"27 kilos of rocks were recovered, making this the largest fall of its kind since similar meteorites fell near Murchison in Australia in 1969," said meteor astronomer Peter Jenniskens of the SETI Institute and NASA Ames Research Center.
That earlier fall coincided with humanity's first Moon landing, setting off a scientific rush to compare space materials. The Aguas Zarcas event proved equally significant in the field of meteoritics. "The recovery of Aguas Zarcas, too, was a small step for man, but a giant leap for meteoritics," said Gerardo Soto, a geologist at the University of Costa Rica. "76 papers have since been written about this meteorite."
Jenniskens and Soto jointly investigated the fall, which generated widespread public interest. "The fall of Aguas Zarcas was huge news in the country. No other fireball was as widely reported and then recovered as stones on the ground in Costa Rica in the last 150 years," Soto added.
Using video evidence, the researchers determined that the meteorite entered the atmosphere from the west-northwest at 14.6 kilometers per second, striking nearly vertically. While the intense atmospheric entry vaporized some of the rock, there was little fragmentation. "It penetrated deep into Earth's atmosphere, until the surviving mass shattered at 25 km above the Earth's surface," said Jenniskens, "where it produced a bright flash that was detected by satellites in orbit."
Thanks to the end of a prolonged dry season, many of the recovered fragments were in pristine condition. "The Aguas Zarcas fall produced an amazing selection of fusion-crusted stones with a wide range of shapes," said Laurence Garvie, a co-author and meteoriticist at Arizona State University's Buseck Center. "Some stones have a beautiful blue iridescence to the fusion crust."
Most of the stones remained intact upon landing due to the soft ground of the jungle and grassy fields. Their aerodynamic shapes and lack of angular breakage hinted at minimal secondary fragmentation. Jenniskens noted, "Other meteorites of this type are often described as mudballs, as they contain water-rich minerals. Apparently, that does not mean they are weak."
The team concluded that the rock retained its strength by avoiding violent collisions in space, which often introduce fractures. "The last collision experienced by this rock was 2 million years ago," said UC Berkeley cosmochemist Kees Welten. His team measured the rock's exposure to cosmic rays, a method that reveals the time it spent traveling through space.
Welten added, "We know of other Murchison-like meteorites that broke off at approximately the same time, and likely in the same event, but most broke much more recently."
At the time of atmospheric entry, the object measured about 60 centimeters across. By reconstructing its flight path, the scientists traced its origin to the outer reaches of the asteroid belt. "We can tell that this object came from a larger asteroid low in the asteroid belt, likely from its outer regions," Jenniskens explained. "After getting loose, it took two million years to hit the tiny target of Earth, all the time avoiding getting cracked."
Its steep entry and internal strength ensured that a significant portion of its mass reached the surface, giving scientists an extraordinary opportunity to examine one of the most pristine samples of early solar system material.
Research Report:Orbit, meteoroid size, and cosmic ray exposure history of the Aguas Zarcas CM2 breccia
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