A 22-Million-Year Journey From the Asteroid Belt to Botswana

This was a surreal place to go meteorite hunting. Bat-eared foxes and warthogs strolled past, lions stealthily stalked and slaughtered giraffes while leopards lounged in trees. Wardens from Botswana’s Department of Wildlife and National Parks protected the search party in case a fanged predator got too close for comfort. The meteorites also looked a lot like animal poop, meaning the team were frequently bamboozled by coprological impostors.

“It was a totally unusual experience for all of us,” said Mr. Gabadirwe.

Only on June 23, the last day of the initial search mission, was the first meteorite found — a small piece of the stars weighing less than an ounce. It was named Motopi Pan, after a local watering hole. “It became a national treasure of Botswana, this little rock,” Dr. Jenniskens said.

The meteorites’ compositions were matched to those found on Vesta, with the help of data from NASA’s Dawn spacecraft. Now lifelessly orbiting the dwarf planet Ceres after running out of fuel in late 2018, Dawn documented the geology of Vesta from 2011 to 2012. Scientists corroborated this origin story by reverse engineering the asteroid’s Earthbound trajectory.

Cosmic rays imprint traces on asteroids by altering atomic nuclei. The traces on these meteorites suggested the asteroid that crashed into Earth bathed in this radiation for 22 million years as it traveled to Earth. That meant an impact crater 22 million years old would mark the spot where this asteroid was liberated from Vesta.

A six-mile-long crater named Rubria on Vesta was the best candidate. The asteroid’s surprising lack of contamination by the solar wind — the stream of plasma and particles coming from the sun — suggested the asteroid’s material was shielded from space for billions of years. Unlike one other similarly aged crater, Rubria sat on a hill undisturbed by landslides, a tranquil place 2018 LA could remain buried before an impact set it free.

“The study has it all, in terms of cosmic drama,” said Katherine Joy, a meteorite expert at the University of Manchester in England not involved with the work. But linking 2018 LA to a specific place on Vesta relies on many underlying assumptions, so no one can be sure that Rubria is the correct spot.

For now, scientists will continue to monitor the skies and scout Earth’s deserts, hoping to find more enlightening fragments of our cosmic cradle’s past. Meteorite hunts “are always incredible adventures,” said Dr. Jenniskens — an exhausting but thrilling way to spend a lifetime.

source: nytimes.com