Tracing a falling star

28th November 2018

By: Rebecca Campbell

Creamer Media Senior Deputy Editor

     

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International cooperation involving researchers at South Africa’s University of the Witwatersrand (Wits) and Madagascar’s University of Antananarivo (UA), supported by an Italian laboratory, has successfully described, classified and traced the history of a meteorite that fell in the area of the south-western Madagascan town of Benenitra on July 27 this year. The project involved Wits’ School of Geosciences, UA’s Laboratory of Seismology and Infrasound/Observatory of Geophysics, the Africa Array Seismic Network (AASN, which is coordinated from Wits) and Italy’s Laborattori Nazionali del Gran Sasso (Gran Sasso National Laboratory) of the Istituto Nazionale di Fisica Nucleare (National Institute of Nuclear Physics).

The fall of the meteorite on the evening of the 27th was observed by the local people, who, a few days later, told Wits Geosciences graduate Tim Marais (who happened to be in that part of the country at the time) of a bright meteor fireball, with a loud explosion, followed by a rain of meteor fragments in and around Benenitra. Fortunately, no people or buildings were hit. Marais was able to get several small fragments which had been collected by people in the area. These fragments displayed signs of a dark fusion crust.

The local people also informed the Malagasy-language Madagascan newspaper “Triatra Gazette”, which published the story on September 4. This story included a photo of a large fragment that had a similar fusion crust.

Marais took his fragments back to Wits and handed them over to Geosciences School professors Roger Gibson and Lewis Ashwal to see if they were in fact from the meteorite. In addition to the fusion crust, small spheres were visible in broken surfaces in the rock matrix. As these were encouraging signs, the professors handed the samples over to School senior technician Caiphas Majola who prepared a thin section of one of them, for microscopic analysis.

The analysis confirmed that the fragment came from a common form of meteorite called a chrondrite (because these meteorites contain small spheres called chondrules). These meteorites date from the formation of the Solar System, some 4.56-billion years ago. However, further research was required to scientifically confirm the event and so to be able to have the meteorite officially named and registered (as the Benenitra meteorite) on the database of the International Meteoritical Society.

Consequently, the Wits team contacted Dr Andry Ramanantsoa of the UA Seismology and Infrasound Lab to see if he could establish whether there had been a noticeable explosion in the atmosphere over his country on the evening in question. He was able to use infrasound data from Comprehensive Nuclear Test Ban Treaty Organisation infrasound station IS33 (located outside Antananarivo) to confirm there had been an “upper atmosphere energy release event” at 7.16 PM local time (5.16 PM Greenwich Mean Time – GMT) on July 27. Moreover, he was also able to establish that the explosion had occurred on the exact bearing of Benenitra.

The next question was: had that explosion generated a shock wave strong enough to have created a ground vibration that could have been detected by geophysical seismometers? To this end, postdoctoral fellow Dr Andriamiranto Raveloson, technical manager of the AASN, was consulted. He determined that a very slight seismic tremor was detected at 5.17 PM GMT that same evening.

The final step in the investigation was the examination of the meteorite fragment for rare cosmogenic nuclides (which are created when an asteroid in space is bombarded by high energy cosmic rays). This was done by Dr Matthias Laubenstein at the Gran Sasso Lab. He found high levels of cosmogenic nuclides, indicating that the meteorite had entered Earth’s protecting atmosphere only during recent months.

The meteorite is now classified as an L6 chondrite. “L” means it is low in iron, “6” indicates that it experienced a high level of recrystallisation of its metals and chondrules through heating but experienced only partial melting (which allowed some chondrules to survive). This indicates that the meteorite was once part of a much larger body. “The meteorite also has a thin shock-melt vein that is likely related to a collision with another asteroid that shattered the original body and sent the fragment spinning off on its eventual collision course with Earth,” stated the Wits press release.

Edited by Creamer Media Reporter

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