles streaming into space and either falling back or entering orbit. The pebbles were similar in size to the Aguas Zarcas chondrules—generally 0.5 to 1 centimeter for Bennu, compared to a few millimeters to about a centimeter for Aguas Zarcas.
OSIRIS-REx recorded more than 300 ejection events, or an average of almost one per day. “The events ranged from quite explosive, with numerous particles shooting out, to popcorn-like, with a few small particles popping off the surface,” said Harold Connolly Jr., a professor of geology at Rowan University in Glassboro, N.J., and leader of the sample science team for OSIRIS-REx, who was not involved in the study. “Some went into orbit but usually for no more than a few days, so what goes up must come down.”
Mission scientists devised a range of explanations for the ejections, including impacts, cracking caused by the temperature extremes of Bennu’s day-night cycle, and small outbursts caused by the vaporization of ices just below the surface.
Day-Night Cycle May Free Pebbles
The dynamics of the ejected pebbles suggested that the asteroid that gave birth to Aguas Zarcas must have been like Bennu, Yang said, with chondrules popping off into space then settling back in different locations, mingling with other chondrules that also were transported from elsewhere. (The process shouldn’t work on larger asteroids because their surface gravity would limit how far an ejected bit of rock could travel.)
Large impacts can excavate a lot of material, spraying it all across the asteroid’s surface, but the relocated chondrules then should be alike—all similarly deformed and aligned. Because Aguas Zarcas contains a mixture of shapes and orientations, however, that scenario doesn’t seem to work, Yang said. Instead, “this deformation appears to be local,” with less powerful impacts compressing the chondrules across a smaller area and leaving many of them buried beneath the surface.
This observation favors the day-night cycle as the cause of the scattering of both the Bennu and Aguas Zarcas pebbles, Heck suggested. The brittle rock may fracture, turning it into a loosely consolidated pit of rocks and dust and perhaps kicking away some debris. In addition, smaller impacts could splash some of the rock away, scattering it across the asteroid. Another impact could smash mingled material back together, forming solid rock like Aguas Zarcas. This impact could also blast chunks into space to drift through the solar system—and eventually land on Earth.
The Chicago team is examining additional carbonaceous chondrites, as well as reviewing old CT scans from previous studies, to verify the scenario outlined in the study. In addition, Heck is a member of the team that will analyze the roughly 60 grams of Bennu collected by OSIRIS-REx, which may shed some light on the formation and redistribution of the pebbles.
“It will contribute to answering parts of their hypothesis,” said Connolly. “And it’ll undoubtedly open up new questions we’ve never thought about. That’s the wonder of looking at a new rock for the first time—you’ll never be bored because you always find something you don’t understand.”
—Damond Benningfield, Science Writer