Aguas Zarcas meteorite with irregular surface features. This 146g stone is on loan to the Buseck Center for Meteorite Studies from Michael Farmer. Credit: Arizona State University / SETI Institute.

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

In April 2019, rare primitive meteorites fell near the town of Aguas Zarcas in northern Costa Rica.

Now, an international team of researchers have described the circumstances of the fall and show that ‘mudball’ meteorites are not necessarily weak.

Meteor astronomer Peter Jenniskens of the SETI Institute and NASA Ames Research Center, said: “27 kilos of rocks were recovered, making this the largest fall of its kind since similar meteorites fell near Murchison in Australia in 1969.”

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.

Geologist Gerardo Soto of the University of Costa Rica in San José, paraphrasing Moon astronaut Neil Armstrong’s words, said: “The recovery of Aguas Zarcas, too, was a small step for man, but a giant leap for meteoritics. 76 papers have since been written about this meteorite.”

Jenniskens teamed up with Soto to investigate the new fall.

Soto said: “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.”

Video footage

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 kilometres per second.

The intense heat of collisions with the atmosphere melted (ablated) much of the rock, but there was surprisingly little sign of fragmentation.

Jenniskins said: “It penetrated deep into Earth’s atmosphere, until the surviving mass shattered at 25km above the Earth’s surface, 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.

Co-author and meteoriticist Laurence Garvie, of the Buseck Center for Meteorite Studies at Arizona State University, said: “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.”

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.

Jenniskins said: “Other meteorites of this type are often described as ‘mudballs’, as they contain water-rich minerals. Apparently, that does not mean they are weak.”

Cosmic rays

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.

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

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

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.”

The team determined the rock was about 60cm in diameter when it hit the Earth’s atmosphere.

From the path it travelled through the atmosphere, the team traced the meteorite back to the asteroid belt.

Jenniskens said: “We can tell that this object came from a larger asteroid low in the asteroid belt, likely from its outer regions.

“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.

The article is published in Meteoritics & Planetary Science.