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April 23, 2021
by Rebecca McDonald, SETI Institute
An international team of researchers looked for pieces of a small asteroid that was being tracked in space and then watched it hit Botswana on June 2, 2018. Under the direction of meteor astronomer Peter Jenniskens from the SETI Institute, they found 23 meteorites deep in the central Kalahari game reserve and have now published their results online in the journal Meteoritics and Planetary Science.
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« The combination of observations of the small asteroid in space with information from the meteorites shows that it probably came from Vesta, the second largest asteroid in our solar system and target of NASA’s DAWN mission, » said Jenniskens. « Billions of years ago, two huge impacts on Vesta produced a family of larger, more dangerous asteroids. The newly recovered meteorites gave us an indication of when these impacts might have occurred. »
The small asteroid that hit Botswana and in 2018 LA was first discovered by the University of Arizona’s Catalina Sky Survey as a faint point of light between the stars. The Catalina Sky Survey searches for Earth-crossing asteroids as part of NASA’s Planetary Defense program.
« Small meter-sized asteroids are not a threat to us, but they improve our ability to detect approaching asteroids, » said Eric Christensen, director of the Catalina Sky Survey program.
The team restored archival data from the SkyMapper Southern Survey program in Australia that showed the asteroid spinning in space, spinning every four minutes, and alternating one wide and one narrow side pointed as it reflected sunlight.
On its journey to Earth, cosmic rays bombarded the asteroid, creating radioactive isotopes. By analyzing these isotopes, the researchers determined that in 2018 LA was a 1.5 m solid rock that reflected about 25% of sunlight.
« This is the second time we’ve discovered an asteroid in space before it hits the earth over land, « said Jenniskens. « The first was the 2008 TC3 asteroid in Sudan ten years ago. » Jenniskens also led the search for fragments from 2008 TC3.
This time, fewer observations led to more uncertainty in the asteroid’s position in its orbit. Davide Farnocchia of NASA’s JPL Center for Near-Earth Object Studies combined astronomical observations of the asteroid with satellite data from the US government fireball to calculate the area of fall. At the same time, Esko Lyytinen from the Finnish fireball network undertook Ursa.
« When Jenniskens first arrived in Maun, he needed our help to narrow down the area of the case, » says Oliver Moses of the Okavango Research Institute. « We then tracked down further video recordings in Rakops and Maun and were able to triangulate the position of the fireball. »
After confirming the fall area, Moses and Jenniskens searched together with geologist Alexander Proyer from the Botswana International University of Science and Technology (BIUST ) in Palapye and the geoscientist Mohutsiwa Gabadirwe from the Botswana Geoscience Institute (BGI) in Lobatse and her colleagues looked for the meteorites.
« On the fifth day, our last day of the search, Lesedi Seitshiro from BIUST found the first meteorite only 30 meters away from camp, « said Jenniskens. « It was 18 grams and about 3 cm tall. »
The search area was in the Central Kalahari Game Reserve, which is home to various wildlife such as leopards and lions. The researchers were brought to safety by the staff of the Botswana Department of Wildlife and National Parks. BGI coordinated the search with the Department of National Museum and Monuments in Botswana.
« The meteorite is called ‘Motopi Pan’ after a local watering hole, » said Gabadirwe, now curator of this rare sample of an asteroid that hit Earth before it hit observed in space. « This meteorite is a national treasure of Botswana. »
Non-destructive analysis at the University of Helsinki, Finland, showed that Motopi Pan belongs to the group of Howardite-Eucrite-Diogenite Meteorites (HED), which are known to that they probably came from the giant asteroid Vesta, which was recently examined in detail by NASA’s Dawn Mission.
« We managed to measure the metal content, as well as to obtain a reflection spectrum and an X-ray element analysis from a thinly encrusted part of the exposed interior of the meteorite « said Tomas Kohout from the University of Helsinki. « All measurements added up well and showed values that are typical of HED-type meteorites. »
Dynamic studies show that the orbit of 2018 LA coincides with an origin from the inner part of the asteroid belt in which Vesta is located . The asteroid was brought into an earth-impacting orbit via the resonance in the inside of the asteroid belt.
« Another HED meteorite fall that we investigated in Turkey in 2015, Sariçiçek, hit a similarly short orbit and mostly produced smaller 2 to 5 gram meteorites, « said Jenniskens.
When Jenniskens returned to Botswana in October 2018, the team found 22 more small meteorites. Gabadirwe was the first to discover another out of this world stone. Surprisingly, subsequent meteorite finds showed great diversity in their external appearance.
« We examined 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 contained in the 2018 LA asteroid as Howardite, but some individual fragments had greater affinity for Diogenites and Eucrites. » Other studies also confirmed the surprising diversity of the team’s findings, such as this Reflectance spectroscopy and the content of polyaromatic hydrocarbons in the sample. The asteroid was a breccia, a mixture of cemented pieces of rock from different parts of Vesta.
An earlier hypothesis was that Sariçiçek came from Vesta in the collision that formed the Antonia impact crater pictured by DAWN. This young crater, still showing a visible ejecta blanket, was formed about 22 million years ago. A third of all HED meteorites falling to Earth were ejected 22 million years ago. Does Motopi Pan come from the same crater?
« Noble gas isotope measurements at the ETH in Zurich, Switzerland, and radioactive isotopes at Purdue University showed that this meteorite had also been a small object in space for about 23 million years, » said Kees Welten from UC Berkeley It takes 4 million years, so it could have come from the same source crater on Vesta. «
The researchers found that Motopi Pan and Sariçiçek are similar in some ways but different in others. Like Motopi Pan, Sariçiçek exploded at a height of 27.8 km, but produced less light in this separation.
« The infrasound shock wave measured in South Africa was not as strong as expected from the US government’s bright light sensor detections, » said Peter Brown of the University of Western Ontario, Canada.
Researchers found lead isotopes in zirconia minerals reported that both Sariçiçek and Motopi Pan solidified on the surface of Vesta about 4563 million years ago. But phosphate grains in Motopi Pan have seen another melting event more recently. Sariçiçek did not.
« Located about 4234 million years ago the material in Motopi Pan near the center of a major impact event, « said Qing-zhu Yin of UC Davis, » Sariçiçek was not. «
Vesta experienced two major impacts events that caused the Rheasilvia impingement pool and the underlying and therefore older Veneneia impingement pool.
« We now suspect that Motopi Pan was heated by the Veneneia impact, while the subsequent Rheasilvia impact dispersed this material », said Jenniskens. « If so, this would date the Veneneia impact to about 4,234 million years. In addition to the Rheasylvia impact ejection, there is the Rubria impact crater, 10.3 km in diameter, which is slightly smaller than the 16.7 km long Antonia crater and is a little younger at 19 / -. 3 million years, but a good candidate for the original crater of Motopi Pan. «
In November 2020, an expedition led by Fulvio Franchi from BIUST discovered another Motopi-Pan- Meteorites. This 92-gram meteorite is today the largest fragment of the asteroid 2018 LA recovered to date and another small piece of the giant asteroid Vesta.
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