An international team of researchers searched for pieces of a small asteroid tracked in space, then observed its impact on Botswana on June 2, 2018. Guided by SETI Institute meteorite astronomer Peter Jenniskens, they found 23 meteorites in the heart of the Central Kalahari Game Reserve and have now published their results online in the journal Meteorites and planetary science.
“Combining observations of the small asteroid in space with information gleaned from meteorites shows that it likely originated from Vesta, the second largest asteroid in our solar system and target of NASA’s DAWN mission,” Jenniskens said. “Billions of years ago, two giant impacts on Vesta created a family of larger and more dangerous asteroids. The newly recovered meteorites have given us a clue as to when these impacts could have occurred.”
The small asteroid that struck Botswana, called 2018 LA, was first spotted by the University of Arizona’s Catalina Sky Survey as a faint point of light moving among the stars. The Catalina Sky Survey searches for asteroids passing through Earth as part of NASA’s planetary defense program.
“Small asteroids the size of a meter are not a danger to us, but they hone our skills in detecting approaching asteroids,” said Eric Christensen, director of the Catalina Sky Survey program.
The team retrieved archival data from the SkyMapper Southern Survey program in Australia that showed the asteroid spinning in space, spinning once every 4 minutes, alternately showing us a wide side and a narrow side while reflecting the sunlight.
During its journey to Earth, cosmic rays bombarded the asteroid and created radioactive isotopes. By analyzing these isotopes, the researchers determined that 2018 LA was a solid rock about 1.5m in size, which reflected about 25% of sunlight.
“This is only the second time that we have spotted an asteroid in space before it hits Earth above Earth,” Jenniskens said. “The first was the 2008 asteroid TC3 in Sudan ten years earlier.” Jenniskens also guided the search for 2008 TC3 fragments.
This time, fewer observations have led to more uncertainty about the position of the asteroid in its orbit. Davide Farnocchia of the Center for Near-Earth Object Studies at NASA’s JPL combined astronomical observations of the asteroid with data from the US government satellite on the fireball to calculate the drop zone. Esko Lyytinen of the Ursa Finnish Fireball Network made a parallel effort.
“When Jenniskens first arrived in Maun, he needed our help to reduce the drop zone,” says Oliver Moses of the Okavango Research Institute. “We subsequently found other video recordings in Rakops and Maun and were able to triangulate the position of the fireball.”
After confirming the drop zone, Moses and Jenniskens joined geologist Alexander Proyer from Botswana International University of Science and Technology (BIUST) in Palapye and geoscientist Mohutsiwa Gabadirwe from Botswana Geoscience Institute (BGI) in Lobatse and their colleagues. to search for meteorites.
“On the fifth day, our last day of research, Lesedi Seitshiro of BIUST found the first meteorite just 30 meters from the camp,” said Jenniskens. “It was 18 grams and measured about 3 cm.”
The research area was in the Central Kalahari Game Reserve, which is home to diverse wildlife including leopards and lions. The researchers were brought to safety by staff from Botswana’s Department of Wildlife and National Parks. BGI coordinated the research with the Department of National Museums and Monuments of Botswana.
“The meteorite is named ‘Motopi Pan’ after a local waterhole,” said Gabadirwe, now curator of this rare sample of an asteroid observed in space before touching Earth. “This meteorite is a national treasure of Botswana.”
The type of meteorite
Non-destructive analysis at the University of Helsinki, Finland, showed that Motopi Pan belongs to the Howardite-Eucrite-Diogenite (HED) meteorite group, known to have probably originated from the giant asteroid Vesta, which was recently studied in detail by NASA. DAWN mission.
“We were successful in measuring the metal content and obtaining a reflectance spectrum and an elemental x-ray analysis of a thin crusted part of the interior of the exposed meteorite,” said Tomas Kohout of the University of ‘Helsinki. “All measurements added up nicely and gave values typical of HED-type meteorites.”
Dynamic studies show that the 2018 LA orbit is consistent with an origin in the inner part of the asteroid belt where Vesta is located. The asteroid was delivered into an orbit impacting Earth via the resonance located in the inner face of the asteroid belt.
“Another HED meteorite fall that we investigated in Turkey in 2015, called Sariçiçek, impacted an equally short orbit and produced mostly small meteorites of 2-5 grams,” Jenniskens said.
When Jenniskens returned to Botswana in October 2018, the team found 22 other small meteorites. Gabadirwe was the first to spot another rock out of the ordinary. Surprisingly, later meteorite discoveries showed great diversity in their outward appearance.
“We have studied the petrography and mineral chemistry of five of these meteorites and confirmed that they belong to the HED group,” said Roger Gibson of Witts University in Johannesburg, South Africa. “Overall, we classified the material in asteroid 2018 LA as Howardite, but some individual fragments had more affinity for diogenites and eucrites.”
Other studies also confirmed the surprising diversity of the team’s findings, such as reflection spectroscopy and the content of polyaromatic hydrocarbons in the sample. The asteroid was a breach, a mixture of pieces of cemented rock from different parts of Vesta.
Origin of meteorites
A previous hypothesis has proposed that Sariçiçek originated from Vesta in the collision that created the Antonia impact crater imagined by DAWN. Still showing visible ejecta blanket, this young crater formed about 22 million years ago. A third of all HED meteorites that fall to Earth were ejected 22 million years ago. Does Motopi Pan originate from the same crater?
“Measurements of noble gas isotopes at ETH in Zurich, Switzerland, and radioactive isotopes measured at Purdue University showed that this meteorite had also been in space as a small object for about 23 million years, “said Kees Welten of UC Berkeley,” but give or It may take 4 million years, so it could have come from the same source crater on Vesta. “
The researchers found that Motopi Pan and Sariçiçek were similar in some ways, but different in others. Like Motopi Pan, Sariçiçek exploded at an altitude of 27.8 km, but produced less light during this rupture.
“The infrasound shock wave measured in South Africa was not as strong as expected from bright light detections by US government sensors,” said Peter Brown of the University of Western Ontario, in Canada.
From the lead isotopes in zircon minerals, the researchers found that Sariçiçek and Motopi Pan solidified on the surface of Vesta around 4,563 million years ago. But Motopi Pan’s phosphate grains have experienced another melting event more recently. Sariçiçek did not.
“Around 4234 million years ago, the Motopi Pan material was near the center of a big impact event,” UC Davis’s Qing-zhu Yin said, “Sariçiçek was not.”
Vesta experienced two significant impact events that created the Rheasilvia Impact Basin and the underlying, and therefore older, Veneneia Impact Basin.
“We now suspect that Motopi Pan was heated by the Veneneia impact, while the subsequent impact of Rheasilvia dispersed this material,” Jenniskens said. If so, that would date the impact of Veneneia to around 4,234 million years ago. Above the Rheasilvia impact ejecta is the Rubria impact crater 10.3 km in diameter, slightly smaller than the 16.7 km Antonia crater and slightly younger at 19 +/- 3 million years old, but a good candidate for the original Motopi Pan crater. “
In November 2020, an expedition led by Fulvio Franchi of BIUST discovered another Motopi Pan meteorite. This 92-gram meteorite is now the largest 2018 LA asteroid fragment recovered to date and another small piece of the giant asteroid Vesta.