In 2019, a rather large meteorite fell in Costa Rica. Researchers attributed it to the Murchison type. Usually such space rocks are considered primitive because they are essentially pieces of space dirt. But this time it became clear that it wasn’t so simple.
Meteorite from Costa Rica
In April 2019, several meteorites fell near the town of Aguas Zarcas in northern Costa Rica and were categorized as the Murchison type. In a paper published in the journal Meteoritics & Planetary Science, an international team of researchers describes the circumstances of the fall and shows that space rocks that are usually thought of as just pieces of debris are not so primitive.
“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 that gave its name to this type of space rock fell only two months after the first manned lunar landing in 1969, when researchers were ready to study lunar rocks and excitedly practiced their tools on this other rock from space.
“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,” said geologist Gerardo Soto of the University of Costa Rica in San José.
Circumstances of the meteorite impact
Analysis of the team’s video camera records showed that the rock entered the Earth’s atmosphere at a near-vertical angle from a northwesterly direction at a speed of 14.6 kilometers per second. The intense heat from the collision with the atmosphere melted (ablation) most of the rock, but there were surprisingly few traces of fragmentation.
“It penetrated deep into the Earth’s atmosphere until the surviving mass broke up 25 kilometers above the Earth’s surface,” Jenniskens said, “where there was a bright flash that was detected by satellites in orbit.
Nature was favorable to this meteorite as the fall occurred at the end of an unusually long dry season in Costa Rica.
“The Aguas Zarcas fall created an amazing collection of melted crustal rocks with a wide range of shapes,” said co-author and meteoriticist Laurence Garvie of the Buseck Center for Meteorite Studies at Arizona State University. “Some rocks have beautiful blue shimmers on the melted crust.”
Solid rocks
Many of the rocks did not break as they landed on relatively soft jungle and grassy surfaces. The researchers were surprised by the unusual shape of many of the rocks caused by ablation, with no relatively flat surfaces resulting from secondary fragmentation.
“Other meteorites of this type are often described as mud balls because they contain water-rich minerals,” Jenniskens said. “Obviously, that doesn’t mean they’re fragile.
The research team now believes Aguas Zarcas is not such because it avoided collisions in space and did not have the cracks that weaken many meteorites.
“The last impact this rock experienced was two million years ago,” says cosmochemist Kees Welten of the University of California, Berkeley.
Welten and his team measured how long the rock was exposed to cosmic rays after it broke away 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 that the rock was about 60 centimeters in diameter when it entered Earth’s atmosphere. Based on the trajectory it took through the atmosphere, the team traced the meteorite’s path to the asteroid belt.
“We can say that this object broke away from a larger asteroid lower in the asteroid belt, probably from its outer regions,” Jenniskens said. “After getting loose, it took two million years to hit the tiny target of Earth, all the time avoiding getting cracked.”
Since the rock was solid and entered at a steep angle, a relatively large portion of its mass survived on Earth.
According to phys.org
