"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 Murchison event occurred just two months after humans first walked on the Moon, capturing the attention of scientists worldwide. In similar spirit, the Aguas Zarcas meteorite fall has become a focal point for planetary science. "The recovery of Aguas Zarcas, too, was a small step for man, but a giant leap for meteoritics," said geologist Gerardo Soto of the University of Costa Rica in San Jose. "76 papers have since been written about this meteorite."

Soto and Jenniskens collaborated to document the event, which became a nationwide sensation. "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.

The researchers used video camera recordings to reconstruct the meteorite's entry path. They found the rock hurtled into Earth's atmosphere at 14.6 kilometers per second from a west-northwest direction, descending at a steep angle. While much of the outer layer melted due to intense friction, fragmentation was unexpectedly minimal.

"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."

Unusually dry conditions in Costa Rica at the time aided recovery efforts. According to Laurence Garvie, a meteoriticist at Arizona State University's Buseck Center for Meteorite Studies, "The Aguas Zarcas fall produced an amazing selection of fusion-crusted stones with a wide range of shapes. Some stones have a beautiful blue iridescence to the fusion crust."

Soft ground in the region helped preserve the meteorite fragments, many of which remained intact. Their irregular forms, sculpted by ablation, lacked the flattened surfaces typically caused by in-air fragmentation.

"Other meteorites of this type are often described as mudballs, as they contain water-rich minerals," said Jenniskens, "Apparently, that does not mean they are weak."

Researchers concluded that the rock's surprising strength likely came from its relatively untouched past in space, with no major impacts to compromise its integrity.

"The last collision experienced by this rock was 2 million years ago," said cosmochemist Kees Welten of UC Berkeley, who led the team analyzing the rock's cosmic ray exposure history.

"We know of other Murchison-like meteorites that broke off at approximately the same time, and likely in the same event," said Welten, "but most broke much more recently."

The researchers estimate the meteoroid was about 60 centimeters wide before entering the atmosphere. By tracing its trajectory, they concluded it originated in the outer parts of a larger asteroid located in the low region of the main asteroid belt.

"We can tell that this object came from a larger asteroid low in the asteroid belt, likely from its outer regions," said Jenniskens. "After getting loose, it took two million years to hit the tiny target of Earth, all the time avoiding getting cracked."

Its strong structure and steep descent allowed a sizable portion of the meteorite to survive the fiery entry and reach the ground.

Research Report:Orbit, meteoroid size, and cosmic ray exposure history of the Aguas Zarcas CM2 breccia

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