A cosmic survivor that avoided collisions in the pinball world of asteroids

In April 2019, rare primitive meteorites fell near the town of Aguas Zarcas in northern Costa Rica. In an article published in the journal Meteoritics & Planetary Science, an international team of researchers describes the circumstances of the fall and show that mudball meteorites are not necessarily weak.

“Twenty-seven 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.

The Murchison meteorite fell only two months after the first manned landing on the moon in 1969, when researchers were ready to study moon rocks and eagerly trained their instruments on this other rock from space.

“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 José, paraphrasing Neil Armstrong’s words. “Seventy-six papers have since been written about this meteorite.”

Jenniskens teamed up with Soto to investigate the new fall.

“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.

Analysis of video camera footage by the team showed that the rock entered at a near-vertical angle into Earth’s atmosphere from a WNW direction at a speed of 14.6 kilometers per second. The intense heat of collisions with the atmosphere melted (ablated) much of the rock, but there was surprisingly little sign of 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.”

Nature was kind to this meteorite in that the fall occurred at the end of an unusually long dry season in Costa Rica.

“The Aguas Zarcas fall produced an amazing selection of fusion-crusted stones with a wide range of shapes,” said co-author and meteoriticist Laurence Garvie at the Buseck Center for Meteorite Studies at Arizona State University. “Some stones have a beautiful blue iridescence to the fusion crust.”

Many of the stones are unbroken as they landed on the relatively soft jungle and grassy surfaces. Researchers were surprised by the unusual shape of many of the rocks caused by ablation, without the relatively flat surfaces that result from secondary 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.”

The research team now believes that Aguas Zarcas is strong because it avoided collisions in space and did not have the cracks that weaken many meteorites.

“The last collision experienced by this rock was two million years ago,” said cosmochemist Kees Welten of UC Berkeley.

He and his team measured how long the rock was exposed to cosmic rays after it had broken off from a larger asteroid.

“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 team determined the rock was about 60 centimeters in diameter when it hit the Earth’s atmosphere. From the path it traveled through the atmosphere, the team traced the meteorite back to the 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.”

Because the rock was strong and entered at a steep angle, a relatively large fraction of its mass survived to the ground.